Abstract Classes and InterfacesAbstract methodsAbstract classes IAbstract classes IIWhy have abstract classes?An example abstract classAnother reason for abstract classesInterfacesImplementing an interface IImplementing an interface IIImplementing an interface with an abstract classWhat are interfaces for?instanceofInterfaces, againAdapter classesVocabularyThe EndAbstract Classesand InterfacesAbstract methods•You can decl are 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 methodAbstract 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 classAbstract 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 instantiatedWhy 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 par t i cular shapes make sense, not gene ric 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” classesAn example abstract class•public abstract class Animal { abstract int decideMove(); ...}•This class cannot be instantiated•Any non-abstract subclass must provide the decideMove() methodAnother 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 mi g ht contain a generic Figure •Solution: give Figure an abst ract method draw()•Now the class Figure is abstract, so you cannot instantiate it–You can’t have a generic FigureInterfaces•An interface declare s (describes) methods but does not supply bodies for theminterface 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 def i ne 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 MyKeyListenerImplementing 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 abstractWhat 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, ifclass Dog extends Animal implements Pet {...}Animal fido = new Dog(); then the following are all true: fido instanceof Dog fido instanceof Animal fido instanceof PetInterfaces, 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 classesVocabulary•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 bodiesThe
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