Computer Object Oriented Programming - Chapter 4 - Excption Handling.ppt

Exceptions Handling
Compiled by Hashim S.
Chapter 4

Exceptions
 An exception is an object that describes an unusual or
erroneous situation
 Exceptions are thrown by a program, and may be caught and
handled by another part of the program

Exception Hierarchy
 Errors:
 Errors are fairly rare and usually fatal. They are caused by
bugs in the Java VM, or by the program running out of
memory or other resources.
 Errors are usually not handled by programs.
 Exceptions:
 Exceptions can be caused by bugs in the program or improper
data supplied to the program
 Exceptions should be handled, or caught
 An exception is either checked or unchecked
 Checked exceptions must be fixed or declared as thrown.
 You are not required to handle unchecked exceptions.

Attack of the Exception
 What happens when this method is used to take the average of
an array of length zero?
 Program throws an Exception and fails
java.lang.ArithmeticException: / by zero
public static int average(int[] a) {
int total = 0;
int total1=0;
for(int i = 0; i < a.length; i++) {
total += a[i];
}
return total /total1;
}

What is an Exception?
 An error event that disrupts the program flow and may
cause a program to fail.
 Some examples:
 Performing illegal arithmetic
 Illegal arguments to methods
 Accessing an out-of-bounds array element
 Hardware failures
 Writing to a read-only file

Another Exception Example
 What is the output of this program?
public class ExceptionExample {
public static void main(String args[]) {
String[] greek = {"Alpha", "Beta"};
System.out.println(greek[2]);
}
}
Output:
Exception in thread "main“ java.lang.ArrayIndexOutOfBoundsException: 2
at ExceptionExample.main(ExceptionExample.java:4)

Exception Message Details
 What exception class?
 Which array index is out of bounds?
 What method throws the exception?
 What file contains the method?
 What line of the file throws the exception?
Exception message format:
[exception class]: [additional description of exception]
at [class].[method]([file]:[line number])
Example:
java.lang.ArrayIndexOutOfBoundsException: 2
at ExceptionExample.main(ExceptionExample.java:4)
ArrayIndexOutOfBoundsException
2
main
ExceptionExample.java
4

8
Classifying Java Exceptions
 Unchecked Exceptions
 All the exceptions we've seen so far have been Unchecked
Exceptions, or Runtime Exceptions
 Usually occur because of programming errors, when code is
not robust enough to prevent them
 They are numerous and can be ignored by the programmer
 Common Unchecked Exceptions

NullPointerException: reference is null and should not be

IllegalArgumentException: method argument is improper is
some way

IllegalStateException: method called when class is in improper
state

 Checked Exceptions
 Usually occur because of errors programmer cannot control

examples: hardware failures, unreadable files
 They are less frequent and they cannot be ignored by the
programmer . . .
 Every method must catch (handle) checked exceptions or
specify that it may throw them
 Specify with the throws keyword

Checked and Unchecked Exceptions
Checked Exception Unchecked Exception
not subclass of RuntimeException subclass of RuntimeException
if not caught, method must specify it
to be thrown
if not caught, method may specify it
to be thrown
for errors that the programmer
cannot directly prevent from
occurring
for errors that the programmer can
directly prevent from occurring,
IOException,
FileNotFoundException,
SocketException
NullPointerException,
IllegalArgumentException,
IllegalStateException

Exception Class Hierarchy
Exception
RuntimeException IOException
FileNotFoundException
MalformedURLException
SocketException
ArrayIndexOutofBounds
NullPointerException
etc. etc.
SQLException
IllegalArgumentException
Unchecked Exceptions Checked Exceptions
 All exceptions are instances of classes that are subclasses
of Exception

12
Keywords for Java Exceptions
 throws: - Describes the exceptions which can be raised by a
method.
 throw: - Raises an exception to the first available handler in
the call stack, unwinding the stack along the way.
 try: - Marks the start of a block associated with a set of
exception handlers.
 catch: - If the block enclosed by the try generates an
exception of this type, control moves here; watch out for
implicit subsumption .
 finally: - Always called when the try block concludes, and
after any necessary catch handler is complete.

Exceptions Terminology
 When an exception happens we say it was thrown or
raised
 When an exception is dealt with, we say the exception is
handled or caught

Exception Handling
 Use a try-catch block to handle exceptions that are thrown
try {
// code that might throw exception
}
catch ([Type of Exception] e) {
// what to do if exception is thrown
}

Exception Handling Example
int total = 0;
int tot1=0;
total += a[i];
}
return total / tot1;
}
public static void printAverage(int[] a) {
try {
int avg = average(a);
System.out.println("the average is: " + avg);
}
catch (ArithmeticException e) {
System.out.println("error calculating average");
}
}

Catching Multiple Exceptions
 Handle multiple possible exceptions by multiple successive
catch blocks
try {
// code that might throw multiple exception
}
catch (IOException e) {
// handle IOException and all subclasses
}
catch (ClassNotFoundException e2) {
// handle ClassNotFoundException
}

Exception Constructors
 Exceptions have at least two constructors:
1. no arguments
NullPointerException e = new
NullPointerException();
2. single String argument descriptive message that appears when
exception error message is printed
IllegalArgumentExceptione e =
new IllegalArgumentException("number must be positive");

Writing Your Own Exception
 To write your own exception, write a subclass of Exception
and write both types of constructors
public class MyCheckedException extends IOException {
public MyCheckedException() {}
public MyCheckedException(String m) {
super(m);
}
}
public class MyUncheckedException extends RuntimeException {
public MyUncheckedException() {}
public MyUncheckedException(String m) {
super(m);
}
}

Throwing Exceptions
 Throw exception with the throw keyword
if (a.length == 0) {
throw new IllegalArgumentException("array is empty");
}
int total = 0;
total += a[i];
}
return total / a.length;
}

Throw Statement and Throws Clause
public class MyClass {
public void aMethod(int i, int j) throws MyException {
// ...
throw new MyException(reason);
// ...
}
}

Throws and Inheritance
 A method can throw less exceptions, but not more, than the
method it is overriding
public class MyClass {
public void doSomething() throws IOException, SQLException {
// do something here
}
}
public class MySubclass extends MyClass {
public void doSomething() throws IOException {
// do here
}
}

Computer Object Oriented Programming - Chapter 4 - Excption Handling.ppt

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