JAVA-PPT'S.pdf

UNIT I
 OOP concepts- Data abstraction- encapsulation-
inheritance- benefits of inheritance- polymorphism-classes
and objects- procedural and object oriented programming
paradigm.
 Java programming – History of java- comments data
types-variables-constants-scope and life time of variables-
operators-operator hierarchy-expressions-type conversion
and casting- enumerated types- control flow – block scope-
conditional statements-loops-break and continue
statements- simple java stand alone programs-arrays-
console input and output- formatting output-constructors-
methods-parameter passing- static fields and methods-
access control- this reference- overloading methods and
constructors-recursion-garbage collection- building
strings- exploring string class

Need for OOP Paradigm
 OOP is an approach to program organization and development,
which attempts to eliminate some of the drawbacks of conventional
programming methods by incorporating the best of structured
programming features with several new concepts.
 OOP allows us to decompose a problem into number of entities
called objects and then build data and methods (functions) around
these entities.
 The data of an object can be accessed only by the methods
associated with the object.

Introduction
 Object-oriented programming (OOP) is a programming
paradigm that uses “Objects “and their interactions to design
applications.
 It simplifies the software development and maintenance by
providing some concepts:
 Object
 Class
 Data Abstraction & Encapsulation
 Inheritance
 Polymorphism
 Dynamic Binding
 Message Passing

Object
 Objects are the basic run time entities in an object-
oriented system. They may represent a person, a place,
a bank account, a table of data or any item that the
program has to handle.

Class
 The entire set of data and code of an object can be made of a
user defined data type with the help of a class.
 In fact, Objects are variables of the type class. Once a class has
been defined, we can create any number of objects belonging to
that class.
 Classes are data types based on which objects are created.
Objects with similar properties and methods are grouped
together to form a Class. Thus a Class represents a set of
individual objects.

 Characteristics of an object are represented in a class as
Properties. The actions that can be performed by objects
become functions of the class and is referred to as
Methods.
 A class is thus a collection of objects of similar type . for
example: mango, apple, and orange are members of the
class fruit . ex: fruit mango; will create an object mango
belonging to the class fruit.

Example for class
 class Human
 {
private:
 EyeColor IColor;
 NAME personname;
 public:
 void SetName(NAME anyName);
 void SetIColor(EyeColor eyecolor);
 };

Data abstraction
 Abstraction refers to the act of representing essential features
without including the background details or explanations. since the
classes use the concept of data abstraction ,they are known as
abstraction data type(ADT).
For example, a class Car would be made up of an Engine, Gearbox,
Steering objects, and many more components. To build the Car
class, one does not need to know how the different components
work internally, but only how to interface with them, i.e., send
messages to them, receive messages from them, and perhaps make
the different objects composing the class interact with each other.

An example for abstraction
 Humans manage complexity through abstraction. When you drive
your car you do not have to be concerned with the exact internal
working of your car(unless you are a mechanic). What you are
concerned with is interacting with your car via its interfaces like
steering wheel, brake pedal, accelerator pedal etc. Various
manufacturers of car has different implementation of car working but
its basic interface has not changed (i.e. you still use steering wheel,
brake pedal, accelerator pedal etc to interact with your car). Hence the
knowledge you have of your car is abstract.

Some of the Object-Oriented Paradigm are:
1. Emphasis is on data rather than procedure.
2. Programs are divided into objects.
3. Data Structures are designed such that they Characterize
the objects.
4 Methods that operate on the data of an object are tied
together in the data structure.
5 Data is hidden and can not be accessed by external
functions.
6 Objects may communicate with each other through
methods.

A way of viewing world – Agents
 OOP uses an approach of treating a real world agent as an
object.
 Object-oriented programming organizes a program around its
data (that is, objects) and a set of well-defined interfaces to
that data.
 An object-oriented program can be characterized as data
controlling access to code by switching the controlling entity to
data.

Responsibility
 primary motivation is the need for a platform-independent
(that is, architecture- neutral) language that could be used to
create software to be embedded in various consumer
electronic devices, such as microwave ovens and remote
controls.
 Objects with clear responsibilities.
 Each class should have a clear responsibility.
 If you can't state the purpose of a class in a single, clear
sentence, then perhaps your class structure needs some
thought.

Messages
 We all like to use programs that let us know what's going on.
Programs that keep us informed often do so by displaying
status and error messages.
 These messages need to be translated so they can be
understood by end users around the world.

 The Section discusses translatable text messages. Usually,
you're done after you move a message String into a
ResourceBundle.
 If you've embedded variable data in a message, you'll have to
take some extra steps to prepare it for translation.

Methods
 A method is a group of instructions that is given a name and can be
called up at any point in a program simply by quoting that name.
 Drawing a Triangle require draw of three straight lines. This
instruction three times to draw a simple triangle.
 We can define a method to call this instruction three times and draw
the triangle(i.e. create a method drawLine() to draw lines and this
method is called repeatedly to achieve the needed task)
 The idea of methods appears in all programming languages, although
sometimes it goes under the name functions and sometimes under
the name procedures.
 The name methods is a throw-back to the language C++, from which
Java was developed.
 In C++, there is an object called a class which can contain methods.
However, everything in Java is enclosed within a class .so the
functions within it are called methods

CLASSES
• Class is blue print or an idea of an Object
• From One class any number of Instances can be created
• It is an encapsulation of attributes and methods
FIGURE
CIRCLE
RECTANGLE
SQUARE
Ob1
Ob2
Ob3
class

syntax of CLASS
class <ClassName>
{
attributes/variables;
Constructors();
methods();
}

INSTANCE
• Instance is an Object of a class which is an entity with its own
attribute values and methods.
• Creating an Instance
ClassName refVariable;
refVariable = new Constructor();
or
ClassName refVariable = new Constructor();

Java Class Hierarchy
• In Java, class “Object” is the base class to all other classes
– If we do not explicitly say extends in a new class definition,
it implicitly extends Object
– The tree of classes that extend from Object and all of its
subclasses are is called the class hierarchy
– All classes eventually lead back up to Object
– This will enable consistent access of objects of different
classes.

Inheritance
 Methods allows to reuse a sequence of statements
 Inheritance allows to reuse classes by deriving a new class
from an existing one
 The existing class is called the parent class, or superclass, or
base class
 The derived class is called the child class or subclass.
 The child class inherits characteristics of the parent class(i.e
the child class inherits the methods and data defined for the
parent class

Inheritance
 Inheritance relationships are often shown graphically
in a class diagram, with the arrow pointing to the
parent class
Animal
weight : int
+ getWeight() : int
Bird
+ fly() : void

Method Binding
 Objects are used to call methods.
 MethodBinding is an object that can be used to call an arbitrary
public method, on an instance that is acquired by evaluating the
leading portion of a method binding expression via a value
binding.
 It is legal for a class to have two or more methods with the same
name.
 Java has to be able to uniquely associate the invocation of a
method with its definition relying on the number and types of
arguments.
 Therefore the same-named methods must be distinguished:
1) by the number of arguments, or
2) by the types of arguments
 Overloading and inheritance are two ways to implement
polymorphism.

Method Overriding.
 There may be some occasions when we want an object to
respond to the same method but have different behavior
when that method is called.
 That means, we should override the method defined in the
super class. This is possible by defining a method in a sub class
that has the same name, same arguments and same return
type as a method in the super class.
 Then when that method is called, the method defined in the
sub class is invoked and executed instead of the one in the
super class. This is known as overriding.

Exceptions in Java
• Exception is an abnormal condition that arises in the code
sequence.
• Exceptions occur during compile time or run time.
• “throwable” is the super class in exception hierarchy.
• Compile time errors occurs due to incorrect syntax.
• Run-time errors happen when
– User enters incorrect input
– Resource is not available (ex. file)
– Logic error (bug) that was not fixed

Exception classes
 In Java, exceptions are objects. When you throw an exception, you
throw an object. You can't throw just any object as an exception,
however -- only those objects whose classes descend from Throwable.
 Throwable serves as the base class for an entire family of classes,
declared in java.lang, that your program can instantiate and throw.
 Throwable has two direct subclasses, Exception and Error.
 Exceptions are thrown to signal abnormal conditions that can often be
handled by some catcher, though it's possible they may not be caught
and therefore could result in a dead thread.
 Errors are usually thrown for more serious problems, such as
OutOfMemoryError, that may not be so easy to handle. In general,
code you write should throw only exceptions, not errors.
 Errors are usually thrown by the methods of the Java API, or by the
Java virtual machine itself.

Summary of OOPS
The following are the basic oops concepts: They are as follows:
1. Objects.
2. Classes.
3. Data Abstraction.
4. Data Encapsulation.
5. Inheritance.
6. Polymorphism.
7. Dynamic Binding.
8. Message Passing.

Abstraction in Object-Oriented Programming
Procedural Abstraction
• Procedural Abstractions organize instructions.
Function Power
Give me two numbers (base & exponent)
I’ll return baseexponent
Implementation

Data Abstraction
• Data Abstractions organize data.
Name (string)
Marks (num)
Grade (char)
Student Number (num)
StudentType

Behavioral Abstraction
• Behavioral Abstractions combine procedural and
data abstractions.
Data State
Enqueue
Is Full
Is Empty Dequeue
Initialize
Queue Object

Java History
 Computer language innovation and development occurs
for two fundamental reasons:
1) to adapt to changing environments and uses
2) to implement improvements in the art of
programming
 The development of Java was driven by both in equal
measures.
 Many Java features are inherited from the earlier
languages:
B  C  C++  Java

Before Java: C
 Designed by Dennis Ritchie in 1970s.
 Before C: BASIC, COBOL, FORTRAN, PASCAL
 C- structured, efficient, high-level language that could
replace assembly code when creating systems programs.
 Designed, implemented and tested by programmers.

Before Java: C++
 Designed by Bjarne Stroustrup in 1979.
 Response to the increased complexity of programs and
respective improvements in the programming
paradigms and methods:
1) assembler languages
2) high-level languages
3) structured programming
4) object-oriented programming (OOP)
 OOP – methodology that helps organize complex
programs through the use of inheritance, encapsulation
and polymorphism.
 C++ extends C by adding object-oriented features.

Java: History
 In 1990, Sun Microsystems started a project called Green.
 Objective: to develop software for consumer electronics.
 Project was assigned to James Gosling, a veteran of classic
network software design. Others included Patrick
Naughton, ChrisWarth, Ed Frank, and Mike Sheridan.
 The team started writing programs in C++ for embedding
into
– toasters
– washing machines
– VCR’s
 Aim was to make these appliances more “intelligent”.

Java: History (contd.)
 C++ is powerful, but also dangerous. The power and popularity of
C derived from the extensive use of pointers. However, any
incorrect use of pointers can cause memory leaks, leading the
program to crash.
 In a complex program, such memory leaks are often hard to
detect.
 Robustness is essential. Users have come to expect that Windows
may crash or that a program running under Windows may crash.
(“This program has performed an illegal operation and will be
shut down”)
 However, users do not expect toasters to crash, or washing
machines to crash.
 A design for consumer electronics has to be robust.
 Replacing pointers by references, and automating memory
management was the proposed solution.
L 1.5

Java: History (contd.)
 Hence, the team built a new programming language called Oak,
which avoided potentially dangerous constructs in C++, such as
pointers, pointer arithmetic, operator overloading etc.
 Introduced automatic memory management, freeing the
programmer to concentrate on other things.
 Architecture neutrality (Platform independence)
 Many different CPU’s are used as controllers. Hardware chips are
evolving rapidly. As better chips become available, older chips
become obsolete and their production is stopped. Manufacturers
of toasters and washing machines would like to use the chips
available off the shelf, and would not like to reinvest in compiler
development every two-three years.
 So, the software and programming language had to be architecture
neutral.

Java: History (contd)
 It was soon realized that these design goals of consumer electronics perfectly
suited an ideal programming language for the Internet and WWW, which
should be:
 object-oriented (& support GUI)
 – robust
 – architecture neutral
 Internet programming presented a BIG business opportunity. Much bigger
than programming for consumer electronics.
 Java was “re-targeted” for the Internet
 The team was expanded to include Bill Joy (developer of Unix), Arthur van
Hoff, Jonathan Payne, Frank Yellin, Tim Lindholm etc.
 In 1994, an early web browser called WebRunner was written in Oak.
WebRunner was later renamed HotJava.
 In 1995, Oak was renamed Java.
 A common story is that the name Java relates to the place from where the
development team got its coffee. The name Java survived the trade mark
search.

Java History
 Designed by James Gosling, Patrick Naughton, Chris
Warth, Ed Frank and Mike Sheridan at Sun
Microsystems in 1991.
 The original motivation is not Internet: platform-
independent software embedded in consumer
electronics devices.
 With Internet, the urgent need appeared to break the
fortified positions of Intel, Macintosh and Unix
programmer communities.
 Java as an “Internet version of C++”? No.
 Java was not designed to replace C++, but to solve a
different set of problems.

The Java Buzzwords
 The key considerations were summed up by the Java
team in the following list of buzzwords:
 Simple
 Secure
 Portable
 Object-oriented
 Robust
 Multithreaded
 Architecture-neutral
 Interpreted
 High performance
 Distributed
 Dynamic

 simple – Java is designed to be easy for the professional
programmer to learn and use.
 object-oriented: a clean, usable, pragmatic approach to
objects, not restricted by the need for compatibility with
other languages.
 Robust: restricts the programmer to find the mistakes early,
performs compile-time (strong typing) and run-time
(exception-handling) checks, manages memory
automatically.
 Multithreaded: supports multi-threaded programming for
writing program that perform concurrent computations

 Architecture-neutral: Java Virtual Machine provides
a platform independent environment for the execution
of Java byte code
 Interpreted and high-performance: Java programs
are compiled into an intermediate representation –
byte code:
a) can be later interpreted by any JVM
b) can be also translated into the native machine code
for efficiency.

 Distributed: Java handles TCP/IP protocols,
accessing a resource through its URL much like
accessing a local file.
 Dynamic: substantial amounts of run-time type
information to verify and resolve access to objects
at run-time.
 Secure: programs are confined to the Java
execution environment and cannot access other
parts of the computer.

 Portability: Many types of computers and
operating systems are in use throughout the
world—and many are connected to the Internet.
 For programs to be dynamically downloaded to all
the various types of platforms connected to the
Internet, some means of generating portable
executable code is needed. The same mechanism
that helps ensure security also helps create
portability.
 Indeed, Java's solution to these two problems is
both elegant and efficient.
L 1.13

Data Types
 Java defines eight simple types:
1)byte – 8-bit integer type
2)short – 16-bit integer type
3)int – 32-bit integer type
4)long – 64-bit integer type
5)float – 32-bit floating-point type
6)double – 64-bit floating-point type
7)char – symbols in a character set
8)boolean – logical values true and false

 byte: 8-bit integer type.
Range: -128 to 127.
Example: byte b = -15;
Usage: particularly when working with data
streams.
 short: 16-bit integer type.
Range: -32768 to 32767.
Example: short c = 1000;
Usage: probably the least used simple type.

 int: 32-bit integer type.
Range: -2147483648 to 2147483647.
Example: int b = -50000;
Usage:
1) Most common integer type.
2) Typically used to control loops and to index arrays.
3) Expressions involving the byte, short and int values are
promoted to int before calculation.
L 1.16

 long: 64-bit integer type.
Range: -9223372036854775808 to
9223372036854775807.
Example: long l = 10000000000000000;
Usage: 1) useful when int type is not large enough to hold
the desired value
 float: 32-bit floating-point number.
Range: 1.4e-045 to 3.4e+038.
Example: float f = 1.5;
Usage:
1) fractional part is needed
2) large degree of precision is not required

 double: 64-bit floating-point number.
Range: 4.9e-324 to 1.8e+308.
Example: double pi = 3.1416;
Usage:
1) accuracy over many iterative calculations
2) manipulation of large-valued numbers
L 1.18

char: 16-bit data type used to store characters.
Range: 0 to 65536.
Example: char c = ‘a’;
Usage:
1) Represents both ASCII and Unicode character sets;
Unicode defines a
character set with characters found in (almost) all
human languages.
2) Not the same as in C/C++ where char is 8-bit and
represents ASCII only.

 boolean: Two-valued type of logical values.
Range: values true and false.
Example: boolean b = (1<2);
Usage:
1) returned by relational operators, such as 1<2
2) required by branching expressions such as if
or for
L 1.20

Variables
 declaration – how to assign a type to a variable
 initialization – how to give an initial value to a variable
 scope – how the variable is visible to other parts of the
program
 lifetime – how the variable is created, used and destroyed
 type conversion – how Java handles automatic type
conversion
 type casting – how the type of a variable can be narrowed
down

Variables
 Java uses variables to store data.
 To allocate memory space for a variable JVM
requires:
1) to specify the data type of the variable
2) to associate an identifier with the variable
3) optionally, the variable may be assigned an initial
value
 All done as part of variable declaration.
L 2.2

Basic Variable Declaration
 datatype identifier [=value];
 datatype must be
 A simple datatype
 User defined datatype (class type)
 Identifier is a recognizable name confirm to identifier
rules
 Value is an optional initial value.

Variable Declaration
 We can declare several variables at the same time:
type identifier [=value][, identifier [=value] …];
Examples:
int a, b, c;
int d = 3, e, f = 5;
byte g = 22;
double pi = 3.14159;
char ch = 'x';
L 2.4

Variable Scope
 Scope determines the visibility of program elements with respect
to other program elements.
 In Java, scope is defined separately for classes and methods:
1) variables defined by a class have a global scope
2) variables defined by a method have a local scope
A scope is defined by a block:
{
…
}
A variable declared inside the scope is not visible outside:
{
int n;
}
n = 1;// this is illegal

Variable Lifetime
 Variables are created when their scope is entered
by control flow and destroyed when their scope is
left:
 A variable declared in a method will not hold its
value between different invocations of this
method.
 A variable declared in a block looses its value when
the block is left.
 Initialized in a block, a variable will be re-
initialized with every re-entry. Variables lifetime is
confined to its scope!

Arrays
 An array is a group of liked-typed variables referred to by
a common
 name, with individual variables accessed by their index.
 Arrays are:
1) declared
2) created
3) initialized
4) used
 Also, arrays can have one or several dimensions.

Array Declaration
 Array declaration involves:
1) declaring an array identifier
2) declaring the number of dimensions
3) declaring the data type of the array elements
 Two styles of array declaration:
type array-variable[];
or
type [] array-variable;
L 2.8

Array Creation
 After declaration, no array actually exists.
 In order to create an array, we use the new
operator:
type array-variable[];
array-variable = new type[size];
 This creates a new array to hold size elements of
type type, which reference will be kept in the
variable array-variable.

