Abstract Classes and InterfacesJava is “safer” than PythonAbstract methodsAbstract classes IAbstract classes IIWhy have abstract classes?An example abstract classWhy have abstract methods?A problemA solutionInterfacesDesigning interfacesImplementing an interface IImplementing an interface IIPartially implementing an InterfaceWhat are interfaces for?How to use interfacesinstanceofInterfaces, againAdapter classesVocabularyThe EndJan 14, 2019Abstract Classes and InterfacesJava is “safer” than PythonPython is very dynamic—classes and methods can be added, modified, and deleted as the program runsIf you have a call to a function that doesn't exist, Python will give you a runtime error when you try to call itIn Java, everything has to be defined before the program begins to executeIf you have a call to a function that doesn't exist, the compiler marks it as a syntax errorSyntax errors are far better than runtime errorsAmong other things, they won’t make it into distributed codeTo achieve this, Java requires some additional kinds of classes23Abstract methodsYou 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 missingA method that has been declared but not defined is an abstract method4Abstract classes IAny class containing an abstract method is an abstract classYou must declare the class with the keyword abstract: abstract class MyClass {...}An abstract class is incompleteIt has “missing” method bodiesYou cannot instantiate (create a new instance of) an abstract class5Abstract classes IIYou can extend (subclass) an abstract classIf the subclass defines all the inherited abstract methods, it is “complete” and can be instantiatedIf the subclass does not define all the inherited abstract methods, it too must be abstractYou can declare a class to be abstract even if it does not contain any abstract methodsThis prevents the class from being instantiated6Why 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 onesIf Shape is abstract, you can’t create a new ShapeYou can create a new Oval, a new Rectangle, etc.Abstract classes are good for defining a general category containing specific, “concrete” classes7An example abstract classpublic abstract class Animal { abstract int eat(); abstract void breathe();}This class cannot be instantiatedAny non-abstract subclass of Animal must provide the eat() and breathe() methods8Why have abstract methods?Suppose you have a class Shape, but it isn’t abstractShape should not have a draw() methodEach subclass of Shape should have a draw() methodNow suppose you have a variable Shape figure; where figure contains some subclass object (such as a Star)It is a syntax error to say figure.draw(), because the Java compiler can’t tell in advance what kind of value will be in the figure variableA class “knows” its superclass, but doesn’t know its subclassesAn object knows its class, but a class doesn’t know its objectsSolution: Give Shape an abstract method draw()Now the class Shape is abstract, so it can’t be instantiatedThe figure variable cannot contain a (generic) Shape, because it is impossible to create oneAny object (such as a Star object) that is a (kind of) Shape will have the draw() methodThe Java compiler can depend on figure.draw() being a legal call and does not give a syntax error9A problemclass Shape { ... }class Star extends Shape { void draw() { ... } ...}class Crescent extends Shape { void draw() { ... } ...}Shape someShape = new Star();This is legal, because a Star is a ShapesomeShape.draw();This is a syntax error, because some Shape might not have a draw() methodRemember: A class knows its superclass, but not its subclasses10A solutionabstract class Shape { abstract void draw();}class Star extends Shape { void draw() { ... } ...}class Crescent extends Shape { void draw() { ... } ...}Shape someShape = new Star();This is legal, because a Star is a ShapeHowever, Shape someShape = new Shape(); is no longer legalsomeShape.draw();This is legal, because every actual instance must have a draw() method11InterfacesAn 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 abstractYou can add these qualifiers if you like, but why bother?You cannot instantiate an interfaceAn interface is like a very abstract class—none of its methods are definedAn interface may also contain constants (final variables)12Designing interfacesMost of the time, you will use Sun-supplied Java interfacesSometimes you will want to design your ownYou would write an interface if you want classes of various types to all have a certain set of capabilitiesFor example, if you want to be able to create animated displays of objects in a class, you might define an interface as:public interface Animatable { install(Panel p); display();}Now you can write code that will display any Animatable class in a Panel of your choice, simply by calling these methods13Implementing an interface IYou extend a class, but you implement an interfaceA class can only extend (subclass) one other class, but it can implement as many interfaces as you likeExample: class MyListener implements KeyListener, ActionListener { … }14Implementing an interface IIWhen you say a class implements an interface, you are promising to define all the methods that were declared in the interfaceExample: class MyKeyListener implements KeyListener { public void keyPressed(KeyEvent e) {...}; public void keyReleased(KeyEvent e) {...}; public void keyTyped(KeyEvent e) {...};}The “...” indicates actual code that you must supplyNow you can create a new MyKeyListener15Partially implementing an InterfaceIt is possible to define some but not all of the methods defined in an interface: abstract class MyKeyListener implements KeyListener
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