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8.-Abstract-Class-and-Interfaces.pdf vk sir.pdf
- 1. CSE 1201 Object OrientedProgramming Abstract Classes and Interfaces
- 2. Acknowledgement ● For preparingthe slides I took materials from the following sources • Course Slides of Dr. Tagrul Dayar, Bilkent University • Java book “Java Software Solutions” by Lewis & Loftus. 2
- 3. Abstract Classes ● Anabstract class is a placeholder in a class hierarchy that represents a generic concept ● An abstract class cannot be instantiated ● We use the modifier abstract on the class header to declare a class as abstract: public abstract class Whatever { // contents } 3
- 4. Abstract Example ● Agood example is the MovingObject class. MovingObject is just an abstract or synthetic concept to help us capture commonalities. ● To make sure nobody creates an instance of class MovingObject, we need to declare it abstract. 4
- 5. Abstract Example ● Let’ssay you want the MovingObjects to be able to collide with each other, but cannot define a collide method since the outcome of collision depends on specific object MovingObje ct //collide ? MovingRectangle collide(MovingObject mo) Ball collide(. .) MovingIma ge ????????? 5
- 6. Abstract Classes ● Anabstract class often contains abstract methods with no definitions ● In addition to forcing sub-classes to override to become concrete classes, it enables one to write polymorphic methods ● An abstract class typically contains non-abstract methods (with bodies), which can even call abstract methods ● A class declared as abstract does not need to contain abstract methods 6
- 7. Vehicle example public abstractclass Vehicle { private Position position; public getPosition() { return position; } public abstract void start(); public abstract void move(); public abstract void turnLeft(); public abstract void turnRight(); public abstract void stop(); public void goto(Position pos) { start(); if (position.getX() > pos.getX()) turnLeft(); . …… 7
- 8. Abstract Classes ● Thechild of an abstract class must override the abstract methods of the parent, or it too will be considered abstract ● An abstract method cannot be defined as final (because it must be overridden) or static (because it has no definition yet) ● The use of abstract classes is a design decision – it helps us establish common elements in a class that is too general to instantiate 8
- 9. AbstractMethods ● to makesure every MovingObject has a collide() method, you can declare an abstract MovingObject. collide() method without an implementation, to be provided by more specific sub-classes public class MovingObject { ….. public abstract void collide (MovingObject other); ……… } 9
- 10. © 2004 PearsonAddison- Wesley. All rights reserved 6-10 Interfaces ● A Java interface is a collection of abstract methods and constants ● An abstract method is a method header without a method body ● An abstract method can be declared using the modifier abstract, but because all methods in an interface are abstract, usually it is left off ● An interface is used to establish a set of methods that a class will implement
- 11. 6-11 Interfaces public interface Doable { publicvoid doThis(); public int doThat(); public void doThis2 (float value, char ch); public boolean doTheOther (int num); } interface is a reserved word None of the methods in an interface are given a definition (body) A semicolon immediately follows each method header
- 12. 6-12 Interfaces ● An interfacecannot be instantiated ● Methods in an interface have public visibility by default ● A class formally implements an interface by: • stating so in the class header • providing implementations for each abstract method in the interface ● If a class asserts that it implements an interface, it must define all methods in the interface
- 13. 6-13 Interfaces public class CanDoimplements Doable { public void doThis () { // whatever } public void doThat () { // whatever } // etc. } implements is a reserved word Each method listed in Doable is given a definition
- 14. 6-14 Interfaces ● A classthat implements an interface can implement other methods as well ● See Complexity.java ● See Question.java ● See MiniQuiz.java ● In addition to (or instead of) abstract methods, an interface can contain constants ● When a class implements an interface, it gains access to all its constants
- 15. 6-15 Interfaces ● A classcan implement multiple interfaces ● The interfaces are listed in the implements clause ● The class must implement all methods in all interfaces listed in the header class ManyThings implements interface1, interface2 { // all methods of both interfaces }
- 16. 6-16 Interfaces ● The Javastandard class library contains many helpful interfaces ● The Comparable interface contains one abstract method called compareTo, which is used to compare two objects ● The String class implements Comparable, giving us the ability to put strings in lexicographic order
- 17. 6-17 The Comparable Interface ●Any class can implement Comparable to provide a mechanism for comparing objects of that type if (obj1.compareTo(obj2) < 0) System.out.println ("obj1 is less than obj2"); • The value returned from compareTo should be negative is obj1 is less that obj2, 0 if they are equal, and positive if obj1 is greater than obj2 • When a programmer designs a class that implements the Comparable interface, it should follow this intent
- 18. 6-18 The Comparable Interface ●It's up to the programmer to determine what makes one object less than another ● For example, you may define the compareTo method of an Employee class to order employees by name (alphabetically) or by employee number ● The implementation of the method can be as straightforward or as complex as needed for the situation
- 19. 6-19 Interfaces ● You couldwrite a class that implements certain methods (such as compareTo) without formally implementing the interface (Comparable) ● However, formally establishing the relationship between a class and an interface allows Java to deal with an object in certain ways ● Interfaces are a key aspect of object-oriented design in Java
- 20. Polymorphism via Interfaces ●An interface name can be used as the type of an object reference variable Speaker current; ● The current reference can be used to point to any object of any class that implements the Speaker interface ● The version of speak that the following line invokes depends on the type of object that current is referencing current.speak(); 20
- 21. Polymorphism via Interfaces ●Suppose two classes, Philosopher and Dog, both implement the Speaker interface, providing distinct versions of the speak method ● In the following code, the first call to speak invokes one version and the second invokes another: Speaker guest = new Philospher(); guest.speak(); guest = new Dog(); guest.speak(); 21