Array Indexing
 Later we can refer to the elements of this array through
their indexes:
 array-variable[index]
 The array index always starts with zero!
 The Java run-time system makes sure that all array
indexes are in the correct range, otherwise raises a run-
time error.

Array Initialization
 Arrays can be initialized when they are declared:
 int monthDays[] = {31,28,31,30,31,30,31,31,30,31,30,31};
 Note:
1) there is no need to use the new operator
2) the array is created large enough to hold all specified
elements

Multidimensional Arrays
 Multidimensional arrays are arrays of arrays:
1) declaration: int array[][];
2) creation: int array = new int[2][3];
3) initialization
int array[][] = { {1, 2, 3}, {4, 5, 6} };

Operators Types
 Java operators are used to build value expressions.
 Java provides a rich set of operators:
1) assignment
2) arithmetic
3) relational
4) logical
5) bitwise
L 2.13

Arithmetic assignments
+= v += expr; v = v + expr ;
-= v -=expr; v = v - expr ;
*= v *= expr; v = v * expr ;
/= v /= expr; v = v / expr ;
%= v %= expr; v = v % expr ;

Basic Arithmetic Operators
+ op1 + op2 ADD
- op1 - op2 SUBSTRACT
* op1 * op2 MULTIPLY
/ op1 / op2 DIVISION
% op1 % op2 REMAINDER
L 2.15

Relational operator
== Equals to Apply to any type
!= Not equals to Apply to any type
> Greater than Apply to numerical type
< Less than Apply to numerical type
>= Greater than or equal Apply to numerical type
<= Less than or equal Apply to numerical type

Logical operators
& op1 & op2 Logical AND
| op1 | op2 Logical OR
&& op1 && op2 Short-circuit
AND
|| op1 || op2 Short-circuit OR
! ! op Logical NOT
^ op1 ^ op2 Logical XOR
L 2.17

Bit wise operators
~ ~op Inverts all bits
& op1 & op2 Produces 1 bit if both operands are 1
| op1 |op2 Produces 1 bit if either operand is 1
^ op1 ^ op2 Produces 1 bit if exactly one operand is 1
>> op1 >> op2 Shifts all bits in op1 right by the value of
op2
<< op1 << op2 Shifts all bits in op1 left by the value of
op2

 An expression is a construct made up of variables,
operators, and method invocations, which are
constructed according to the syntax of the language, that
evaluates to a single value.
 Examples of expressions are in bold below:
int number = 0;
anArray[0] = 100;
System.out.println ("Element 1 at index 0: " +
anArray[0]);
int result = 1 + 2; // result is now 3 if(value1 ==
value2)
System.out.println("value1 == value2");
L 2.19
Expressions

Expressions
 The data type of the value returned by an expression depends on
the elements used in the expression.
 The expression number = 0 returns an int because the
assignment operator returns a value of the same data type as its
left-hand operand; in this case, number is an int.
 As you can see from the other expressions, an expression can
return other types of values as well, such as boolean or String.
The Java programming language allows you to construct
compound expressions from various smaller expressions as long
as the data type required by one part of the expression matches
the data type of the other.
 Here's an example of a compound expression: 1 * 2 * 3

Control Statements
 Java control statements cause the flow of execution to
advance and branch based on the changes to the state
of the program.
 Control statements are divided into three groups:
 1) selection statements allow the program to choose
different parts of the execution based on the outcome
of an expression
 2) iteration statements enable program execution to
repeat one or more statements
 3) jump statements enable your program to execute in
a non-linear fashion
L 3.1

Selection Statements
 Java selection statements allow to control the flow
of program’s execution based upon conditions
known only during run-time.
 Java provides four selection statements:
1) if
2) if-else
3) if-else-if
4) switch

Iteration Statements
 Java iteration statements enable repeated execution of
part of a program until a certain termination condition
becomes true.
 Java provides three iteration statements:
1) while
2) do-while
3) for
L 3.3

Jump Statements
 Java jump statements enable transfer of control to
other parts of program.
 Java provides three jump statements:
1) break
2) continue
3) return
 In addition, Java supports exception handling that can
also alter the control flow of a program.

L 3.5
Type Conversion
• Size Direction of Data Type
– Widening Type Conversion (Casting down)
• Smaller Data Type  Larger Data Type
– Narrowing Type Conversion (Casting up)
• Larger Data Type  Smaller Data Type
• Conversion done in two ways
– Implicit type conversion
• Carried out by compiler automatically
– Explicit type conversion
• Carried out by programmer using casting

Type Conversion
• Widening Type Converstion
– Implicit conversion by compiler automatically
byte -> short, int, long, float, double
short -> int, long, float, double
char -> int, long, float, double
int -> long, float, double
long -> float, double
float -> double

Type Conversion
• Narrowing Type Conversion
– Programmer should describe the conversion
explicitly
byte -> char
short -> byte, char
char -> byte, short
int -> byte, short, char
long -> byte, short, char, int
float -> byte, short, char, int, long
double -> byte, short, char, int, long, float

Type Conversion
 byte and short are always promoted to int
 if one operand is long, the whole expression is
promoted to long
 if one operand is float, the entire expression is
promoted to float
 if any operand is double, the result is double

Type Casting
 General form: (targetType) value
 Examples:
 1) integer value will be reduced module bytes
range:
int i;
byte b = (byte) i;
 2) floating-point value will be truncated to
integer value:
float f;
int i = (int) f;
L 3.9

Simple Java Program
 A class to display a simple message:
class MyProgram
{
public static void main(String[] args)
{
System.out.println(“First Java program.");
}
}

What is an Object?
 Real world objects are things that have:
1) state
2) behavior
Example: your dog:
 state – name, color, breed, sits?, barks?, wages
tail?, runs?
 behavior – sitting, barking, waging tail, running
 A software object is a bundle of variables (state)
and methods (operations).

What is a Class?
 A class is a blueprint that defines the variables and
methods common to all objects of a certain kind.
 Example: ‘your dog’ is a object of the class Dog.
 An object holds values for the variables defines in the
class.
 An object is called an instance of the Class
L 4.3

Object Creation
 A variable is declared to refer to the objects of
type/class String:
String s;
 The value of s is null; it does not yet refer to any
object.
 A new String object is created in memory with
initial “abc” value:
 String s = new String(“abc”);
 Now s contains the address of this new object.

Object Destruction
 A program accumulates memory through its
execution.
 Two mechanism to free memory that is no longer need
by the program:
1) manual – done in C/C++
2) automatic – done in Java
 In Java, when an object is no longer accessible through
any variable, it is eventually removed from the
memory by the garbage collector.
 Garbage collector is parts of the Java Run-Time
Environment.
L 4.5

Class
 A basis for the Java language.
 Each concept we wish to describe in Java must be
included inside a class.
 A class defines a new data type, whose values are
objects:
 A class is a template for objects
 An object is an instance of a class

Class Definition
 A class contains a name, several variable declarations
(instance variables) and several method declarations. All
are called members of the class.
 General form of a class:
class classname {
type instance-variable-1;
…
type instance-variable-n;
type method-name-1(parameter-list) { … }
type method-name-2(parameter-list) { … }
…
type method-name-m(parameter-list) { … }
}
L 4.7

Example: Class Usage
class Box {
double width;
double height;
double depth;
}
class BoxDemo {
public static void main(String args[]) {
Box mybox = new Box();
double vol;
mybox.width = 10;
mybox.height = 20;
mybox.depth = 15;
vol = mybox.width * mybox.height * mybox.depth;
System.out.println ("Volume is " + vol);
} }

Constructor
 A constructor initializes the instance variables of an object.
 It is called immediately after the object is created but before
the new operator completes.
1) it is syntactically similar to a method:
2) it has the same name as the name of its class
3) it is written without return type; the default
return type of a class
 constructor is the same classWhen the class has no
constructor, the default constructor automatically initializes
all its instance variables with zero.

Example: Constructor
class Box {
double width;
double height;
double depth;
Box() {
System.out.println("Constructing Box");
width = 10; height = 10; depth = 10;
}
double volume() {
return width * height * depth;
}
}
L 5.2

Parameterized Constructor
class Box {
double width;
double height;
double depth;
Box(double w, double h, double d) {
width = w; height = h; depth = d;
}
double volume()
{ return width * height * depth;
}
}

Methods
 General form of a method definition:
type name(parameter-list) {
… return value;
…
}
 Components:
1) type - type of values returned by the method. If a method
does not return any value, its return type must be void.
2) name is the name of the method
3) parameter-list is a sequence of type-identifier lists
separated by commas
4) return value indicates what value is returned by the
method.
L 5.4

Example: Method
 Classes declare methods to hide their internal data
structures, as well as for their own internal use: Within a
class, we can refer directly to its member variables:
class Box {
double width, height, depth;
void volume() {
System.out.print("Volume is ");
System.out.println(width * height * depth);
}
}

Parameterized Method
 Parameters increase generality and applicability of
a method:
 1) method without parameters
int square() { return 10*10; }
 2) method with parameters
int square(int i) { return i*i; }
 Parameter: a variable receiving value at the time
the method is invoked.
 Argument: a value passed to the method when it is
invoked.
L 5.6

Access Control: Data Hiding and
Encapsulation
• Java provides control over the visibility of variables
and methods.
• Encapsulation, safely sealing data within the capsule
of the class Prevents programmers from relying on
details of class implementation, so you can update
without worry
• Helps in protecting against accidental or wrong
usage.
• Keeps code elegant and clean (easier to maintain)

L 6.2
Access Modifiers: Public, Private,
Protected
• Public: keyword applied to a class, makes it
available/visible everywhere. Applied to a
method or variable, completely visible.
• Default(No visibility modifier is specified): it
behaves like public in its package and private
in other packages.
• Default Public keyword applied to a class,
makes it available/visible everywhere.
Applied to a method or variable, completely
visible.

 Private fields or methods for a class only visible within
that class. Private members are not visible within
subclasses, and are not inherited.
 Protected members of a class are visible within the
class, subclasses and also within all classes that are in
the same package as that class.

L 6.4
Visibility
public class Circle {
private double x,y,r;
// Constructor
public Circle (double x, double y, double r) {
this.x = x;
this.y = y;
this.r = r;
}
//Methods to return circumference and area
public double circumference() { return 2*3.14*r;}
public double area() { return 3.14 * r * r; }
}

String Handling
 String is probably the most commonly used class in
Java's class library. The obvious reason for this is that
strings are a very important part of programming.
 The first thing to understand about strings is that
every string you create is actually an object of type
String. Even string constants are actually String
objects.
 For example, in the statement
System.out.println("This is a String, too");
the string "This is a String, too" is a String constant

 Java defines one operator for String objects: +.
 It is used to concatenate two strings. For example, this
statement
 String myString = "I" + " like " + "Java.";
results in myString containing
"I like Java."
L 8.4

 The String class contains several methods that you can use.
Here are a few. You can
 test two strings for equality by using
equals( ). You can obtain the length of a string by calling the
length( ) method. You can obtain the character at a specified
index within a string by calling charAt( ). The general forms
of these three methods are shown here:
 // Demonstrating some String methods.
class StringDemo2 {
String strOb1 = "First String";
String strOb2 = "Second String";
String strOb3 = strOb1;
System.out.println("Length of strOb1: " +
strOb1.length());

System.out.println ("Char at index 3 in strOb1: " +
strOb1.charAt(3));
if(strOb1.equals(strOb2))
System.out.println("strOb1 == strOb2");
else
System.out.println("strOb1 != strOb2");
if(strOb1.equals(strOb3))
System.out.println("strOb1 == strOb3");
else
System.out.println("strOb1 != strOb3");
} }
This program generates the following output:
Length of strOb1: 12
Char at index 3 in strOb1: s
strOb1 != strOb2
strOb1 == strOb3

TOPICS
1 Hierarchical abstractions Base class object.
2 subclass, subtype, substitutability.
3 forms of inheritance- specialization,
4 construction, extension, limitation, combination.
5 Benefits of inheritance, costs of inheritance.
6 Member access rules, super uses, using final with inheritance.
7 polymorphism- method overriding, abstract classes.
1 Defining, Creating and Accessing a Package
2 Importing packages
3 Differences between classes and interfaces
4 Defining an interface
5 Implementing interface
6 Applying interfaces
7 variables in interface and extending interfaces

107
Inheritance
 Methods allows a software developer to reuse a sequence of
statements
 Inheritance allows a software developer to reuse classes by
deriving a new class from an existing one
 The existing class is called the parent class, or superclass,
or base class
 As the name implies, the child inherits characteristics of
the parent
 That is, the child class inherits the methods and data
defined for the parent class

108
Inheritance
 Inheritance relationships are often shown
graphically in a class diagram, with the arrow
pointing to the parent class
Inheritance
should create an
is-a relationship,
meaning the
child is a more
specific version
of the parent
Animal
weight : int
+ getWeight() : int
Bird
+ fly() : void

109
Deriving Subclasses
 In Java, we use the reserved word extends to establish an
inheritance relationship
class Animal
{
// class contents
int weight;
public void int getWeight() {…}
}
class Bird extends Animal
{
// class contents
public void fly() {…};
}

110
Class Hierarchy
 A child class of one parent can be the parent of another
child, forming class hierarchies
Animal
Reptile Bird Mammal
Snake Lizard Bat
Horse
Parrot
 At the top of the hierarchy there’s a default class
called Object.

111
Class Hierarchy
 Good class design puts all common features as high in the
hierarchy as reasonable
 inheritance is transitive
 An instance of class Parrot is also an instance of Bird, an instance of
Animal, …, and an instance of class Object
 The class hierarchy determines how methods are executed:
 Previously, we took the simplified view that when variable v is an
instance of class C, then a procedure call v.proc1() invokes the
method proc1() defined in class C
 However, if C is a child of some superclass C’ (and hence v is both
an instance of C and an instance of C’), the picture becomes more
complex, because methods of class C can override the methods of
class C’ (next two slides).

112
Defining Methods in the Child Class:
Overriding by Replacement
 A child class can override the definition of an inherited method in
favor of its own
 that is, a child can redefine a method that it inherits from its parent
 the new method must have the same signature as the parent's method,
but can have different code in the body
 In java, all methods except of constructors override the methods of
their ancestor class by replacement. E.g.:
 the Animal class has method eat()
 the Bird class has method eat() and Bird extends Animal
 variable b is of class Bird, i.e. Bird b = …
 b.eat() simply invokes the eat() method of the Bird class
 If a method is declared with the final modifier, it cannot be
overridden

113
Overriding by Refinement
 Constructors in a subclass override the definition of an inherited constructor
method by refining them (instead of replacing them)
- Assume class Animal has constructors
Animal(), Animal(int weight), Animal(int weight, int livespan)
- Assume class Bird which extends Animal has constructors
Bird(), Bird(int weight), Bird(int weight, int livespan)
- Let’s say we create a Bird object, e.g. Bird b = Bird(5)
- This will invoke first the constructor of the Animal (the superclass of Bird) and
then the constructor of the Bird
 This is called constructor chaining: If class C0 extends C1 and C1 extends C2
and … Cn-1 extends Cn = Object then when creating an instance of object C0
first constructor of Cn is invoked, then constructors of Cn-1, …, C2, C1, and
finally the constructor of C
- The constructors (in each case) are chosen by their signature, e.g. (), (int), etc…
- If no constructor with matching signature is found in any of the class Ci for i>0 then the
default constructor is executed for that class
- If no constructor with matching signature is found in the class C0 then this causes a
compiler errorFirst the new method must have the same signature as the parent's method,
but can have different code in the body

114
Recap: Class Hierarchy
 In Java, a class can extend a single other class
(If none is stated then it implicitly extends an Object class)
Animal
Reptile Bird Mammal
Snake Lizard Bat
Horse
Parrot
 Imagine what would happen to method handling
rules if every class could extend two others…
(Answer: It would create multiple problems!)

Hierarchical Abstraction
 An essential element of object-oriented programming is
abstraction.
 Humans manage complexity through abstraction. For
example, people do not think of a car as a set of tens of
thousands of individual parts. They think of it as a well-
defined object with its own unique behavior.
 This abstraction allows people to use a car without being
overwhelmed by the complexity of the parts that form the
car. They can ignore the details of how the engine,
transmission, and braking systems work.
 Instead they are free to utilize the object as a whole.

Class Hierarchy
 A child class of one parent can be the parent of another
child, forming class hierarchies
Animal
Reptile Bird Mammal
Snake Lizard Bat
Horse
Parrot
 At the top of the hierarchy there’s a default class called Object.

Class Hierarchy
 Good class design puts all common features as high in
the hierarchy as reasonable
 The class hierarchy determines how methods are
executed
 inheritance is transitive
 An instance of class Parrot is also an instance of Bird,
an instance of Animal, …, and an instance of class
Object

Base Class Object
 In Java, all classes use inheritance.
 If no parent class is specified explicitly, the base class Object is
implicitly inherited.
 All classes defined in Java, is a child of Object class, which provides
minimal functionality guaranteed to e common to all objects.
 Methods defined in Object class are;
1. equals(Object obj): Determine whether the argument object is the
same as the receiver.
2. getClass(): Returns the class of the receiver, an object of type Class.
3. hashCode(): Returns a hash value for this object. Should be
overridden when the equals method is changed.
4. toString(): Converts object into a string value. This method is also
often overridden.

Base class
1) a class obtains variables and methods from another class
2) the former is called subclass, the latter super-class (Base class)
3) a sub-class provides a specialized behavior with respect to its
super-class
4) inheritance facilitates code reuse and avoids duplication of
data
Extends
 Is a keyword used to inherit a class from another class
 Allows to extend from only one class
class One
{ int a=5;
}
class Two extends One
{
int b=10;
}

Subclass, Subtype and Substitutability
 A subtype is a class that satisfies the principle of
substitutability.
 A subclass is something constructed using inheritance,
whether or not it satisfies the principle of substitutability.
 The two concepts are independent. Not all subclasses are
subtypes, and (at least in some languages) you can
construct subtypes that are not subclasses.
 Substitutability is fundamental to many of the powerful
software development techniques in OOP.
 The idea is that, declared a variable in one type may hold
the value of different type.
 Substitutability can occur through use of inheritance,
whether using extends, or using implements keywords.

Subclass, Subtype, and Substitutability
When new classes are constructed using inheritance, the argument
used to justify the validity of substitutability is as follows;
• Instances of the subclass must possess all data fields associated
with its parent class.
• Instances of the subclass must implement, through inheritance
at least, all functionality defined for parent class. (Defining new
methods is not important for the argument.)
• Thus, an instance of a child class can mimic the behavior of the
parent class and should be indistinguishable from an instance of
parent class if substituted in a similar situation.

Subclass, Subtype, and
Substitutability
The term subtype is used to describe the relationship between
types that explicitly recognizes the principle of substitution. A type
B is considered to be a subtype of A if an instances of B can legally
be assigned to a variable declared as of type A.
The term subclass refers to inheritance mechanism made by
extends keyword.
Not all subclasses are subtypes. Subtypes can also be formed
using interface, linking types that have no inheritance relationship.

