Abstract Classes and Interfaces Abstract methods You can declare an object without defining it Person p Similarly you can declare a method without defining it public abstract void draw int size Notice that the body of the method is missing A method that has been declared but not defined is an abstract method Abstract classes I Any class containing an abstract method is an abstract class You must declare the class with the keyword abstract abstract class MyClass An abstract class is incomplete It has missing method bodies You cannot instantiate create a new instance of an abstract class Abstract classes II You can extend subclass an abstract class If the subclass defines all the inherited abstract methods it is complete and can be instantiated If the subclass does not define all the inherited abstract methods it too must be abstract You can declare a class to be abstract even if it does not contain any abstract methods This prevents the class from being instantiated Why have abstract classes Suppose you wanted to create a class Shape with subclasses Oval Rectangle Triangle Hexagon etc You don t want to allow creation of a Shape Only particular shapes make sense not generic ones If Shape is abstract you can t create a new Shape You can create a new Oval a new Rectangle etc Abstract classes are good for defining a general category containing specific concrete classes An example abstract class public abstract class Animal abstract int decideMove This class cannot be instantiated Any non abstract subclass must provide the decideMove method Another reason for abstract classes Suppose you have an ordinary class Figure Figure should not have a draw method Each subclass of Figure should have a draw method Now suppose you have a variable Figure figure where figure contains some subclass object such as a Star It is illegal to say figure draw even if figure happens to contain a Star because it might contain a generic Figure Solution give Figure an abstract method draw Now the class Figure is abstract so you cannot instantiate it You can t have a generic Figure Interfaces An interface declares describes methods but does not supply bodies for them interface KeyListener public void keyPressed KeyEvent e public void keyReleased KeyEvent e public void keyTyped KeyEvent e All the methods are implicitly public and abstract You can add these qualifiers if you like but why bother You cannot instantiate an interface An interface may also contain constants final variables Implementing an interface I You extend a class but you implement an interface A class can only extend subclass one other class but it can implement as many interfaces as you like Example class MyListener implements KeyListener ActionListener Implementing an interface II When you say a class implements an interface you are promising to define all the methods that were declared in the interface Example class MyKeyListener implements KeyListener public void keyPressed KeyEvent e public void keyReleased KeyEvent e public void keyTyped KeyEvent e Now you can create a new MyKeyListener Implementing an interface with an abstract class It is possible to define some but not all of the methods defined in an interface abstract class MyKeyListener implements KeyListener public void keyTyped KeyEvent e This is an abstract class and you must supply the keyword abstract What are interfaces for A class can only extend one other class but it can implement multiple interfaces This lets the class fill multiple roles In writing Applets it is common to have one class implement several different listeners Example class MyApplet extends Applet implements ActionListener KeyListener instanceof instanceof is a keyword that tells you whether a variable is a member of a class or interface For example if class Dog extends Animal implements Pet Animal fido new Dog then the following are all true fido instanceof Dog fido instanceof Animal fido instanceof Pet Interfaces again When you implement an interface you promise to define all the functions it declares There can be a lot of methods interface KeyListener public void keyPressed KeyEvent e public void keyReleased KeyEvent e public void keyTyped KeyEvent e What if you only care about a couple of these methods Adapter classes Solution use an adapter class An adapter class implements an interface and provides empty method bodies class KeyAdapter implements KeyListener public void keyPressed KeyEvent e public void keyReleased KeyEvent e public void keyTyped KeyEvent e You can override only the methods you care about This isn t elegant but it does work Java provides a number of adapter classes Vocabulary abstract method a method which is declared but not defined it has no method body abstract class a class which either 1 contains abstract methods or 2 has been declared abstract instantiate to create an instance object of a class interface similar to a class but contains only abstract methods and possibly constants adapter class a class that implements an interface but has only empty method bodies The End
View Full Document
Unlocking...