Oop interfaces
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The document discusses Java interfaces. It explains that interfaces declare methods but do not provide implementations, and that classes implement interfaces to provide those method implementations. It provides examples of how to declare an interface and how a class implements an interface. It also discusses why interfaces are useful for allowing classes to have multiple roles and behave in standard ways.
Oop interfaces
- 2. Interfaces An interface declares (describes) methods but does not supply bodies for them interface interFaceName{ public void function1(Class1 obj1); public void function2(Class2 obj2); } 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 is like a very abstract class—none of its methods are defined An interface may also contain constants (final variables) which act like constants 2
- 3. Implementin g an interface 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 otherClass implements Interface1, Interface2 { //declarations } 3
- 4. What are interfaces for? Reason 1: A class can only extend one other class, but it can implement multiple interfaces This lets the class fill multiple “roles” This concept could be explained as partial multiple inheritance In writing Applets, it is common to have one class implement several different listeners Example: class otherClass extends Applet implements Interface1, Interface2 { //Statements } Reason 2: You can write methods that work for more than one kind of class 4
- 5. 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 instanceof is seldom used When you find yourself wanting to use instanceof, think about whether the method you are writing should be moved to the individual subclasses 5 Instance of
- 6. 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? 6 Interfaces again
- 7. 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 7 Adapter class
- 8. 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 8