Subclass
 Methods allows to reuse a sequence of statements
 Inheritance allows to reuse classes by deriving a new class from
an existing one
 The existing class is called the parent class, or superclass, or base
class
 As the name implies, the child inherits characteristics of the
parent(i.e the child class inherits the methods and data defined
for the parent class

Subtype
 Inheritance relationships are often shown graphically in
a class diagram, with the arrow pointing to the parent
class
Animal
weight : int
+ getWeight() : int
Bird
+ fly() : void

Substitutability (Deriving Subclasses)
 In Java, we use the reserved word extends to establish an
inheritance relationship
class Animal
{
// class contents
int weight;
public void int getWeight() {…}
}
class Bird extends Animal
{
// class contents
public void fly() {…};
}

Overriding by Replacement
 A child class can override the definition of an inherited method in
favor of its own
 that is, a child can redefine a method that it inherits from its parent
 the new method must have the same signature as the parent's
method, but can have different code in the body
 In java, all methods except of constructors override the methods
of their ancestor class by replacement. E.g.:
 the Animal class has method eat()
 the Bird class has method eat() and Bird extends Animal
 variable b is of class Bird, i.e. Bird b = …
 b.eat() simply invokes the eat() method of the Bird class
 If a method is declared with the final modifier, it cannot be
overridden

Forms of Inheritance
Inheritance is used in a variety of way and for a variety of different
purposes .
• Inheritance for Specialization
• Inheritance for Specification
• Inheritance for Construction
• Inheritance for Extension
• Inheritance for Limitation
• Inheritance for Combination
One or many of these forms may occur in a single case.

(- Inheritance for Specialization -)
Most commonly used inheritance and sub classification is for
specialization.
Always creates a subtype, and the principles of substitutability
is explicitly upheld.
It is the most ideal form of inheritance.
An example of subclassification for specialization is;
public class PinBallGame extends Frame {
// body of class
}

Specialization
 By far the most common form of inheritance is for specialization.
 Child class is a specialized form of parent class
 Principle of substitutability holds
 A good example is the Java hierarchy of Graphical components in the AWT:
• Component
 Label
 Button
 TextComponent
 TextArea
 TextField
 CheckBox
 ScrollBar

(- Inheritance for Specification -)
This is another most common use of inheritance. Two different
mechanisms are provided by Java, interface and abstract, to make use of
subclassification for specification. Subtype is formed and substitutability
is explicitly upheld.
Mostly, not used for refinement of its parent class, but instead is used for
definitions of the properties provided by its parent.
class FireButtonListener implements ActionListener {
// body of class
}
class B extends A {
// class A is defined as abstract specification class
}

Specification
 The next most common form of inheritance involves
specification. The parent class specifies some behavior, but
does not implement the behavior
 Child class implements the behavior
 Similar to Java interface or abstract class
 When parent class does not implement actual behavior but
merely defines the behavior that will be implemented in child
classes
 Example, Java 1.1 Event Listeners:
ActionListener, MouseListener, and so on specify behavior, but
must be subclassed.

(- Inheritance for Construction -)
Child class inherits most of its functionality from parent, but may
change the name or parameters of methods inherited from
parent class to form its interface.
This type of inheritance is also widely used for code reuse
purposes. It simplifies the construction of newly formed
abstraction but is not a form of subtype, and often violates
substitutability.
Example is Stack class defined in Java libraries.

Construction
 The parent class is used only for its behavior, the child class
has no is-a relationship to the parent.
 Child modify the arguments or names of methods

 An example might be subclassing the idea of a Set from an
existing List class.
 Child class is not a more specialized form of parent class;
no substitutability

(- Inheritance for Extension -)
Subclassification for extension occurs when a child class only
adds new behavior to the parent class and does not modify or
alter any of the inherited attributes.
Such subclasses are always subtypes, and substitutability can be
used.
Example of this type of inheritance is done in the definition of
the class Properties which is an extension of the class HashTable.

Generalization or Extension
 The child class generalizes or extends the parent class by
providing more functionality
 In some sense, opposite of subclassing for specialization
 The child doesn't change anything inherited from the
parent, it simply adds new features
 Often used when we cannot modify existing base parent
class
 Example, ColoredWindow inheriting from Window
 Add additional data fields
 Override window display methods

(- Inheritance for Limitation -)
Subclassification for limitation occurs when the behavior of the
subclass is smaller or more restrictive that the behavior of its
parent class.
Like subclassification for extension, this form of inheritance
occurs most frequently when a programmer is building on a
base of existing classes.
Is not a subtype, and substitutability is not proper.

Limitation
 The child class limits some of the behavior of the parent class.
 Example, you have an existing List data type, and you want a
Stack
 Inherit from List, but override the methods that allow access
to elements other than top so as to produce errors.

(- Inheritance for Combination -)
This types of inheritance is known as multiple inheritance in
Object Oriented Programming.
Although the Java does not permit a subclass to be formed be
inheritance from more than one parent class, several
approximations to the concept are possible.
Example of this type is Hole class defined as;
class Hole extends Ball implements PinBallTarget{
// body of class
}

Combimnation
 Two or more classes that seem to be related, but its not clear
who should be the parent and who should be the child.
 Example: Mouse and TouchPad and JoyStick
 Better solution, abstract out common parts to new parent
class, and use subclassing for specialization.

Summary of Forms of Inheritance
• Specialization. The child class is a special case of the parent class; in other words, the
child class is a subtype of the parent class.
• Specification. The parent class defines behavior that is implemented in the child class
but not in the parent class.
• Construction. The child class makes use of the behavior provided by the parent class,
but is not a subtype of the parent class.
• Generalization. The child class modifies or overrides some of the methods of the
parent class.
• Extension. The child class adds new functionality to the parent class, but does not
change any inherited behavior.
• Limitation. The child class restricts the use of some of the behavior inherited from
the parent class.
• Variance. The child class and parent class are variants of each other, and the class-
subclass relationship is arbitrary.
• Combination. The child class inherits features from more than one parent class. This
is multiple inheritance and will be the subject of a later chapter.

The Benefits of Inheritance
 Software Reusability (among projects)
 Increased Reliability (resulting from reuse and sharing
of well-tested code)
 Code Sharing (within a project)
 Consistency of Interface (among related objects)
 Software Components
 Rapid Prototyping (quickly assemble from pre-existing
components)
 Polymorphism and Frameworks (high-level reusable
components)
 Information Hiding

The Costs of Inheritance
 Execution Speed
 Program Size
 Message-Passing Overhead
 Program Complexity (in overuse of inheritance)

Types of inheritance
 Acquiring the properties of an existing Object into newly
creating Object to overcome the re-declaration of
properties in deferent classes.
 These are 3 types:
1.Simple Inheritance
SUPER
SUB
SUPER
SUB 1 SUB 2
extends
extends

2. Multi Level
Inheritance
3. Multiple
Inheritance
SUPER
SUB
SUB SUB
SUPER 1
SUPER 2
extends
extends
implement
s
SUB
SUPER 1 SUPER 2
implement
s
SUB
extends

Member access rules
 Visibility modifiers determine which class members are
accessible and which do not
 Members (variables and methods) declared with public visibility
are accessible, and those with private visibility are not
 Problem: How to make class/instance variables visible only to
its subclasses?
 Solution: Java provides a third visibility modifier that helps in
inheritance situations: protected

Modifiers and Inheritance
(cont.)
Visibility Modifiers for class/interface:
public : can be accessed from outside the class definition.
protected : can be accessed only within the class definition in
which it appears, within other classess in the same package,
or within the definition of subclassess.
private : can be accessed only within the class definition in
which it appears.
default-access (if omitted) features accessible from inside the
current Java package

The protected Modifier
 The protected visibility modifier allows a member of a base
class to be accessed in the child
 protected visibility provides more encapsulation than
public does
 protected visibility is not as tightly encapsulated as
private visibility
Book
protected int pages
+ getPages() : int
+ setPages(): void
Dictionary
+ getDefinitions() : int
+ setDefinitions(): void
+ computeRatios() : double

“super” uses
 ‘super’ is a keyword used to refer to hidden variables of super
class from sub class.
 super.a=a;
 It is used to call a constructor of super class from constructor of
sub class which should be first statement.
 super(a,b);
 It is used to call a super class method from sub class method to
avoid redundancy of code
 super.addNumbers(a, b);

Super and Hiding
 Why is super needed to access super-class members?
 When a sub-class declares the variables or methods with
the same names and types as its super-class:
class A {
int i = 1;
}
class B extends A {
int i = 2;
System.out.println(“i is “ + i);
}
 The re-declared variables/methods hide those of the
super-class.

Example: Super and Hiding
class A {
int i;
}
class B extends A {
int i;
B(int a, int b) {
super.i = a; i = b;
}
void show() {
System.out.println("i in superclass: " + super.i);
System.out.println("i in subclass: " + i);
}
}

Example: Super and Hiding
 Although the i variable in B hides the i variable in A,
super allows access to the hidden variable of the
super-class:
class UseSuper {
B subOb = new B(1, 2);
subOb.show();
}
}

Using final with inheritance
 final keyword is used declare constants which can not
change its value of definition.
 final Variables can not change its value.
 final Methods can not be Overridden or Over Loaded
 final Classes can not be extended or inherited

Preventing Overriding with final
 A method declared final cannot be overridden in
any sub-class:
class A {
final void meth() {
System.out.println("This is a final method.");
}
}
This class declaration is illegal:
class B extends A {
void meth() {
System.out.println("Illegal!");
}
}

Preventing Inheritance with final
 A class declared final cannot be inherited – has no sub-
classes.
final class A { … }
 This class declaration is considered illegal:
class B extends A { … }
 Declaring a class final implicitly declares all its methods
final.
 It is illegal to declare a class as both abstract and final.

Polymorphism
 Polymorphism is one of three pillars of object-
orientation.
 Polymorphism: many different (poly) forms of objects
that share a common interface respond differently when
a method of that interface is invoked:
1) a super-class defines the common interface
2) sub-classes have to follow this interface
(inheritance), but are also permitted to provide their
own implementations (overriding)
 A sub-class provides a specialized behaviors relying on
the common elements defined by its super-class.

Polymorphism
 A polymorphic reference can refer to different types of
objects at different times
 In java every reference can be polymorphic except of
references to base types and final classes.
 It is the type of the object being referenced, not the
reference type, that determines which method is invoked
 Polymorphic references are therefore resolved at run-
time, not during compilation; this is called dynamic
binding
 Careful use of polymorphic references can lead to elegant,
robust software designs

Method Overriding
 When a method of a sub-class has the same name
and type as a method of the super-class, we say that
this method is overridden.
 When an overridden method is called from within
the sub-class:
1) it will always refer to the sub-class method
2) super-class method is hidden

Example: Hiding with Overriding 1
class A {
int i, j;
A(int a, int b) {
i = a; j = b;
}
void show() {
System.out.println("i and j: " + i + " " + j);
}
}

class B extends A {
int k;
B(int a, int b, int c) {
super(a, b);
k = c;
}
void show() {
System.out.println("k: " + k);
}
}

 When show() is invoked on an object of type B,
the version of show() defined in B is used:
class Override {
B subOb = new B(1, 2, 3);
subOb.show();
}
}
 The version of show() in A is hidden through
overriding.

Overloading vs. Overriding
 Overloading deals with
multiple methods in the
same class with the same
name but different
signatures
 Overloading lets you
define a similar operation
in different ways for
different data
 Overriding deals with two
methods, one in a parent
class and one in a child
class, that have the same
signature
o Overriding lets you define a
similar operation in
different ways for different
object types

Abstract Classes
 Java allows abstract classes
 use the modifier abstract on a class header to declare an
abstract class
abstract class Vehicle
{ … }
 An abstract class is a placeholder in a class hierarchy
that represents a generic concept
Vehicle
Car Boat Plane

Abstract Class: Example
public abstract class Vehicle
{
String name;
public String getName()
{ return name; } method body
abstract public void move();
no body!
}
 An abstract class often contains abstract methods,
though it doesn’t have to
 Abstract methods consist of only methods declarations,
without any method body

Abstract Classes
 An abstract class often contains abstract methods, though it
doesn’t have to
 Abstract methods consist of only methods declarations, without any
method body
 The non-abstract child of an abstract class must override
the abstract methods of the parent
 An abstract class cannot be instantiated
 The use of abstract classes is a design decision; it helps us
establish common elements in a class that is too general to
instantiate

Abstract Method
 Inheritance allows a sub-class to override the methods of its
super-class.
 A super-class may altogether leave the implementation details
of a method and declare such a method abstract:
 abstract type name(parameter-list);
 Two kinds of methods:
1) concrete – may be overridden by sub-classes
2) abstract – must be overridden by sub-classes
 It is illegal to define abstract constructors or static methods.

Defining a Package
A package is both a naming and a visibility control
mechanism:
1) divides the name space into disjoint subsets It is possible
to define classes within a package that are not accessible by
code outside the package.
2) controls the visibility of classes and their members It is
possible to define class members that are only exposed to
other members of the same package.
Same-package classes may have an intimate knowledge of
each other, but not expose that knowledge to other
packages

Creating a Package
 A package statement inserted as the first line of the source
file:
package myPackage;
class MyClass1 { … }
 means that all classes in this file belong to the myPackage
package.
 The package statement creates a name space where such
classes are stored.
 When the package statement is omitted, class names are
put into the default package which has no name.

Multiple Source Files
 Other files may include the same package
instruction:
1. package myPackage;
2. package myPackage;
class MyClass3{ … }
 A package may be distributed through several
source files

Packages and Directories
 Java uses file system directories to store packages.
 Consider the Java source file:
package myPackage;
 The byte code files MyClass1.class and MyClass2.class must
be stored in a directory myPackage.
 Case is significant! Directory names must match package
names exactly.

Package Hierarchy
 To create a package hierarchy, separate each package name
with a dot:
package myPackage1.myPackage2.myPackage3;
 A package hierarchy must be stored accordingly in the file
system:
1) Unix myPackage1/myPackage2/myPackage3
2) Windows myPackage1myPackage2myPackage3
3) Macintosh myPackage1:myPackage2:myPackage3
 You cannot rename a package without renaming its directory!

Accessing a Package
 As packages are stored in directories, how does the Java
run-time system know where to look for packages?
 Two ways:
1) The current directory is the default start point - if
packages are stored in the current directory or sub-
directories, they will be found.
2) Specify a directory path or paths by setting the
CLASSPATH environment variable.

CLASSPATH Variable
 CLASSPATH - environment variable that points to the root
directory of the system’s package hierarchy.
 Several root directories may be specified in CLASSPATH,
 e.g. the current directory and the C:rajumyJava directory:
.;C:rajumyJava
 Java will search for the required packages by looking up
subsequent directories described in the CLASSPATH
variable.

Finding Packages
 Consider this package statement:
package myPackage;
In order for a program to find myPackage, one of the following
must be true:
1) program is executed from the directory immediately above
myPackage (the parent of myPackage directory)
2) CLASSPATH must be set to include the path to myPackage

Example: Package
package MyPack;
class Balance {
String name;
double bal;
Balance(String n, double b) {
name = n; bal = b;
}
void show() {
if (bal<0) System.out.print("-->> ");
System.out.println(name + ": $" + bal);
} }

Example: Package
class AccountBalance
{
public static void main(String args[])
{
Balance current[] = new Balance[3];
current[0] = new Balance("K. J. Fielding", 123.23);
current[1] = new Balance("Will Tell", 157.02);
current[2] = new Balance("Tom Jackson", -12.33);
for (int i=0; i<3; i++) current[i].show();
}
}

Example: Package
 Save, compile and execute:
1) call the file AccountBalance.java
2) save the file in the directory MyPack
3) compile; AccountBalance.class should be also in
MyPack
4) set access to MyPack in CLASSPATH variable, or make
the
parent of MyPack your current directory
5) run: java MyPack.AccountBalance
 Make sure to use the package-qualified class name.

Importing of Packages
 Since classes within packages must be fully-qualified with
their package names, it would be tedious to always type
long dot-separated names.
 The import statement allows to use classes or whole
packages directly.
 Importing of a concrete class:
import myPackage1.myPackage2.myClass;
 Importing of all classes within a package:
import myPackage1.myPackage2.*;

Import Statement
 The import statement occurs immediately after the
package
statement and before the class statement:
package myPackage;
 import otherPackage1;otherPackage2.otherClass;
class myClass { … }
 The Java system accepts this import statement by default:
import java.lang.*;
 This package includes the basic language functions.
Without such functions, Java is of no much use.

Example: Packages 1
 A package MyPack with one public class Balance.
The class has two same-package variables: public constructor and a
public show method.
package MyPack;
public class Balance {
String name;
double bal;
public Balance(String n, double b) {
name = n; bal = b;
}
public void show() {
if (bal<0) System.out.print("-->> ");
System.out.println(name + ": $" + bal);
}
}

Example: Packages 2
The importing code has access to the public class Balance of
the
MyPack package and its two public members:
import MyPack.*;
class TestBalance {
Balance test = new Balance("J. J. Jaspers", 99.88);
test.show();
}
}

Java Source File
Finally, a Java source file consists of:
1) a single package instruction (optional)
2) several import statements (optional)
3) a single public class declaration (required)
4) several classes private to the package (optional)
At the minimum, a file contains a single public class
declaration.

Differences between classes and interfaces
 Interfaces are syntactically similar to classes, but they lack instance
variables, and their methods are declared without any body.
 One class can implement any number of interfaces.
 Interfaces are designed to support dynamic method resolution at run
time.
 Interface is little bit like a class... but interface is lack in instance
variables....that's u can't create object for it.....
 Interfaces are developed to support multiple inheritance...
 The methods present in interfaces r pure abstract..
 The access specifiers public,private,protected are possible with classes,
but the interface uses only one spcifier public.....
 interfaces contains only the method declarations.... no definitions.......
 A interface defines, which method a class has to implement. This is
way - if you want to call a method defined by an interface - you don't
need to know the exact class type of an object, you only need to know
that it implements a specific interface.
 Another important point about interfaces is that a class can implement
multiple interfaces.

Defining an interface
 Using interface, we specify what a class must do, but not how it does this.
 An interface is syntactically similar to a class, but it lacks instance
variables and its methods are declared without any body.
 An interface is defined with an interface keyword.
 An interface declaration consists of modifiers, the keyword
interface,the interface name, a comma-separated list of parent
interfaces (if any), and the interface body.
For example:
public interface GroupedInterface extends Interface1, Interface2,
Interface3 {
// constant declarations double E = 2.718282;
// base of natural logarithms //
//method signatures
void doSomething (int i, double x);
int doSomethingElse(String s);
}
 The public access specifier indicates that the interface can be used by any
class in any package. If you do not specify that the interface is public, your
interface will be accessible only to classes defined in the same package as
the interface.
 An interface can extend other interfaces, just as a class can extend or
subclass another class. However, whereas a class can extend only one
other class, an interface can extend any number of interfaces. The
interface declaration includes a comma-separated list of all the interfaces
that it extends

Implementing interface
General format:
access interface name {
type method-name1(parameter-list);
type method-name2(parameter-list);
…
type var-name1 = value1;
type var-nameM = valueM;
…
}

 Two types of access:
1) public – interface may be used anywhere in a program
2) default – interface may be used in the current package
only
 Interface methods have no bodies – they end with the
semicolon after the parameter list.
 They are essentially abstract methods.
 An interface may include variables, but they must be final,
static and initialized with a constant value.
 In a public interface, all members are implicitly public.

Interface Implementation
 A class implements an interface if it provides a complete set
of methods defined by this interface.
1) any number of classes may implement an interface
2) one class may implement any number of interfaces
 Each class is free to determine the details of its
implementation.
 Implementation relation is written with the implements
keyword.

Implementation Format
 General format of a class that includes the implements
clause:
 Syntax:
access class name extends super-class implements
interface1, interface2, …, interfaceN {
…
}
 Access is public or default.

Implementation Comments
 If a class implements several interfaces, they are separated
with a comma.
 If a class implements two interfaces that declare the same
method, the same method will be used by the clients of
either interface.
 The methods that implement an interface must be declared
public.
 The type signature of the implementing method must
match exactly the type signature specified in the interface
definition.

Example: Interface
Declaration of the Callback interface:
interface Callback
{
void callback(int param);
}
Client class implements the Callback interface:
class Client implements Callback
{
public void callback(int p)
{
System.out.println("callback called with " + p);
}
}

More Methods in Implementation
 An implementing class may also declare its own
methods:
class Client implements Callback {
public void callback(int p) {
System.out.println("callback called with " + p);
}
void nonIfaceMeth() {
System.out.println("Classes that implement “ +
“interfaces may also define ” +
“other members, too.");
}
}

Applying interfaces
A Java interface declares a set of method signatures i.e., says what
behavior exists Does not say how the behavior is implemented
i.e., does not give code for the methods
Does not describe any state (but may include “final” constants)
A concrete class that implements an interface Contains “implements
InterfaceName” in the class declaration
Must provide implementations (either directly or inherited from a
superclass) of all methods declared in the interface
An abstract class can also implement an interface
Can optionally have implementations of some or all interface
methods

 Interfaces and Extends both describe an “is- a” relation.
 If B implements interface A, then B inherits the (abstract)
method signatures in A
 If B extends class A, then B inherits everything in A.
 which can include method code and instance variables as well
as abstract method signatures.
 Inheritance” is sometimes used to talk about the superclass /
subclass “extends” relation only

Variables in interface
 Variables declared in an interface must be constants.
 A technique to import shared constants into multiple
classes:
1) declare an interface with variables initialized to the
desired
values
2) include that interface in a class through
implementation.
 As no methods are included in the interface, the class does
not implement.
 anything except importing the variables as constants.

Example: Interface Variables 1
An interface with constant values:
import java.util.Random;
interface SharedConstants {
int NO = 0;
int YES = 1;
int MAYBE = 2;
int LATER = 3;
int SOON = 4;
int NEVER = 5;
}

 Question implements SharedConstants, including all its constants.
 Which constant is returned depends on the generated random
number:
class Question implements SharedConstants {
Random rand = new Random();
int ask() {
int prob = (int) (100 * rand.nextDouble());
if (prob < 30) return NO;
else if (prob < 60) return YES;
else if (prob < 75) return LATER;
else if (prob < 98) return SOON;
else return NEVER;
}
}

 AskMe includes all shared constants in the same way, using them
to display the result, depending on the value received:
class AskMe implements SharedConstants {
static void answer(int result) {
switch(result) {
case NO: System.out.println("No"); break;
case YES: System.out.println("Yes"); break;
case MAYBE: System.out.println("Maybe"); break;
case LATER: System.out.println("Later"); break;
case SOON: System.out.println("Soon"); break;
case NEVER: System.out.println("Never"); break;
}
}

 The testing function relies on the fact that both ask and
answer methods.
 defined in different classes, rely on the same constants:
Question q = new Question();
answer(q.ask());
answer(q.ask());
answer(q.ask());
answer(q.ask());
}
}

Extending interfaces
 One interface may inherit another interface.
 The inheritance syntax is the same for classes and
interfaces.
interface MyInterface1 {
void myMethod1(…) ;
}
interface MyInterface2 extends MyInterface1 {
void myMethod2(…) ;
}
 When a class implements an interface that inherits another
interface, it must provide implementations for all methods
defined within the interface inheritance chain.

Example: Interface Inheritance 1
 Consider interfaces A and B.
interface A {
void meth1();
void meth2();
}
B extends A:
interface B extends A {
void meth3();
}

 MyClass must implement all of A and B methods:
class MyClass implements B {
public void meth1() {
System.out.println("Implement meth1().");
}
}
} }

 Create a new MyClass object, then invoke all interface
methods on it:
class IFExtend {
public static void main(String arg[]) {
MyClass ob = new MyClass();
ob.meth1();
ob.meth2();
ob.meth3();
}
}

Exceptions
 Exception is an abnormal condition that arises when
executing a program.
 In the languages that do not support exception handling,
errors must be checked and handled manually, usually
through the use of error codes.
 In contrast, Java:
1) provides syntactic mechanisms to signal, detect and
handle errors
2) ensures a clean separation between the code executed
in the
absence of errors and the code to handle various kinds of
errors
3) brings run-time error management into object-oriented
programming

Exception Handling
 An exception is an object that describes an exceptional
condition (error) that has occurred when executing a
program.
 Exception handling involves the following:
1) when an error occurs, an object (exception) representing
this error is created and thrown in the method that caused
it
2) that method may choose to handle the exception itself or
pass it on
3) either way, at some point, the exception is caught and
processed

Exception Sources
 Exceptions can be:
1) generated by the Java run-time system Fundamental errors that
violate the rules of the Java language or the constraints of the Java
execution environment.
2) manually generated by programmer’s code Such exceptions are
typically used to report some error conditions to the caller of a
method.
Exception Constructs
 Five constructs are used in exception handling:
1) try – a block surrounding program statements to monitor for
exceptions
2) catch – together with try, catches specific kinds of exceptions and
handles them in some way
3) finally – specifies any code that absolutely must be executed
whether or not an exception occurs
4) throw – used to throw a specific exception from the program
5) throws – specifies which exceptions a given method can throw

Exception-Handling Block
General form:
try { … }
catch(Exception1 ex1) { … }
catch(Exception2 ex2) { … }
…
finally { … }
where:
1) try { … } is the block of code to monitor for exceptions
2) catch(Exception ex) { … } is exception handler for the
exception Exception
3) finally { … } is the block of code to execute before the try
block ends

Benefits of exception handling
 Separating Error-Handling code from “regular” business logic
code
 Propagating errors up the call stack
 Grouping and differentiating error types

Using Java Exception Handling
method1 {
try {
call method2;
} catch (exception e) {
doErrorProcessing;
}
}
method2 throws exception {
call method3;
}
method3 throws exception {
call readFile;
}
 Any checked exceptions
that can be thrown within a
method must be specified in
its throws clause.

Grouping and Differentiating Error Types
Because all exceptions thrown within a program are objects, the
grouping or categorizing of exceptions is a natural outcome of
the class hierarchy.
An example of a group of related exception classes in the Java
platform are those defined in java.io.IOException and its
descendants.
IOException is the most general and represents any type of error
that can occur when performing I/O.
Its descendants represent more specific errors. For example,
FileNotFoundException means that a file could not be located on
disk.

 A method can write specific handlers that can handle a very
specific exception.
The FileNotFoundException class has no descendants, so the
following handler can handle only one type of exception.
catch (FileNotFoundException e) {
...
}
A method can catch an exception based on its group or general
type by specifying any of the exception's super classes in the
catch statement.
For example, to catch all I/O exceptions, regardless of their
specific type, an exception handler specifies an IOException
argument.
// Catch all I/O exceptions, including
// FileNotFoundException, EOFException, and so on.
catch (IOException e) {
...
}

Termination vs. Resumption
 There are two basic models in exception-handling theory.
 In termination the error is so critical there’s no way to get
back to where the exception occurred. Whoever threw the
exception decided that there was no way to salvage the
situation, and they don’t want to come back.
 The alternative is called resumption. It means that the
exception handler is expected to do something to rectify the
situation, and then the faulting method is retried, presuming
success the second time. If you want resumption, it means
you still hope to continue execution after the exception is
handled.

 In resumption a method call that want resumption-like
behavior (i.e don’t throw an exception all a method that fixes
the problem.)
 Alternatively, place your try block inside a while loop that
keeps reentering the try block until the result is satisfactory.
 Operating systems that supported resumptive exception
handling eventually ended up using termination-like code and
skipping resumption.

Exception Hierarchy
 All exceptions are sub-classes of the build-in class Throwable.
 Throwable contains two immediate sub-classes:
1) Exception – exceptional conditions that programs should catch
The class includes:
a) RuntimeException – defined automatically for user
programs to include: division by zero, invalid array
indexing, etc.
b) use-defined exception classes
2) Error – exceptions used by Java to indicate errors with the
runtime environment; user programs are not supposed to catch
them

Hierarchy of Exception Classes

Usage of try-catch Statements
 Syntax:
try {
<code to be monitored for exceptions>
} catch (<ExceptionType1> <ObjName>) {
<handler if ExceptionType1 occurs>
} ...
} catch (<ExceptionTypeN> <ObjName>) {
<handler if ExceptionTypeN occurs>
}

Catching Exceptions:
The try-catch Statements
class DivByZero {
try {
System.out.println(3/0);
System.out.println(“Please print me.”);
} catch (ArithmeticException exc) {
//Division by zero is an ArithmeticException
System.out.println(exc);
}
System.out.println(“After exception.”);
}
}

Multiple catch
class MultipleCatch {
try {
int den = Integer.parseInt(args[0]);
System.out.println(3/den);
} catch (ArithmeticException exc) {
System.out.println(“Divisor was 0.”);
} catch (ArrayIndexOutOfBoundsException exc2) {
System.out.println(“Missing argument.”);
}
System.out.println(“After exception.”);
}
}

Nested try's
class NestedTryDemo {
public static void main(String args[]){
try {
int a = Integer.parseInt(args[0]);
try {
int b = Integer.parseInt(args[1]);
System.out.println(a/b);
} catch (ArithmeticException e) {
System.out.println(“Div by zero error!");
} } catch (ArrayIndexOutOfBoundsException) {
System.out.println(“Need 2 parameters!");
} } }

Nested try's with methods
class NestedTryDemo2 {
static void nestedTry(String args[]) {
try {
int a = Integer.parseInt(args[0]);
int b = Integer.parseInt(args[1]);
System.out.println(a/b);
} catch (ArithmeticException e) {
System.out.println("Div by zero error!");
} }
public static void main(String args[]){
try {
nestedTry(args);
} catch (ArrayIndexOutOfBoundsException e) {
System.out.println("Need 2 parameters!");
} } }

Throwing Exceptions(throw)
 So far, we were only catching the exceptions thrown by the Java
system.
 In fact, a user program may throw an exception explicitly:
throw ThrowableInstance;
 ThrowableInstance must be an object of type Throwable or its
subclass.
Once an exception is thrown by:
throw ThrowableInstance;
1) the flow of control stops immediately.
2) the nearest enclosing try statement is inspected if it has a catch
statement that matches the type of exception:
1) if one exists, control is transferred to that statement
2) otherwise, the next enclosing try statement is examined
3) if no enclosing try statement has a corresponding catch clause,
the default exception handler halts the program and prints the
stack

Creating Exceptions
Two ways to obtain a Throwable instance:
1) creating one with the new operator
All Java built-in exceptions have at least two Constructors:
One without parameters and another with one String
parameter:
throw new NullPointerException("demo");
2) using a parameter of the catch clause
try { … } catch(Throwable e) { … e … }

Example: throw 1
class ThrowDemo {
//The method demoproc throws a NullPointerException
exception which is immediately caught in the try block and
re-thrown:
static void demoproc() {
try {
throw new NullPointerException("demo");
} catch(NullPointerException e) {
System.out.println("Caught inside demoproc.");
throw e;
}
}

Example: throw 2
The main method calls demoproc within the try block
which catches and handles the NullPointerException
exception:
try {
demoproc();
} catch(NullPointerException e) {
System.out.println("Recaught: " + e);
}
}
}

throws Declaration
 If a method is capable of causing an exception that it does not
handle, it must specify this behavior by the throws clause in its
declaration:
type name(parameter-list) throws exception-list {
…
}
 where exception-list is a comma-separated list of all types of
exceptions that a method might throw.
 All exceptions must be listed except Error and RuntimeException
or any of their subclasses, otherwise a compile-time error occurs.

Example: throws 1
 The throwOne method throws an exception that it does not
catch, nor declares it within the throws clause.
class ThrowsDemo {
static void throwOne() {
System.out.println("Inside throwOne.");
throw new IllegalAccessException("demo");
}
throwOne();
}
}
 Therefore this program does not compile.

Example: throws 2
 Corrected program: throwOne lists exception, main catches it:
class ThrowsDemo {
static void throwOne() throws IllegalAccessException {
System.out.println("Inside throwOne.");
throw new IllegalAccessException("demo");
}
try {
throwOne();
} catch (IllegalAccessException e) {
System.out.println("Caught " + e);
} } }

finally
 When an exception is thrown:
1) the execution of a method is changed
2) the method may even return prematurely.
 This may be a problem is many situations.
 For instance, if a method opens a file on entry and closes on
exit; exception handling should not bypass the proper closure
of the file.
 The finally block is used to address this problem.

finally Clause
 The try/catch statement requires at least one catch or finally
clause, although both are optional:
try { … }
catch(Exception1 ex1) { … } …
finally { … }
 Executed after try/catch whether of not the exception is thrown.
 Any time a method is to return to a caller from inside the
try/catch block via:
1) uncaught exception or
2) explicit return
the finally clause is executed just before the method returns.

Example: finally 1
 Three methods to exit in various ways.
class FinallyDemo {
//procA prematurely breaks out of the try by throwing an
exception, the finally clause is executed on the way out:
static void procA() {
try {
System.out.println("inside procA");
throw new RuntimeException("demo");
} finally {
System.out.println("procA's finally");
} }

Example: finally 2
// procB’s try statement is exited via a return statement, the
finally clause is executed before procB returns:
static void procB() {
try {
System.out.println("inside procB");
return;
} finally {
System.out.println("procB's finally");
}
}

Example: finally 3
 In procC, the try statement executes normally without error,
however the finally clause is still executed:
static void procC() {
try {
System.out.println("inside procC");
} finally {
System.out.println("procC's finally");
}
}

Example: finally 4
 Demonstration of the three methods:
try {
procA();
} catch (Exception e) {
System.out.println("Exception caught");
}
procB();
procC();
}
}

Java Built-In Exceptions
 The default java.lang package provides several exception classes, all
sub-classing the RuntimeException class.
 Two sets of build-in exception classes:
1) unchecked exceptions – the compiler does not check if a method
handles or throws there exceptions
2) checked exceptions – must be included in the method’s throws
clause if the method generates but does not handle them

Unchecked Built-In Exceptions
Methods that generate but do not handle those exceptions need not
declare them in the throws clause:
1) ArithmeticException
2) ArrayIndexOutOfBoundsException
3) ArrayStoreException
4) ClassCastException
5) IllegalStateException
6) IllegalMonitorStateException
7) IllegalArgumentException
8. StringIndexOutOfBounds
9. UnsupportedOperationException
10. SecurityException
11. NumberFormatException
12. NullPointerException
13. NegativeArraySizeException
14. IndexOutOfBoundsException
15. IllegalThreadStateException

Checked Built-In Exceptions
Methods that generate but do not handle those exceptions must
declare them in the throws clause:
1. NoSuchMethodException NoSuchFieldException
2. InterruptedException
3. InstantiationException
4. IllegalAccessException
5. CloneNotSupportedException
6. ClassNotFoundException

Creating Own Exception Classes
 Build-in exception classes handle some generic errors.
 For application-specific errors define your own exception classes.
How? Define a subclass of Exception:
class MyException extends Exception { … }
 MyException need not implement anything – its mere existence in
the type system allows to use its objects as exceptions.

Example: Own Exceptions 1
 A new exception class is defined, with a private detail
variable, a one parameter constructor and an overridden
toString method:
class MyException extends Exception {
private int detail;
MyException(int a) {
detail = a;
}
public String toString() {
return "MyException[" + detail + "]";
}
}

class ExceptionDemo {
The static compute method throws the MyException
exception whenever its a argument is greater than 10:
static void compute(int a) throws MyException {
System.out.println("Called compute(" + a + ")");
if (a > 10) throw new MyException(a);
System.out.println("Normal exit");
}

The main method calls compute with two arguments within a try
block that catches the MyException exception:
try {
compute(1);
compute(20);
} catch (MyException e) {
System.out.println("Caught " + e);
}
}
}

Differences between multi threading and
multitasking
Multi-Tasking
 Two kinds of multi-tasking:
1) process-based multi-tasking
2) thread-based multi-tasking
 Process-based multi-tasking is about allowing several programs to execute
concurrently, e.g. Java compiler and a text editor.
 Processes are heavyweight tasks:
1) that require their own address space
2) inter-process communication is expensive and limited
3) context-switching from one process to another is expensive
and limited

Thread-Based Multi-Tasking
 Thread-based multi-tasking is about a single program
executing concurrently
 several tasks e.g. a text editor printing and spell-checking
text.
 Threads are lightweight tasks:
1) they share the same address space
2) they cooperatively share the same process
3) inter-thread communication is inexpensive
4) context-switching from one thread to another
is low-cost
 Java multi-tasking is thread-based.

Reasons for Multi-Threading
 Multi-threading enables to write efficient programs that
make the maximum use of the CPU, keeping the idle time to
a minimum.
 There is plenty of idle time for interactive, networked
applications:
1) the transmission rate of data over a network is much
slower than the rate at which the computer can process it
2) local file system resources can be read and written at a
much slower rate than can be processed by the CPU
3) of course, user input is much slower than the computer

Thread Lifecycle
 Thread exist in several states:
1) ready to run
2) running
3) a running thread can be suspended
4) a suspended thread can be resumed
5) a thread can be blocked when waiting for a resource
6) a thread can be terminated
 Once terminated, a thread cannot be resumed.

Thread Lifecycle
Born
Blocked
Runnable
Dead
stop()
start()
stop()
Active
block on I/O
I/O available
JVM
sleep(500)
wake up
suspend()
resume()
wait
notify

 New state – After the creations of Thread instance the thread is in this
state but before the start() method invocation. At this point, the thread
is considered not alive.
 Runnable (Ready-to-run) state – A thread start its life from Runnable
state. A thread first enters runnable state after the invoking of start()
method but a thread can return to this state after either running,
waiting, sleeping or coming back from blocked state also. On this state a
thread is waiting for a turn on the processor.
 Running state – A thread is in running state that means the thread is
currently executing. There are several ways to enter in Runnable state
but there is only one way to enter in Running state: the scheduler select
a thread from runnable pool.
 Dead state – A thread can be considered dead when its run() method
completes. If any thread comes on this state that means it cannot ever
run again.
 Blocked - A thread can enter in this state because of waiting the
resources that are hold by another thread.

Creating Threads
 To create a new thread a program will:
1) extend the Thread class, or
2) implement the Runnable interface
 Thread class encapsulates a thread of execution.
 The whole Java multithreading environment is based on
the Thread class.

Thread Methods
 Start: a thread by calling start its run method
 Sleep: suspend a thread for a period of time
 Run: entry-point for a thread
 Join: wait for a thread to terminate
 isAlive: determine if a thread is still running
 getPriority: obtain a thread’s priority
 getName: obtain a thread’s name

New Thread: Runnable
 To create a new thread by implementing the Runnable
interface:
1) create a class that implements the run method (inside this
method, we define the code that constitutes the new
thread):
public void run()
2) instantiate a Thread object within that class, a possible
constructor is:
Thread(Runnable threadOb, String threadName)
3) call the start method on this object (start calls run):
void start()

Example: New Thread 1
 A class NewThread that implements Runnable:
class NewThread implements Runnable {
Thread t;
//Creating and starting a new thread. Passing this to the
// Thread constructor – the new thread will call this
// object’s run method:
NewThread() {
t = new Thread(this, "Demo Thread");
System.out.println("Child thread: " + t);
t.start();
}

//This is the entry point for the newly created thread – a five-iterations loop
//with a half-second pause between the iterations all within try/catch:
public void run() {
try {
for (int i = 5; i > 0; i--) {
System.out.println("Child Thread: " + i);
Thread.sleep(500);
}
} catch (InterruptedException e) {
System.out.println("Child interrupted.");
}
System.out.println("Exiting child thread.");
}
}

class ThreadDemo {
//A new thread is created as an object of
// NewThread:
new NewThread();
//After calling the NewThread start method,
// control returns here.

//Both threads (new and main) continue concurrently.
//Here is the loop for the main thread:
try {
for (int i = 5; i > 0; i--) {
System.out.println("Main Thread: " + i);
Thread.sleep(1000);
}
System.out.println("Main thread interrupted.");
}
System.out.println("Main thread exiting.");
}
}

New Thread: Extend Thread
 The second way to create a new thread:
1) create a new class that extends Thread
2) create an instance of that class
 Thread provides both run and start methods:
1) the extending class must override run
2) it must also call the start method

 The new thread class extends Thread:
class NewThread extends Thread {
//Create a new thread by calling the Thread’s
// constructor and start method:
NewThread() {
super("Demo Thread");
System.out.println("Child thread: " + this);
start();
}

NewThread overrides the Thread’s run method:
public void run() {
try {
for (int i = 5; i > 0; i--) {
System.out.println("Child Thread: " + i);
Thread.sleep(500);
}
System.out.println("Child interrupted.");
}
System.out.println("Exiting child thread.");
}
}

class ExtendThread {
//After a new thread is created:
new NewThread();
//the new and main threads continue
//concurrently…

//This is the loop of the main thread:
try {
for (int i = 5; i > 0; i--) {
System.out.println("Main Thread: " + i);
Thread.sleep(1000);
}
System.out.println("Main thread interrupted.");
}
System.out.println("Main thread exiting.");
}
}

Threads: Synchronization
 Multi-threading introduces asynchronous behavior to a program.
 How to ensure synchronous behavior when we need it?
 For instance, how to prevent two threads from simultaneously
writing and reading the same object?
 Java implementation of monitors:
1) classes can define so-called synchronized methods
2) each object has its own implicit monitor that is automatically
entered when one of the object’s synchronized methods is called
3) once a thread is inside a synchronized method, no other thread
can call any other synchronized method on the same object

Thread Synchronization
 Language keyword: synchronized
 Takes out a monitor lock on an object
 Exclusive lock for that thread
 If lock is currently unavailable, thread will block

Thread Synchronization
 Protects access to code, not to data
 Make data members private
 Synchronize accessor methods
 Puts a “force field” around the locked object so no other
threads can enter
 Actually, it only blocks access to other synchronizing threads

Daemon Threads
 Any Java thread can be a daemon thread.
 Daemon threads are service providers for other threads running in the
same process as the daemon thread.
 The run() method for a daemon thread is typically an infinite loop that
waits for a service request. When the only remaining threads in a
process are daemon threads, the interpreter exits. This makes sense
because when only daemon threads remain, there is no other thread
for which a daemon thread can provide a service.
 To specify that a thread is a daemon thread, call the setDaemon
method with the argument true. To determine if a thread is a daemon
thread, use the accessor method isDaemon.

Thread Groups
o Every Java thread is a member of a thread group.
o Thread groups provide a mechanism for collecting multiple threads into a single
object and manipulating those threads all at once, rather than individually.
o For example, you can start or suspend all the threads within a group with a single
method call.
o Java thread groups are implemented by the “ThreadGroup” class in the java.lang
package.
 The runtime system puts a thread into a thread group during thread construction.
 When you create a thread, you can either allow the runtime system to put the new
thread in some reasonable default group or you can explicitly set the new thread's
group.
 The thread is a permanent member of whatever thread group it joins upon its
creation--you cannot move a thread to a new group after the thread has been
created

The ThreadGroup Class
 The “ThreadGroup” class manages groups of threads for Java
applications.
 A ThreadGroup can contain any number of threads.
 The threads in a group are generally related in some way, such
as who created them, what function they perform, or when they
should be started and stopped.
 ThreadGroups can contain not only threads but also other
ThreadGroups.
 The top-most thread group in a Java application is the thread
group named main.
 You can create threads and thread groups in the main group.
 You can also create threads and thread groups in subgroups of
main.

Creating a Thread Explicitly in a Group
 A thread is a permanent member of whatever thread group it joins when its
created--you cannot move a thread to a new group after the thread has been
created. Thus, if you wish to put your new thread in a thread group other than the
default, you must specify the thread group explicitly when you create the thread.
 The Thread class has three constructors that let you set a new thread's group:
public Thread(ThreadGroup group, Runnable target) public
Thread(ThreadGroup group, String name)
public Thread(ThreadGroup group, Runnable target, String name)
 Each of these constructors creates a new thread, initializes it based on the Runnable
and String parameters, and makes the new thread a member of the specified group.
For example:
ThreadGroup myThreadGroup = new ThreadGroup("My Group of Threads");
Thread myThread = new Thread(myThreadGroup, "a thread for my group");

UNIT-4
 Collection Framework in java – Introduction to java
collections- overview of java collection frame work-
generics-commonly used collection classes- Array List-
vector -hash table-stack-enumeration-iterator-string
tokenizer -random -scanner -calendar and properties
 Files – streams – byte streams- character stream- text
input/output- binary input/output- random access file
operations- file management using file class.
 Connecting to Database – JDBC Type 1 to 4 drivers-
connecting to a database- quering a database and
processing the results- updating data with JDBC

collection
 A collection — sometimes called a container — is simply an
object that groups multiple elements into a single unit.
 Collections are used to store, retrieve, manipulate, and
communicate aggregate data.
 Typically, they represent data items that form a natural group, such
as a poker hand (a collection of cards), a mail folder (a collection
of letters), or a telephone directory (a mapping of names to phone
numbers).

268
Collections Framework Diagram
•Interfaces, Implementations, and
Algorithms
•From Thinking in Java, page 462

The collections framework was designed to meet several goals:
 The framework had to be high-performance. The
implementations for the fundamental collections (dynamic
arrays, linked lists, trees, and hashtables) are highly efficient.
 The framework had to allow different types of collections to
work in a similar manner and with a high degree of
interoperability.
 Extending and/or adapting a collection had to be easy.

Collections Framework
 A collections framework is a unified architecture for representing
and manipulating collections.
All collections frameworks contain the following:
 Interfaces: These are abstract data types that represent collections.
Interfaces allow collections to be manipulated independently of the
details of their representation. In object-oriented languages,
interfaces generally form a hierarchy.
 Implementations: These are the concrete implementations of the
collection interfaces. In essence, they are reusable data structures.
 Algorithms: These are the methods that perform useful
computations, such as searching and sorting, on objects that
implement collection interfaces. The algorithms are said to be
polymorphic: that is, the same method can be used on many
different implementations of the appropriate collection interface. In
essence, algorithms are reusable functionality.

Benefits of the Java Collections Framework
 Reduces programming effort: By providing useful data structures
and algorithms, the Collections Framework frees you to concentrate
on the important parts of your program rather than on the low-level
"plumbing" required to make it work. By facilitating interoperability
among unrelated APIs, the Java Collections Framework frees you
from writing adapter objects or conversion code to connect APIs.
 Increases program speed and quality: This Collections
Framework provides high-performance, high-quality
implementations of useful data structures and algorithms. The
various implementations of each interface are interchangeable, so
programs can be easily tuned by switching collection
implementations. Because you're freed from the drudgery of writing
your own data structures, you'll have more time to devote to
improving programs' quality and performance.

 Allows interoperability among unrelated APIs: The collection interfaces are the
vernacular by which APIs pass collections back and forth. If my network administration API
furnishes a collection of node names and if your GUI toolkit expects a collection of column
headings, our APIs will interoperate seamlessly, even though they were written
independently.
 Reduces effort to learn and to use new APIs: Many APIs naturally take collections on
input and furnish them as output. In the past, each such API had a small sub-API devoted to
manipulating its collections. There was little consistency among these ad hoc collections
sub-APIs, so you had to learn each one from scratch, and it was easy to make mistakes when
using them. With the advent of standard collection interfaces, the problem went away.
 Reduces effort to design new APIs: This is the flip side of the previous advantage.
Designers and implementers don't have to reinvent the wheel each time they create an API
that relies on collections; instead, they can use standard collection interfaces.
 Fosters software reuse: New data structures that conform to the standard collection
interfaces are by nature reusable. The same goes for new algorithms that operate on objects
that implement these interfaces.

Collections Framework Diagram
 Each collection class implements an interface from a
hierarchy
 Each class is designed for a
specific type of storage

Lists and Sets
A list is a collection that maintains
the order of its elements.
 Ordered Lists
 ArrayList
 Stores a list of items in a dynamically sized array
 LinkedList
 Allows speedy insertion and removal of items from the list

Lists and Sets
A set is an unordered collection
of unique elements.
 Unordered Sets
 HashSet
 Uses hash tables to speed up finding, adding, and removing
elements
 TreeSet
 Uses a binary tree to speed up finding, adding, and removing
elements

Stacks and Queues
 Another way of gaining efficiency in a
collection is to reduce the number of
operations available
 Two examples are:
 Stack
 Remembers the order of its elements, but it does not
allow you to insert elements in every position
 You can only add and remove elements at the top
 Queue
 Add items to one end (the tail)
 Remove them from the other end (the head)
 Example: A line of people waiting for a bank teller

Maps
 A map stores keys, values, and the associations
between them
 Example:
 Barcode keys and books
 Keys
 Provides an easy way to represent an object (such as a
numeric bar code)
 Values
 The actual object that is associated with the key
A map keeps associations
between key and value objects.

collection interfaces
The following list describes the core collection interfaces:
 Collection — the root of the collection hierarchy. A collection represents a group of objects known as its
elements. The Collection interface is the least common denominator that all collections implement and is
used to pass collections around and to manipulate them when maximum generality is desired. Some types
of collections allow duplicate elements, and others do not. Some are ordered and others are unordered.
The Java platform doesn't provide any direct implementations of this interface but provides
implementations of more specific sub interfaces, such as Set and List.
 Set — a collection that cannot contain duplicate elements. This interface models the mathematical set
abstraction and is used to represent sets, such as the cards comprising a poker hand, the courses making
up a student's schedule, or the processes running on a machine..
 List — an ordered collection (sometimes called a sequence). Lists can contain duplicate elements. The
user of a List generally has precise control over where in the list each element is inserted and can access
elements by their integer index (position). If you've used Vector, you're familiar with the general flavor of
List.
 Queue — a collection used to hold multiple elements prior to processing. Besides basic Collection
operations, a Queue provides additional insertion, extraction, and inspection operations.

 Map — an object that maps keys to values. A Map cannot
contain duplicate keys; each key can map to at most one
value. If you've used Hashtable.
The last two core collection interfaces are merely sorted
versions of Set and Map:
 SortedSet — a Set that maintains its elements in ascending
order. Several additional operations are provided to take
advantage of the ordering. Sorted sets are used for naturally
ordered sets, such as word lists and membership rolls
 .
 SortedMap — a Map that maintains its mappings in
ascending key order. This is the Map analog of SortedSet.
Sorted maps are used for naturally ordered collections of
key/value pairs, such as dictionaries and telephone directories

Summary of interfaces
 The core collection interfaces are the foundation of the Java Collections Framework.
The Java Collections Framework hierarchy consists of two distinct interface trees:
 1.The first tree starts with the Collection interface, which provides for the basic
functionality used by all collections, such as add and remove methods. Its subinterfaces
— Set, List, and Queue — provide for more specialized collections.
 The Set interface does not allow duplicate elements. This can be useful for storing
collections such as a deck of cards or student records. The Set interface has a
subinterface, SortedSet, that provides for ordering of elements in the set.
 The List interface provides for an ordered collection, for situations in which you need
precise control over where each element is inserted. You can retrieve elements from a
List by their exact position.
 The Queue interface enables additional insertion, extraction, and inspection operations.
Elements in a Queue are typically ordered in on a FIFO basis.
 The Deque interface enables insertion, deletion, and inspection operations at both the
ends. Elements in a Deque can be used in both LIFO and FIFO.
 2.The second tree starts with the Map interface, which maps keys and values similar to a
Hashtable.
 Map's subinterface, SortedMap, maintains its key-value pairs in ascending order or in an
order specified by a Comparator.
 These interfaces allow collections to be manipulated independently of the details of their
representation.

The Collection Classes
 Java provides a set of standard collection classes that
implement Collection interfaces. Some of the classes provide
full implementations that can be used as-is and others are
abstract class, providing skeletal implementations that are used
as starting points for creating concrete collections.

Classes with Description
1AbstractCollection -Implements most of the Collection interface.
2AbstractList -Extends AbstractCollection and implements most of the List interface.
3AbstractSequentialList Extends AbstractList for use by a collection that uses sequential
rather than random access of its elements.
4LinkedList -Implements a linked list by extending AbstractSequentialList.
5ArrayList -Implements a dynamic array by extending AbstractList.
6AbstractSet -Extends AbstractCollection and implements most of the Set interface.
7HashSet -Extends AbstractSet for use with a hash table.
8LinkedHashSet -Extends HashSet to allow insertion-order iterations.
9TreeSet -Implements a set stored in a tree. Extends AbstractSet.
10AbstractMap -Implements most of the Map interface.
11HashMap -Extends AbstractMap to use a hash table.
12TreeMap -Extends AbstractMap to use a tree.
13WeakHashMap -Extends AbstractMap to use a hash table with weak keys.
14LinkedHashMap-Extends HashMap to allow insertion-order iterations.
15IdentityHashMap -Extends AbstractMap and uses reference equality when comparing
documents.

15.2 Linked Lists
 Linked lists use references to maintain an ordered
lists of ‘nodes’
 The ‘head’ of the list references the first node
 Each node has a value and a reference to the next node
 They can be used to implement
 A List Interface
 A Queue Interface

Linked Lists Operations
 Efficient Operations
 Insertion of a node
 Find the elements it goes between
 Remap the references
 Removal of a node
 Find the element to remove
 Remap neighbor’s references
 Visiting all elements in order
 Inefficient Operations
 Random access Each instance variable is declared just like
other variables we have used.

Sets
 A set is an unordered collection
 It does not support duplicate elements
 The collection does not keep track of the order in which
elements have been added
 Therefore, it can carry out its operations more efficiently than an
ordered collection
The HashSet and TreeSet classes both
implement the Set interface.

Sets
 HashSet: Stores data in a Hash Table
 TreeSet: Stores data in a Binary Tree
 Both implementations arrange the set
elements so that finding, adding, and
removing elements is efficient
Set implementations arrange the elements so
that they can locate them quickly

Hash Table Concept
 Set elements are grouped into smaller collections of
elements that share the same characteristic
 It is usually based on the result of a mathematical
calculation on the contents that results in an integer value
 In order to be stored in a hash table, elements must have a
method to compute their integer values
100
101
102

hashCode
 The method is called hashCode
 If multiple elements have the same hash code, they are stored
in a Linked list
 The elements must also have an equals method for
checking whether an element equals another like:
 String, Integer, Point, Rectangle, Color, and all collection
classes
Set<String> names = new HashSet<String>();

Tree Concept
 Set elements are kept in sorted order
 Nodes are not arranged in a linear sequence
but in a tree shape
 In order to use a TreeSet, it must be possible to compare
the elements and determine which one is “larger”

TreeSet
 Use TreeSet for classes that implement
the Comparable interface
 String and Integer, for example
 The nodes are arranged in a ‘tree’ fashion so
that each ‘parent’ node has up to two child
nodes.
 The node to the left always has a ‘smaller’ value
 The node to the right always has a ‘larger’ value
Set<String> names = new TreeSet<String>();

Iterators and Sets
 Iterators are also used when processing sets
 hasNext returns true if there is a next element
 next returns a reference to the value of the next element
 add via the iterator is not supported for TreeSet and HashSet
 Note that the elements are not visited in the order in which you
inserted them.
 They are visited in the order in which the set keeps them:
 Seemingly random order for a HashSet
 Sorted order for a TreeSet
Iterator<String> iter = names.iterator();
while (iter.hasNext())
{
String name = iter.next();
// Do something with name
}
for (String name : names)
{
// Do something with name
}

Package java.util
 Contains the collections framework, legacy collection
classes, event model, date and time facilities,
internationalization, and miscellaneous utility classes
(a string tokenizer, a random-number generator, and a
bit array).

Interface Summary
 Collection<E>: The root interface in the collection
hierarchy.
 Comparator<T>: A comparison function, which imposes a
total ordering on some collection of objects.
 Enumeration<E>: An object that implements the
Enumeration interface generates a series of elements, one
at a time.
 EventListener: A tagging interface that all event listener
interfaces must extend.
 Iterator<E>: An iterator over a collection
 List<E>An ordered collection (also known as a sequence).
 ListIterator<E>: An iterator for lists that allows the
programmer to traverse the list in either direction, modify
the list during iteration, and obtain the iterator's current
position in the list.

 Map<K,V>: An object that maps keys to values.
 Observer: A class can implement the Observer interface
when it wants to be informed of changes in observable
objects.
 Queue<E>: A collection designed for holding elements
prior to processing.
 Set<E>: A collection that contains no duplicate elements.
 SortedMap<K,V>: A Map that further provides a total
ordering on its keys.
 SortedSet<E>: A Set that further provides a total ordering
on its elements.

Class Summary
 AbstractCollection<E>: This class provides a skeletal
implementation of the Collection interface, to minimize the effort
required to implement this interface.
 AbstractList<E>: This class provides a skeletal implementation of
the List interface to minimize the effort required to implement this
interface backed by a "random access" data store (such as an array).
 AbstractMap<K,V>: This class provides a skeletal implementation
of the Map interface, to minimize the effort required to implement
this interface.
 AbstractQueue<E>: This class provides skeletal implementations
of some Queue operations.
 AbstractSequentialList<E>: This class provides a skeletal
implementation of the List interface to minimize the effort required
to implement this interface backed by a "sequential access" data
store (such as a linked list).
 AbstractSet<E>: This class provides a skeletal implementation of
the Set interface to minimize the effort required to implement this
interface.

 ArrayList<E>: Resizable-array implementation
of the List interface
 Arrays: This class contains various methods for
manipulating arrays (such as sorting and
searching).
 BitSet: This class implements a vector of bits that
grows as needed
 Calendar: The Calendar class is an abstract class
that provides methods for converting between a
specific instant in time and a set of calendar fields:
such as YEAR, MONTH, DAY_OF_MONTH,
HOUR, and so on, and for manipulating the
calendar fields, such as getting the date of the next
week

 Collections: This class consists exclusively of static methods that
operate on or return collections
 Currency: Represents a currency.
 Date: The class Date represents a specific instant in time, with
millisecond precision.
 Dictionary<K,V>: The Dictionary class is the abstract parent of any
class, such as Hashtable, which maps keys to values.
 EventObject: The root class from which all event state objects shall
be derived.

 GregorianCalendar: GregorianCalendar is a concrete subclass of
Calendar and provides the standard calendar system used by most
of the world.
 HashMap<K,V>: Hash table based implementation of the Map
interface.
 HashSet<E>: This class implements the Set interface, backed by a
hash table (actually a HashMap instance)
 .Hashtable<K,V>: This class implements a hashtable, which maps
keys to values.
 LinkedList<E>: Linked list implementation of the List interface
 Locale: A Locale object representsa specific geographical, political,
or cultural region.
 Observable: This class representsan observable object, or "data" in
the model-view paradigm
 Properties: The Properties class representsa persistent set of
properties.

 Random: An instance of this class is used to generate a stream of
pseudorandom numbers.
 ResourceBundle: Resource bundles contain locale-specific
objects.
 SimpleTimeZone: SimpleTimeZone is a concrete subclass of
TimeZone that represents a time zone for use with a Gregorian
calendar.
 Stack<E>: The Stack class represents a last-in-first-out (LIFO)
stack of objects.
 StringTokenizer: The string tokenizer class allows an application
to break a string into tokens.
 TimeZone: TimeZone represents a time zone offset, and also
figures out daylight savings.
 TreeMap<K,V>: A Red-Black tree based NavigableMap
implementation.
 TreeSet<E>: A NavigableSet implementation based on a
TreeMap.UUIDA class that represents an immutable universally
unique identifier (UUID).
 Vector<E>: The Vector class implements a growable array of objects

Exception Summary
 EmptyStackException: Thrown by methods in the Stack class
to indicate that the stack is empty.
 InputMismatchException: Thrown by a Scanner to indicate that
the token retrieved does not match the pattern for the expected
type, or that the token is out of range for the expected type.
 InvalidPropertiesFormatException: Thrown to indicate that an
operation could not complete because the input did not conform to
the appropriate XML document type for a collection of properties,
as per the Properties specification.
 NoSuchElementException: Thrown by the nextElement method
of an Enumeration to indicate that there are no more elements in
the enumeration.
 TooManyListenersException: The TooManyListenersException
Exception is used as part of the Java Event model to annotate and
implement a unicast special case of a multicast Event Source.
 UnknownFormatConversionException: Unchecked exception
thrown when an unknown conversion is given.

 Provides for system input and output through data streams,
serialization and the file system.
Interface Summary
 DataInput The DataInput interface provides for reading
bytes from a binary stream and reconstructing from them
data in any of the Java primitive types.
 DataOutputThe DataOutput interface provides for
converting data from any of the Java primitive types to a series
of bytes and writing these bytes to a binary stream
 .Externalizable Only the identity of the class of an
Externalizable instance is written in the serialization stream
and it is the responsibility of the class to save and restore the
contents of its instances.
 SerializableSerializability of a class is enabled by the class
implementing the java.io.Serializable interface.

Class Summary
 BufferedInputStream: A BufferedInputStream adds
functionality to another input stream-namely, the ability to
buffer the input and to support the mark and reset
methods.
 BufferedOutputStream: The class implements a buffered
output stream.
 BufferedReader: Reads text from a character-input
stream, buffering characters so as to provide for the
efficient reading of characters, arrays, and lines.
 BufferedWriter: Writes text to a character-output stream,
buffering characters so as to provide for the efficient
writing of single characters, arrays, and strings
 ByteArrayInputStream: A ByteArrayInputStream
contains an internal buffer that contains bytes that may be
read from the stream.
 ByteArrayOutputStream: This class implements an
output stream in which the data is written into a byte array.

 CharArrayReader: This class implements a character
buffer that can be used as a character-input stream
 .CharArrayWriter: This class implements a character
buffer that can be used as an Writer
 Console: Methods to access the character-based console
device, if any, associated with the current Java virtual
machine.
 DataInputStream: A data input stream lets an application
read primitive Java data types from an underlying input
stream in a machine-independent way.
 DataOutputStream: A data output stream lets an
application write primitive Java data types to an output
stream in a portable way.

 File: An abstract representation of file and directory pathnames.
 FileInputStream: A FileInputStream obtains input bytes from a file in a file
system.
 FileOutputStream: A file output stream is an output stream for writing data to a
File or to a FileDescriptor.
 FileReader: Convenience class for reading character files.
 FileWriter: Convenience class for writing character files.
 FilterInputStream: A FilterInputStream contains some other input stream, which
it uses as its basic source of data, possibly transforming the data along the way or
providing additional functionality.
 FilterOutputStream: This class is the superclass of all classes that filter output
streams
 .FilterReader: Abstract class for reading filtered character streams
 .FilterWriter: Abstract class for writing filtered character streams
 .InputStream: This abstract class is the superclass of all classes representing an
input stream of bytes.
 InputStreamReader: An InputStreamReader is a bridge from byte streams to

 ObjectInputStream: An ObjectInputStream deserializes
primitive data and objects previously written using an
ObjectOutputStream
 ObjectOutputStream: An ObjectOutputStream writes
primitive data types and graphs of Java objects to an
OutputStream.
 OutputStream: This abstract class is the superclass of all
classes representing an output stream of bytes.
 OutputStreamWriter: An OutputStreamWriter is a bridge
from character streams to byte streams: Characters written to
it are encoded into bytes using a specified charset.
 PrintWriter: Prints formatted representations of objects to a
text-output stream.
 RandomAccessFile: Instances of this class support both
reading and writing to a random access file.
 StreamTokenizer: The StreamTokenizer class takes an input
stream and parses it into "tokens", allowing the tokens to be
read one at a time.

Exception Summary
 FileNotFoundException: Signals that an attempt to open
the file denoted by a specified pathname has failed.
 InterruptedIOException: Signals that an I/O operation
has been interrupted
 InvalidClassException: Thrown when the Serialization
runtime detects one of the following problems with a Class.
 InvalidObjectException: Indicates that one or more
deserialized objects failed validation tests.
 IOException: Signals that an I/O exception of some sort
has occurred.
L 8.6

Introduction
 JDBC stands for Java Database Connectivity, which is a
standard Java API for database-independent connectivity
between the Java programming language and a wide
range of databases.
 The JDBC library includes APIs for each of the tasks
commonly associated with database usage:
Making a connection to a database
Creating SQL or MySQL statements
Executing that SQL or MySQL queries in the database
Viewing & Modifying the resulting records

Required Steps:
 There are following steps required to create a new Database using JDBC
application:
 Import the packages . Requires that you include the packages containing the
JDBC classes needed for database programming. Most often, using import
java.sql.* will suffice.
 Register the JDBC driver . Requires that you initialize a driver so you can open a
communications channel with the database.
 Open a connection . Requires using the DriverManager.getConnection() method
to create a Connection object, which represents a physical connection with
database server.
 To create a new database, you need not to give any database name while preparing
database URL as mentioned in the below example.
 Execute a query . Requires using an object of type Statement for building and
submitting an SQL statement to the database.
 Clean up the environment . Requires explicitly closing all database resources
versus relying on the JVM's garbage collection.

Creating JDBC Application
 There are six steps involved in building a JDBC application which I'm going to brief
in this tutorial:
1.Import the packages:
 To use the standard JDBC package, which allows you to select, insert, update, and delete data in SQL
tables, add the following imports to your source code:
 //STEP 1. Import required packages
 Syntax :import java.sql.*;
2.Register the JDBC driver:
 This requires that you initializea driver so you can open a communications channel with the
database.
 Registering the driver is the process by which the Oracle driver's class file is loaded into memory so
it can be utilized as an implementation of the JDBC interfaces.
 You need to do this registration only once in your program
 //STEP 2: Register JDBC driver
 Syntax:Class.forName("com.mysql.jdbc.Driver");

 Open a connection:
 After you've loaded the driver, you can establish a connection using the
DriverManager.getConnection() method, which represents a physical
connection with the database as follows:
 //STEP 3: Open a connection // Database credentials
 static final String USER = "username";
 static final String PASS = "password";
 System.out.println("Connecting to database...");
 conn = DriverManager.getConnection(DB_URL,USER,PASS);
 Execute a query:
 This requires using an object of type Statement or PreparedStatement for building
and submitting an SQL statement to the database as follows:
 //STEP 4: Execute a query
 System.out.println("Creating statement...");
 stmt = conn.createStatement();
 String sql; sql = "SELECT id, first, last, age FROM Employees";
 ResultSet rs = stmt.executeQuery(sql);

 Following table lists down popular JDBC driver names and
database URL.
RDBMS JDBC driver name URL format
 MySQL com.mysql.jdbc.Driver jdbc:mysql://hostname/ databaseName
ORACLE oracle.jdbc.driver.OracleDriver jdbc:oracle:thin:@hostname:port
Number:databaseName
 DB2 COM.ibm.db2.jdbc.net.DB2Driver jdbc:db2:hostname:port
Number/databaseName
 Sybase com.sybase.jdbc.SybDriver jdbc:sybase:Tds:hostname: port
Number/databaseName
 All the highlighted part in URL format is static and you need to change only remaining
part as per your database setup.

 If there is an SQL UPDATE,INSERT or DELETE statement
required, then following code snippet would be required:
 //STEP 4: Execute a query
 System.out.println("Creating statement...");
 stmt = conn.createStatement();
 String sql;
 sql = "DELETE FROM Employees";
 ResultSet rs = stmt.executeUpdate(sql);

 Extract data from result set:
 This step is required in case you are fetching data from the database.
You can use the appropriate ResultSet.getXXX() method to retrieve the
data from the result set as follows:
 //STEP 5: Extract data from result set
 while(rs.next())
 {
 //Retrieve by column name
 int id = rs.getInt("id");
 int age = rs.getInt("age");
 String first = rs.getString("first");
 String last = rs.getString("last");
 //Display values
 System.out.print("ID: " + id);
 System.out.print(", Age: " + age);
 System.out.print(", First: " + first);
 System.out.println(", Last: " + last); }

 Clean up the environment:
 You should explicitly close all database resources versus
relying on the JVM's garbage collection as follows:
 //STEP 6: Clean-up environment
 rs.close();
 stmt.close();
 conn.close();

JDBC Driver
 JDBC drivers implement the defined interfaces in the JDBC API for
interacting with your database server.
 For example, using JDBC drivers enable you to open database
connections and to interact with it by sending SQL or database
commands then receiving results with Java.
 The Java.sql package that ships with JDK contains various classes with
their behaviours defined and their actual implementaions are done in
third-party drivers.
 Third party vendors implements the java.sql.Driver interface in their
database driver.
 JDBC Drivers Types:
 JDBC driver implementations vary because of the wide variety of
operating systems and hardware platforms in which Java operates. Sun
has divided the implementation types into four categories, Types 1, 2, 3,
and 4

Type 1: JDBC-ODBC Bridge
Driver
 In a Type 1 driver, a JDBC bridge is used to access ODBC
drivers installed on each client machine.
 Using ODBC requires configuring on your system a Data
Source Name (DSN) that represents the target database.
 When Java first came out, this was a useful driver because
most databases only supported ODBC access but now this type
of driver is recommended only for experimental use or when
no other alternative is available.
 The JDBC-ODBC bridge that comes with JDK 1.2 is a good
example of this kind of driver.

Type 1: JDBC-ODBC Bridge Driver

Type 2: JDBC-Native API
 In a Type 2 driver, JDBC API calls are converted into native
C/C++ API calls which are unique to the database.
 These drivers typically provided by the database vendors
and used in the same manner as the JDBC-ODBC Bridge,
the vendor-specific driver must be installed on each client
machine.
 If we change the Database we have to change the native API
as it is specific to a database and they are mostly obsolete
now but you may realize some speed increase with a Type 2
driver, because it eliminates ODBC's overhead.
 The Oracle Call Interface (OCI) driver is an example of a
Type 2 driver.

Type 3: JDBC-Net pure Java
 In a Type 3 driver, a three-tier approach is used to
accessing databases.
 The JDBC clients use standard network sockets to
communicate with an middleware application server. The
socket information is then translated by the middleware
application server into the call format required by the
DBMS, and forwarded to the database server.
 This kind of driver is extremely flexible, since it requires
no code installed on the client and a single driver can
actually provide access to multiple databases

Type 4: 100% pure Java
 In a Type 4 driver, a pure Java-based driver that
communicates directly with vendor's database through
socket connection.
 This is the highest performance driver available for the
database and is usually provided by the vendor itself.
 This kind of driver is extremely flexible, you don't need to
install special software on the client or server. Further,
these drivers can be downloaded dynamically.
 MySQL's Connector/J driver is a Type 4 driver. Because
of the proprietary nature of their network protocols,
database vendors usually supply type 4 drivers.

 Which Driver should be used?
 If you are accessing one type of database, such as
Oracle, Sybase, or IBM, the preferred driver type is 4.
 If your Java application is accessing multiple types of
databases at the same time, type 3 is the preferred
driver.
 Type 2 drivers are useful in situations where a type 3
or type 4 driver is not available yet for your database.
 The type 1 driver is not considered a deployment-
level driver and is typically used for development and
testing purposes only.

TOPICS
1 Events, Event sources, Event classes,
2 Event Listeners, Delegation event model
3 Handling mouse and keyboard events, Adapter classes, inner classes.
4 The AWT class hierarchy,
5 user interface components- labels, button, canvas, scrollbars, text
6 components, check box, check box groups, choices
7 lists panels – scrollpane, dialogs
8 menubar, graphics
9 layout manager – layout manager types –boarder, grid, flow, card and
grib bag

TOPICS
 Concepts of Applets, differences between applets and applications
 Life cycle of an applet, types of applets
 Creating applets, passing parameters to applets.
 Introduction to swings, limitations of AWT
 components, containers
 Exploring swing- JApplet, JFrame and JComponent
 Icons and Labels, text fields, buttons
 Check boxes, Combo boxes,RadioButton,JButton
 Tabbed Panes, Scroll Panes, Trees, and Tables

Event handling
 For the user to interact with a GUI, the underlying
operating system must support event handling.
1) operating systems constantly monitor events such as
keystrokes, mouse clicks, voice command, etc.
2) operating systems sort out these events and report
them to the appropriate application programs
3) each application program then decides what to do in
response to these events

Events
 An event is an object that describes a state change in a
source.
 It can be generated as a consequence of a person
interacting with the elements in a graphical user interface.
 Some of the activities that cause events to be generated are
pressing a button, entering a character via the keyboard,
selecting an item in a list, and clicking the mouse.
 Events may also occur that are not directly caused by
interactions with a user interface.
 For example, an event may be generated when a timer
expires, a counter exceeds a value, a software or hardware
failure occurs, or an operation is completed.
 Events can be defined as needed and appropriate by
application.

Event sources
 A source is an object that generates an event.
 This occurs when the internal state of that object changes
in some way.
 Sources may generate more than one type of event.
 A source must register listeners in order for the listeners to
receive notifications about a specific type of event.
 Each type of event has its own registration method.
 General form is:
public void addTypeListener(TypeListener el)
Here, Type is the name of the event and el is a reference to
the event listener.
 For example,
1. The method that registers a keyboard event listener is
called addKeyListener().
2. The method that registers a mouse motion listener is
called
addMouseMotionListener( ).

 When an event occurs, all registered listeners are notified
and receive a copy of the event object. This is known as
multicasting the event.
 In all cases, notifications are sent only to listeners that
register to receive them.
 Some sources may allow only one listener to register. The
general form is:
public void addTypeListener(TypeListener el) throws
java.util.TooManyListenersException
Here Type is the name of the event and el is a reference to
the event listener.
 When such an event occurs, the registered listener is
notified. This is known as unicasting the event.

 A source must also provide a method that allows a listener to
unregister an interest in a specific type of event.
 The general form is:
public void removeTypeListener(TypeListener el)
Here, Type is the name of the event and el is a reference to the
event listener.
 For example, to remove a keyboard listener, you would call
removeKeyListener( ).
 The methods that add or remove listeners are provided by the
source that generates events.
 For example, the Component class provides methods to add
and remove keyboard and mouse event listeners.

Event classes
 The Event classes that represent events are at the core of
Java's event handling mechanism.
 Super class of the Java event class hierarchy is
EventObject, which is in java.util. for all events.
 Constructor is :
EventObject(Object src)
Here, src is the object that generates this event.
 EventObject contains two methods: getSource( ) and
toString( ).
 1. The getSource( ) method returns the source of the
event. General form is : Object getSource( )
 2. The toString( ) returns the string equivalent of the
event.

 EventObject is a superclass of all events.
 AWTEvent is a superclass of all AWT events that are
handled by the delegation event model.
 The package java.awt.event defines several types of events
that are generated by various user interface elements.

Event Classes in java.awt.event
 ActionEvent: Generated when a button is pressed, a list
item is double clicked, or a menu item is selected.
 AdjustmentEvent: Generated when a scroll bar is
manipulated.
 ComponentEvent: Generated when a component is hidden,
moved, resized, or becomes visible.
 ContainerEvent: Generated when a component is added to
or removed from a container.
 FocusEvent: Generated when a component gains or loses
keyboard focus.

 InputEvent: Abstract super class for all component input
event classes.
 ItemEvent: Generated when a check box or list item is
clicked; also
 occurs when a choice selection is made or a checkable
menu item is selected or deselected.
 KeyEvent: Generated when input is received from the
keyboard.
 MouseEvent: Generated when the mouse is dragged,
moved, clicked, pressed, or released; also generated when
the mouse enters or exits a component.
 TextEvent: Generated when the value of a text area or text
field is changed.
 WindowEvent: Generated when a window is activated,
closed, deactivated, deiconified, iconified, opened, or quit.

Event Listeners
 A listener is an object that is notified when an event occurs.
 Event has two major requirements.
1. It must have been registered with one or more sources
to receive
notifications about specific types of events.
2. It must implement methods to receive and process
these
notifications.
 The methods that receive and process events are defined in
a set of interfaces found in java.awt.event.
 For example, the MouseMotionListener interface defines
two methods to receive notifications when the mouse is
dragged or moved.
 Any object may receive and process one or both of these
events if it provides an implementation of this interface.

Delegation event model
 The modern approach to handling events is based on the
delegation event model, which defines standard and
consistent mechanisms to generate and process events.
 Its concept is quite simple: a source generates an event and
sends it to one or more listeners.
 In this scheme, the listener simply waits until it receives an
event.
 Once received, the listener processes the event and then
returns.
 The advantage of this design is that the application logic
that processes events is cleanly separated from the user
interface logic that generates those events.
 A user interface element is able to "delegate“ the processing
of an event to a separate piece of code.

 In the delegation event model, listeners must register with
a source in order to receive an event notification. This
provides an important benefit: notifications are sent only
to listeners that want to receive them.
 This is a more efficient way to handle events than the
design used by the old Java 1.0 approach. Previously, an
event was propagated up the containment hierarchy until it
was handled by a component.
 This required components to receive events that they did
not process, and it wasted valuable time.The delegation
event model eliminates this overhead.
Note
 Java also allows you to process events without using the
delegation event model.
 This can be done by extending an AWT component.

Handling mouse events
 mouse events can be handled by implementing the
MouseListener and the MouseMotionListener
interfaces.
 MouseListener Interface defines five methods. The
general forms of these methods are:
1. void mouseClicked(MouseEvent me)
2. void mouseEntered(MouseEvent me)
3. void mouseExited(MouseEvent me)
4. void mousePressed(MouseEvent me)
5. void mouseReleased(MouseEvent me)
 MouseMotionListener Interface. This interface defines
two methods. Their general forms are :
1. void mouseDragged(MouseEvent me)
2. void mouseMoved(MouseEvent me)

Handling keyboard events
 Keyboard events, can be handled by implementing the
KeyListener interface.
 KeyListner interface defines three methods. The
general forms of these methods are :
1. void keyPressed(KeyEvent ke)
2. void keyReleased(KeyEvent ke)
3. void keyTyped(KeyEvent ke)
 To implement keyboard events implementation to the
above methods is needed.
L 3.2

Adapter classes
 Java provides a special feature, called an adapter class, that
can simplify the creation of event handlers.
 An adapter class provides an empty implementation of all
methods in an event listener interface.
 Adapter classes are useful when you want to receive and
process only some of the events that are handled by a
particular event listener interface.
 You can define a new class to act as an event listener by
extending one of the adapter classes and implementing
only those events in which you are interested.

Adapter classes in java.awt.event are.
Adapter Class Listener Interface
ComponentAdapter ComponentListener
ContainerAdapter ContainerListener
FocusAdapter FocusListener
KeyAdapter KeyListener
MouseAdapter MouseListener
MouseMotionAdapter MouseMotionListener
WindowAdapter WindowListener

Inner classes
 Inner classes, which allow one class to be defined within another.
 An inner class is a non-static nested class. It has access to all of
the variables and methods of its outer class and may refer to
them directly in the same way that other non-static members of
the outer class do.
 An inner class is fully within the scope of its enclosing class.
 an inner class has access to all of the members of its enclosing
class, but the reverse is not true.
 Members of the inner class are known only within the scope of
the inner class and may not be used by the outer class

The AWT class hierarchy
 The AWT classes are contained in the java.awt package. It
is one of Java's largest packages. some of the AWT classes.
 AWT Classes
1. AWTEvent:Encapsulates AWT events.
2. AWTEventMulticaster: Dispatches events to multiple
listeners.
3. BorderLayout: The border layout manager. Border
layouts use five components: North, South, East, West,
and Center.
4. Button: Creates a push button control.
5. Canvas: A blank, semantics-free window.
6. CardLayout: The card layout manager. Card layouts
emulate index cards. Only the one on top is showing.

7. Checkbox: Creates a check box control.
8. CheckboxGroup: Creates a group of check box controls.
9. CheckboxMenuItem: Creates an on/off menu item.
10. Choice: Creates a pop-up list.
11. Color: Manages colors in a portable, platform-independent fashion.
12. Component: An abstract super class for various AWT components.
13. Container: A subclass of Component that can hold other components.
14. Cursor: Encapsulates a bitmapped cursor.
15. Dialog: Creates a dialog window.
16. Dimension: Specifies the dimensions of an object. The width is stored
in width, and the height is stored in height.
17. Event: Encapsulates events.
18. EventQueue: Queues events.
19. FileDialog: Creates a window from which a file can be selected.
20. FlowLayout: The flow layout manager. Flow layout positions
components left to right, top to bottom.

21. Font: Encapsulates a type font.
22. FontMetrics: Encapsulates various information related to a font. This
information helps you display text in a window.
23. Frame: Creates a standard window that has a title bar, resize corners,
and a menu bar.
24. Graphics: Encapsulates the graphics context. This context is used by
various output methods to display output in a window.
25. GraphicsDevice: Describes a graphics device such as a screen or
printer.
26. GraphicsEnvironment: Describes the collection of available Font and
GraphicsDevice objects.
21. GridBagConstraints: Defines various constraints relating to the
GridBagLayout class.
22. GridBagLayout: The grid bag layout manager. Grid bag layout displays
components subject to the constraints specified by
GridBagConstraints.
23. GridLayout: The grid layout manager. Grid layout displays
components in a two-dimensional grid.

30. Scrollbar: Creates a scroll bar control.
31. ScrollPane: A container that provides horizontal and/or
vertical scrollbars for another component.
32. SystemColor: Contains the colors of GUI widgets such as
windows, scrollbars, text, and others.
33. TextArea: Creates a multiline edit control.
34. TextComponent: A super class for TextArea and TextField.
35. TextField: Creates a single-line edit control.
36. Toolkit: Abstract class implemented by the AWT.
37. Window: Creates a window with no frame, no menu bar, and
no title.

user interface components
 Labels: Creates a label that displays a string.
 A label is an object of type Label, and it contains a string, which it
displays.
 Labels are passive controls that do not support any interaction with the
user.
 Label defines the following constructors:
1. Label( )
2. Label(String str)
3. Label(String str, int how)
 The first version creates a blank label.
 The second version creates a label that contains the string specified by
str. This string is left-justified.
 The third version creates a label that contains the string specified by str
using the alignment specified by how. The value of how must be one of
these three constants: Label.LEFT, Label.RIGHT, or Label.CENTER.

 Set or change the text in a label is done by using the
setText( ) method.
 Obtain the current label by calling getText( ).
 These methods are shown here:
void setText(String str)
String getText( )
 For setText( ), str specifies the new label. For getText( ),
the current label is returned.
 To set the alignment of the string within the label by
calling setAlignment( ).
 To obtain the current alignment, call getAlignment( ).
 The methods are as follows:
void setAlignment(int how)
int getAlignment( )
Label creation: Label one = new Label("One");

Button
 The most widely used control is the push button.
 A push button is a component that contains a label and that generates
an event when it is pressed.
 Push buttons are objects of type Button. Button defines these two
constructors:
Button( )
Button(String str)
 The first version creates an empty button. The second creates a button
that contains str as a label.
 After a button has been created, you can set its label by calling
setLabel( ).
 You can retrieve its label by calling getLabel( ).
 These methods are as follows:
void setLabel(String str)
String getLabel( )
Here, str becomes the new label for the button.
Button creation: Button yes = new Button("Yes");

canvas
 It is not part of the hierarchy for applet or frame windows
 Canvas encapsulates a blank window upon which you can
draw.
 Canvas creation:
Canvas c = new Canvas();
Image test = c.createImage(200, 100);
 This creates an instance of Canvas and then calls the
createImage( ) method to actually make an Image object.
At this point, the image is blank.

scrollbars
 Scrollbar generates adjustment events when the scroll bar
is manipulated.
 Scrollbar creates a scroll bar control.
 Scroll bars are used to select continuous values between a
specified minimum and maximum.
 Scroll bars may be oriented horizontally or vertically.
 A scroll bar is actually a composite of several individual
parts.
 Each end has an arrow that you can click to move the
current value of the scroll bar one unit in the direction of
the arrow.
 The current value of the scroll bar relative to its minimum
and maximum values is indicated by the slider box (or
thumb) for the scroll bar.
 The slider box can be dragged by the user to a new position.
The scroll bar will then reflect this value.

 Scrollbar defines the following constructors:
Scrollbar( )
Scrollbar(int style)
Scrollbar(int style, int initialValue, int thumbSize, int min, int max)
 The first form creates a vertical scroll bar.
 The second and third forms allow you to specify the orientation
of the scroll bar. If style is Scrollbar.VERTICAL, a vertical scroll
bar is created. If style is Scrollbar.HORIZONTAL, the scroll bar
is horizontal.
 In the third form of the constructor, the initial value of the scroll
bar is passed in initialValue.
 The number of units represented by the height of the thumb is
passed in thumbSize.
 The minimum and maximum values for the scroll bar are
specified by min and max.
 vertSB = new Scrollbar(Scrollbar.VERTICAL, 0, 1, 0, height);
 horzSB = new Scrollbar(Scrollbar.HORIZONTAL, 0, 1, 0, width);

Text
 Text is created by Using a TextField class
 The TextField class implements a single-line text-entry
area, usually called an edit
 control.
 Text fields allow the user to enter strings and to edit the
text using the arrow
 keys, cut and paste keys, and mouse selections.
 TextField is a subclass of TextComponent. TextField
defines the following constructors:
TextField( )
TextField(int numChars)
TextField(String str)
TextField(String str, int numChars)

 The first version creates a default text field.
 The second form creates a text field that is numChars
characters wide.
 The third form initializes the text field with the string
contained in str.
 The fourth form initializes a text field and sets its width.
 TextField (and its superclass TextComponent) provides
several methods that allow you to utilize a text field.
 To obtain the string currently contained in the text field,
call getText().
 To set the text, call setText( ). These methods are as
follows:
String getText( )
void setText(String str)
Here, str is the new string.

Components
 At the top of the AWT hierarchy is the Component class.
 Component is an abstract class that encapsulates all of the
attributes of a visual component.
 All user interface elements that are displayed on the screen
and that interact with the user are subclasses of
Component.
 It defines public methods that are responsible for
managing events, such as mouse and keyboard input,
positioning and sizing the window, and repainting.
 A Component object is responsible for remembering the
current foreground and background colors and the
currently selected text font.

 To add components
Component add(Component compObj)
Here, compObj is an instance of the control that you want
to add. A reference to compObj is returned.
Once a control has been added, it will automatically be
visible whenever its parent window is displayed.
 To remove a control from a window when the control is no
longer needed call remove( ).
 This method is also defined by Container. It has this
general form:
void remove(Component obj)
Here, obj is a reference to the control you want to remove.
You can remove all controls by calling removeAll( ).

check box,
 A check box is a control that is used to turn an option on or
off. It consists of a small box that can either contain a check
mark or not.
 There is a label associated with each check box that
describes what option the box represents.
 You can change the state of a check box by clicking on it.
 Check boxes can be used individually or as part of a group.
 Checkboxes are objects of the Checkbox class.

 Checkbox supports these constructors:
1. Checkbox( )
2. Checkbox(String str)
3. Checkbox(String str, boolean on)
4. Checkbox(String str, boolean on, CheckboxGroup cbGroup)
5. Checkbox(String str, CheckboxGroup cbGroup, boolean on)
 The first form creates a check box whose label is initially blank. The
state of the check box is unchecked.
 The second form creates a check box whose label is specified by str.
The state of the check box is unchecked.
 The third form allows you to set the initial state of the check box. If on
is true, the check box is initially checked; otherwise, it is cleared.
 The fourth and fifth forms create a check box whose label is specified
by str and whose group is specified by cbGroup. If this check box is not
part of a group, then cbGroup must be null. (Check box groups are
described in the next section.) The value of on determines the initial
state of the check box.

 To retrieve the current state of a check box, call getState( ).
 To set its state, call setState( ).
 To obtain the current label associated with a check box by
calling getLabel().
 To set the label, call setLabel( ).
boolean getState( )
void setState(boolean on)
String getLabel( )
void setLabel(String str)
Here, if on is true, the box is checked. If it is false, the box
is
cleared.
Checkbox creation:
CheckBox Win98 = new Checkbox("Windows 98", null,
true);

check box groups
 It is possible to create a set of mutually exclusive check boxes in which one and
only one check box in the group can be checked at any one time.
 These check boxes are oftenccalled radio buttons.
 To create a set of mutually exclusive check boxes, you must first define the
group to which they will belong and then specify that group when you
construct the check boxes.
 Check box groups are objects of type CheckboxGroup. Only the default
constructor is defined, which creates an empty group.
 To determine which check box in a group is currently selected by calling
getSelectedCheckbox( ).
 To set a check box by calling setSelectedCheckbox( ).
Checkbox getSelectedCheckbox( )
void setSelectedCheckbox(Checkbox which)
Here, which is the check box that you want to be selected. The previously
selected checkbox will be turned off.
 CheckboxGroup cbg = new CheckboxGroup();
 Win98 = new Checkbox("Windows 98", cbg, true);
 winNT = new Checkbox("Windows NT", cbg, false);

choices
 The Choice class is used to create a pop-up list of items
from which the user may choose.
 A Choice control is a form of menu.
 Choice only defines the default constructor, which creates
an empty list.
 To add a selection to the list, call addItem( ) or add( ).
void addItem(String name)
void add(String name)
 Here, name is the name of the item being added.
 Items are added to the list in the order to determine which
item is currently selected, you may call either
getSelectedItem( ) or getSelectedIndex( ).
String getSelectedItem( )
int getSelectedIndex( )

lists
 The List class provides a compact, multiple-choice, scrolling selection
list.
 List object can be constructed to show any number of choices in the
visible window.
 It can also be created to allow multiple selections. List provides these
constructors:
List( )
List(int numRows)
List(int numRows, boolean multipleSelect)
 To add a selection to the list, call add( ). It has the following two forms:
void add(String name)
void add(String name, int index)
 Ex: List os = new List(4, true);

panels
 The Panel class is a concrete subclass of Container.
 It doesn't add any new methods; it simply implements Container.
 A Panel may be thought of as a recursively nestable, concrete screen
component. Panel is the superclass for Applet.
 When screen output is directed to an applet, it is drawn on the surface
of a Panel object.
 Panel is a window that does not contain a title bar, menu bar, or
border.
 Components can be added to a Panel object by its add( ) method
(inherited from Container). Once these components have been added,
you can position and resize them manually using the setLocation( ),
setSize( ), or setBounds( ) methods defined by Component.
 Ex: Panel osCards = new Panel();
CardLayout cardLO = new CardLayout();
osCards.setLayout(cardLO);

scrollpane
 A scroll pane is a component that presents a rectangular
area in which a component may be viewed.
 Horizontal and/or vertical scroll bars may be provided if
necessary.
 constants are defined by the ScrollPaneConstants
interface.
1. HORIZONTAL_SCROLLBAR_ALWAYS
2. HORIZONTAL_SCROLLBAR_AS_NEEDED
3. VERTICAL_SCROLLBAR_ALWAYS
4. VERTICAL_SCROLLBAR_AS_NEEDED

dialogs
 Dialog class creates a dialog window.
 constructors are :
Dialog(Frame parentWindow, boolean mode)
Dialog(Frame parentWindow, String title, boolean mode)
 The dialog box allows you to choose a method that should
be invoked when the button is clicked.
 Ex: Font f = new Font("Dialog", Font.PLAIN, 12);

menubar
 MenuBar class creates a menu bar.
 A top-level window can have a menu bar associated with it.
A menu bar displays a list of top-level menu choices. Each
choice is associated with a drop-down menu.
 To create a menu bar, first create an instance of MenuBar.
 This class only defines the default constructor. Next, create
instances of Menu that will define the selections displayed
on the bar.
 Following are the constructors for Menu:
Menu( )
Menu(String optionName)
Menu(String optionName, boolean removable)

 Once you have created a menu item, you must add the item
to a Menu object by using
MenuItem add(MenuItem item)
 Here, item is the item being added. Items are added to a
menu in the order in which the calls to add( ) take place.
 Once you have added all items to a Menu object, you can
add that object to the menu bar by using this version of
add( ) defined by MenuBar:
 Menu add(Menu menu)

Graphics
 The AWT supports a rich assortment of graphics methods.
 All graphics are drawn relative to a window.
 A graphics context is encapsulated by the Graphics class
 It is passed to an applet when one of its various methods, such as paint( )
or update( ), is called.
 It is returned by the getGraphics( ) method of Component.
 The Graphics class defines a number of drawing functions. Each shape
can be drawn edge-only or filled.
 Objects are drawn and filled in the currently selected graphics color,
which is black by default.
 When a graphics object is drawn that exceeds the dimensions of the
window, output is automatically clipped
 Ex:
Public void paint(Graphics g)
{
G.drawString(“welcome”,20,20);
}

Layout manager
 A layout managerautomatically arranges your controls within a
window by using some type of algorithm.
 it is very tedious to manually lay out a large number of
components and sometimes the width and height information is
not yet available when you need to arrange some control, because
the native toolkit components haven't been realized.
 Each Container object has a layout managerassociated with it.
 A layout manager is an instance of any class that implements the
LayoutManager interface.
 The layout manager is set by the setLayout( ) method. If no call
to setLayout( ) is made, then the default layout manager is used.
 Whenevera container is resized (or sized for the first time), the
layout manager is used to position each of the components within
it.

Layout manager types
Layout manager class defines the
following types of layout managers
 Boarder Layout
 Grid Layout
 Flow Layout
 Card Layout
 GridBag Layout

Boarder layout
 The BorderLayout class implements a common layout style for top-
level windows. It has four narrow, fixed-width components at the edges
and one large area in the center.
 The four sides are referred to as north, south, east, and west. The
middle area is called the center.
 The constructors defined by BorderLayout:
BorderLayout( )
BorderLayout(int horz, int vert)
 BorderLayout defines the following constants that specify the regions:
BorderLayout.CENTER
B orderLayout.SOUTH
BorderLayout.EAST
B orderLayout.WEST
BorderLayout.NORTH
 Components can be added by
void add(Component compObj, Object region);

Grid layout
 GridLayout lays out components in a two-dimensional grid. When you
instantiate a
 GridLayout, you define the number of rows and columns. The
constructors are
GridLayout( )
GridLayout(int numRows, int numColumns )
GridLayout(int numRows, int numColumns, int horz, int vert)
 The first form creates a single-column grid layout.
 The second form creates a grid layout
 with the specified number of rows and columns.
 The third form allows you to specify the horizontal and vertical space
left between components in horz and vert, respectively.
 Either numRows or numColumns can be zero. Specifying numRows as
zero allows for unlimited-length columns. Specifying numColumns as
zero allows for unlimited-lengthrows.

Flow layout
 FlowLayout is the default layout manager.
 Components are laid out from the upper-left corner, left to right and
top to bottom. When no more components fit on a line, the next one
appears on the next line. A small space is left between each
component, above and below, as well as left and right.
 The constructors are
FlowLayout( )
FlowLayout(int how)
FlowLayout(int how, int horz, int vert)
 The first form creates the default layout, which centers components
and leaves five pixels of space between each component.
 The second form allows to specify how each line is aligned. Valid values
for are:
FlowLayout.LEFT
FlowLayout.CENTER
FlowLayout.RIGHT
These values specify left, center, and right alignment, respectively.
 The third form allows to specify the horizontal and vertical space left
between components in horz and vert, respectively

Card layout
 The CardLayout class is unique among the other layout managers in
that it stores several different layouts.
 Each layout can be thought of as being on a separate index card in a
deck that can be shuffled so that any card is on top at a given time.
 CardLayout provides these two constructors:
CardLayout( )
CardLayout(int horz, int vert)
 The cards are held in an object of type Panel. This panel must have
CardLayout selected as its layout manager.
 Cards are added to panel using
void add(Component panelObj, Object name);
 methods defined by CardLayout:
void first(Container deck)
void last(Container deck)
void next(Container deck)
void previous(Container deck)
void show(Container deck, String cardName)

GridBag Layout
 The Grid bag layout displays components subject to the
constraints specified by GridBagConstraints.
 GridLayout lays out components in a two-dimensional
grid.
 The constructors are
GridLayout( )
GridLayout(int numRows, int numColumns)
GridLayout(int numRows, int numColumns, int horz, int
vert)

Concepts of Applets
 Applets are small applications that are accessed on an Internet
server, transported over the Internet, automatically installed,
and run as part of a Web document.
 After an applet arrives on the client, it has limited access to
resources, so that it can produce an arbitrary multimedia user
interface and run complex computations without introducing
the risk of viruses or breaching data integrity.
 applets – Java program that runs within a Java-enabled browser,
invoked through a “applet” reference on a web page, dynamically
downloaded to the client computer
import java.awt.*;
import java.applet.*;
public class SimpleApplet extends Applet {
public void paint(Graphics g) {
g.drawString("A Simple Applet", 20, 20);
}
}

There are two ways to run an applet:
1. Executing the applet within a Java-compatible Web
browser, such as NetscapeNavigator.
2. Using an applet viewer, such as the standard JDK tool,
appletviewer.
 An appletviewer executes your applet in a window. This is
generally the fastest and easiest way to test an applet.
 To execute an applet in a Web browser, you need to write
a short HTML text file that contains the appropriate
APPLET tag.
<applet code="SimpleApplet" width=200 height=60>
</applet>

Differences between applets and applications
 Java can be used to create two types of programs: applications
and applets.
 An application is a program that runs on your computer, under
the operating system of that Computer(i.e an application
created by Java is more or less like one created using C or C++).
 When used to create applications, Java is not much different
from any other computer language.
 An applet is an application designed to be transmitted over the
Internet and executed by a Java-compatible Web browser.

 An applet is actually a tiny Java program, dynamically
downloaded across the network, just like an image, sound file, or
video clip.

 The important difference is that an applet is an intelligent
program, not just an animation or media file(i.e an applet
is a program that can react to user input and dynamically
change—not just run the same animation or sound over
and over.
 Applications require main method to execute.
 Applets do not require main method.
 Java's console input is quite limited
 Applets are graphical and window-based.

Life cycle of an applet
 Applets life cycle includes the following methods
1. init( )
2. start( )
3. paint( )
4. stop( )
5. destroy( )
 When an applet begins, the AWT calls the following methods, in
this sequence:
init( )
start( )
paint( )
 When an applet is terminated, the following sequence of method
calls takes place:
stop( )
destroy( )

 init( ): The init( ) method is the first method to be called. This is
where you should initialize variables. This method is called only once
during the run time of your applet.
 start( ): The start( ) method is called after init( ). It is also called to
restart an applet after it has been stopped. Whereas init( ) is called
once—the first time an applet is loaded—start( ) is called each time an
applet's HTML document is displayed onscreen. So, if a user leaves a
web page and comes back, the applet resumes execution at start( ).
 paint( ): The paint( ) method is called each time applet's output must
be redrawn. paint( ) is also called when the applet begins execution.
Whatever the cause, whenever the applet must redraw its output,
paint( ) is called. The paint( ) method has one parameter of type
Graphics. This parameter will contain the graphics context, which
describes the graphics environment in which the applet is running.
This context is used whenever output to the applet is required.
 stop( ): The stop( ) method is called when a web browser leaves the
HTML document containing the applet—when it goes to another page,
for example. When stop( ) is called, the applet is probably running.
Applet uses stop( ) to suspend threads that don't need to run when the
applet is not visible. To restart start( ) is called if the user returns to
the page.
 destroy( ): The destroy( ) method is called when the environment
determines that your applet needs to be removed completely from
memory. The stop( ) method is always called before destroy( ).

Types of applets
 Applets are two types
1.Simple applets
2.JApplets
 Simple applets can be created by extending Applet class
 JApplets can be created by extending JApplet class of
javax.swing.JApplet package

Creating applets
 Applets are created by extending the Applet class.
import java.awt.*;
/*<applet code="AppletSkel" width=300 height=100></applet> */
public class AppletSkel extends Applet {
public void init() {
// initialization
}
public void start() {
// start or resume execution
}
public void stop() {
// suspends execution
}
public void destroy() {
// perform shutdown activities
}
// redisplay contents of window
}
}

passing parameters to applets
 APPLET tag in HTML allows you to pass parameters to applet.
 To retrieve a parameter, use the getParameter( ) method. It returns
the value of the specified parameter in the form of a String object.
// Use Parameters
import java.awt.*;
/*
<applet code="ParamDemo" width=300 height=80>
<param name=fontName value=Courier>
<param name=fontSize value=14>
<param name=leading value=2>
<param name=accountEnabled value=true>
</applet>
*/

public class ParamDemo extends Applet{
String fontName;
int fontSize;
float leading;
boolean active;
// Initialize the string to be displayed.
public void start() {
String param;
fontName = getParameter("fontName");
if(fontName == null)
fontName = "Not Found";
param = getParameter("fontSize");
try {
if(param != null) // if not found
fontSize = Integer.parseInt(param);
else
fontSize = 0;
} catch(NumberFormatException e) {
fontSize = -1;
}
param = getParameter("leading");

try {
if(param != null) // if not found
leading = Float.valueOf(param).floatValue();
else
leading = 0;
} catch(NumberFormatException e) {
leading = -1;
}
param = getParameter("accountEnabled");
if(param != null)
active = Boolean.valueOf(param).booleanValue();
}
// Display parameters.
g.drawString("Font name: " + fontName, 0, 10);
g.drawString("Font size: " + fontSize, 0, 26);
g.drawString("Leading: " + leading, 0, 42);
g.drawString("Account Active: " + active, 0, 58);
}
}

Introduction to swings
 Swing is a set of classes that provides more powerful and flexible
components than are possible with the AWT.
 In addition to the familiar components, such as buttons, check boxes,
and labels, Swing supplies several exciting additions, including tabbed
panes, scroll panes, trees, and tables.
 Even familiar components such as buttons have more capabilities in
Swing.
 For example, a button may have both an image and a text string
associated with it. Also, the image can be changed as the state of the
button changes.
 Unlike AWT components, Swing components are not implemented by
platform-specific code.
 Instead, they are written entirely in Java and, therefore, are platform-
independent.
 The term lightweight is used to describe such elements.

 The Swing component are defined in javax.swing
1. AbstractButton: Abstract superclass for Swing buttons.
2. ButtonGroup: Encapsulates a mutually exclusive set of buttons.
3. ImageIcon: Encapsulates an icon.
4. JApplet: The Swing version of Applet.
5. JButton: The Swing push button class.
6. JCheckBox: The Swing check box class.
7. JComboBox : Encapsulates a combo box (an combination of a
drop-down list and text field).
8. JLabel: The Swing version of a label.
9. JRadioButton: The Swing version of a radio button.
10. JScrollPane: Encapsulates a scrollable window.
11. JTabbedPane: Encapsulates a tabbed window.
12. JTable: Encapsulates a table-based control.
13. JTextField: The Swing version of a text field.
14. JTree: Encapsulates a tree-based control.

Limitations of AWT
 AWT supports limited number of GUI components.
 AWT components are heavy weight components.
 AWT components are developed by using platform specific
code.
 AWT components behaves differently in different
operating systems.
 AWT component is converted by the native code of the
operating system.

 Lowest Common Denominator
 If not available natively on one Java platform, not
available on any Java platform
 Simple Component Set
 Components Peer-Based
 Platform controls component appearance
 Inconsistencies in implementations
 Interfacing to native platform error-prone

components
 Container
 JComponent
 AbstractButton
 JButton
 JMenuItem
 JCheckBoxMenuItem
 JMenu
 JRadioButtonMenuItem
 JToggleButton
 JCheckBox
 JRadioButton

Components (contd…)
 JComponent
 JComboBox
 JLabel
 JList
 JMenuBar
 JPanel
 JPopupMenu
 JScrollBar
 JScrollPane

Components (contd…)
 JComponent
 JTextComponent
 JTextArea
 JTextField
 JPasswordField
 JTextPane
 JHTMLPane

Containers
 Top-Level Containers
 The components at the top of any Swing
containment hierarchy

General Purpose Containers
 Intermediate containers that can be used under many
different circumstances.

Special Purpose Container
 Intermediate containers that play specific roles in the UI.

Exploring swing- JApplet
 If using Swing components in an applet, subclass
JApplet, not Applet
 JApplet is a subclass of Applet
 Sets up special internal component event handling,
among other things
 Can have a JMenuBar
 Default LayoutManager is BorderLayout

JFrame
public class FrameTest {
public static void main (String args[]) {
JFrame f = new JFrame ("JFrame Example");
Container c = f.getContentPane();
c.setLayout (new FlowLayout());
for (int i = 0; i < 5; i++) {
c.add (new JButton ("No"));
c.add (new Button ("Batter"));
}
c.add (new JLabel ("Swing"));
f.setSize (300, 200);
f.show();
}
}

JComponent
 JComponent supports the following components.
 JComponent
 JComboBox
 JLabel
 JList
 JMenuBar
 JPanel
 JPopupMenu
 JScrollBar
 JScrollPane
 JTextComponent
 JTextArea
 JTextField
 JPasswordField
 JTextPane
 JHTMLPane

Icons and Labels
 In Swing, icons are encapsulated by the ImageIcon class,
which paints an icon from an image.
 constructors are:
ImageIcon(String filename)
ImageIcon(URL url)
 The ImageIcon class implements the Icon interface that
declares the methods
1. int getIconHeight( )
2. int getIconWidth( )
3. void paintIcon(Component comp,Graphics g,int x, int y)

 Swing labels are instances of the JLabel class, which extends
JComponent.
 It can display text and/or an icon.
 Constructors are:
JLabel(Icon i)
Label(String s)
JLabel(String s, Icon i, int align)
 Here, s and i are the text and icon used for the label. The align
argument is either LEFT, RIGHT, or CENTER. These constants are
defined in the SwingConstants interface,
 Methods are:
1. Icon getIcon( )
2. String getText( )
3. void setIcon(Icon i)
4. void setText(String s)
 Here, i and s are the icon and text, respectively.

Text fields
 The Swing text field is encapsulated by the
JTextComponent class, which extendsJComponent.
 It provides functionality that is common to Swing text
components.
 One of its subclasses is JTextField, which allows you to
edit one line of text.
 Constructors are:
JTextField( )
JTextField(int cols)
JTextField(String s, int cols)
JTextField(String s)
 Here, s is the string to be presented, and cols is the number
of columns in the text field.

Buttons
 Swing buttons provide features that are not found in the Button class defined
by the AWT.
 Swing buttons are subclasses of the AbstractButton class, which extends
JComponent.
 AbstractButton contains many methods that allow you to control the
behavior of buttons, check boxes, and radio buttons.
 Methods are:
1. void setDisabledIcon(Icon di)
2. void setPressedIcon(Icon pi)
3. void setSelectedIcon(Icon si)
4. void setRolloverIcon(Icon ri)
 Here, di, pi, si, and ri are the icons to be used for these different conditions.
 The text associated with a button can be read and written via the following
methods:
1. String getText( )
2. void setText(String s)
 Here, s is the text to be associated with the button.

JButton
 The JButton class provides the functionality of a push
button.
 JButton allows an icon, a string, or both to be associated
with the push button.
 Some of its constructors are :
JButton(Icon i)
JButton(String s)
JButton(String s, Icon i)
 Here, s and i are the string and icon used for the button.

Check boxes
 The JCheckBox class, which provides the functionality of a check box,
is a concrete implementation of AbstractButton.
 Some of its constructors are shown here:
JCheckBox(Icon i)
JCheckBox(Icon i, boolean state)
JCheckBox(String s)
JCheckBox(String s, boolean state)
JCheckBox(String s, Icon i)
JCheckBox(String s, Icon i, boolean state)
 Here, i is the icon for the button. The text is specified by s. If state is
true, the check box is initially selected. Otherwise, it is not.
 The state of the check box can be changed via the following method:
void setSelected(boolean state)
 Here, state is true if the check box should be checked.

Combo boxes
 Swing provides a combo box (a combination of a text field and a drop-
down list) through the JComboBox class, which extends
JComponent.
 A combo box normally displays one entry. However, it can also display a
drop-down list that allows a user to select a different entry. You can also
type your selection into the text field.
 Two of JComboBox's constructors are :
JComboBox( )
JComboBox(Vector v)
 Here, v is a vector that initializes the combo box.
 Items are added to the list of choices via the addItem( ) method,
whose signature is:
void addItem(Object obj)
 Here, obj is the object to be added to the combo box.

Radio Buttons
 Radio buttons are supported by the JRadioButton class, which is a
concrete implementation of AbstractButton.
 Some of its constructors are :
JRadioButton(Icon i)
JRadioButton(Icon i, boolean state)
JRadioButton(String s)
JRadioButton(String s, boolean state)
JRadioButton(String s, Icon i)
JRadioButton(String s, Icon i, boolean state)
 Here, i is the icon for the button. The text is specified by s. If state is
true, the button is initially selected. Otherwise, it is not.
 Elements are then added to the button group via the following method:
void add(AbstractButton ab)
 Here, ab is a reference to the button to be added to the group.

Tabbed Panes
 A tabbed pane is a component that appears as a group of folders in a file
cabinet.
 Each folder has a title. When a user selects a folder, its contents become visible.
Only one of the folders may be selected at a time.
 Tabbed panes are commonly used for setting configuration options.
 Tabbed panes are encapsulated by the JTabbedPane class, which extends
JComponent. We will use its default constructor. Tabs are defined via the
following method:
void addTab(String str, Component comp)
 Here, str is the title for the tab, and comp is the component that should be
added to the tab. Typically, a JPanel or a subclass of it is added.
 The general procedure to use a tabbed pane in an applet is outlined here:
1. Create a JTabbedPane object.
2. Call addTab( ) to add a tab to the pane. (The arguments to this method
define the
title of the tab and the component it contains.)
3. Repeat step 2 for each tab.
4. Add the tabbed pane to the content pane of the applet.

Scroll Panes
 A scroll pane is a component that presents a rectangular area in which a
component may be viewed. Horizontal and/or vertical scroll bars may be
provided if necessary.
 Scroll panes are implemented in Swing by the JScrollPane class, which
extends JComponent. Some of its constructors are :
JScrollPane(Componentcomp)
JScrollPane(int vsb, int hsb)
JScrollPane(Componentcomp, int vsb, int hsb)
 Here, comp is the component to be added to the scroll pane. vsb and hsb are
int constants that define when vertical and horizontal scroll bars for this scroll
pane areshown.
 These constants are defined by the ScrollPaneConstants interface.
1. HORIZONTAL_SCROLLBAR_ALWAYS
2. HORIZONTAL_SCROLLBAR_AS_NEEDED
3. VERTICAL_SCROLLBAR_ALWAYS
4. VERTICAL_SCROLLBAR_AS_NEEDED
 Here are the steps to follow to use a scroll pane in an applet:
1. Create a JComponent object.
2. Create a JScrollPane object. (The arguments to the constructor specify
thecomponent and the policies for vertical and horizontal scroll bars.)
3. Add the scroll pane to the content pane of the applet.

Trees
 Data Model - TreeModel
 default: DefaultTreeModel
 getChild, getChildCount, getIndexOfChild, getRoot,
isLeaf
 Selection Model – TreeSelectionModel
 View - TreeCellRenderer
 getTreeCellRendererComponent
 Node - DefaultMutableTreeNode

Tables
 A table is a component that displays rows and columns of data. You can drag
the cursor on column boundaries to resize columns. You can also drag a column
to a new position.
 Tables are implemented by the JTable class, which extends JComponent.
 One of its constructors is :
JTable(Object data[ ][ ], Object colHeads[ ])
 Here, data is a two-dimensional array of the information to be presented, and
colHeads is a one-dimensional array with the column headings.
 Here are the steps for using a table in an applet:
1. Create a JTable object.
2. Create a JScrollPane object. (The arguments to the constructor specify
the table and
the policies for vertical and horizontal scroll bars.)
3. Add the table to the scroll pane.
4. Add the scroll pane to the content pane of the applet.

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