Abstract Classes and Interfaces Jan 14 2019 Java is safer than Python Python is very dynamic classes and methods can be added modified and deleted as the program runs If you have a call to a function that doesn t exist Python will give you a runtime error when you try to call it In Java everything has to be defined before the program begins to execute If you have a call to a function that doesn t exist the compiler marks it as a syntax error Syntax errors are far better than runtime errors Among other things they won t make it into distributed code To achieve this Java requires some additional kinds of classes 2 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 3 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 4 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 5 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 6 An example abstract class public abstract class Animal abstract int eat abstract void breathe This class cannot be instantiated Any non abstract subclass of Animal must provide the eat and breathe methods 7 Why have abstract methods Suppose you have a class Shape but it isn t abstract Now suppose you have a variable Shape figure where figure contains some subclass object such as a Star Shape should not have a draw method Each subclass of Shape should have a draw method 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 variable A class knows its superclass but doesn t know its subclasses An object knows its class but a class doesn t know its objects Solution Give Shape an abstract method draw Now the class Shape is abstract so it can t be instantiated The figure variable cannot contain a generic Shape because it is impossible to create one Any object such as a Star object that is a kind of Shape will have the draw method The Java compiler can depend on figure draw being a legal call and does not give a syntax error 8 A problem class 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 Shape someShape draw This is a syntax error because some Shape might not have a draw method Remember A class knows its superclass but not its subclasses 9 A solution abstract 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 Shape However Shape someShape new Shape is no longer legal someShape draw This is legal because every actual instance must have a draw method 10 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 cannot instantiate an interface You can add these qualifiers if you like but why bother An interface is like a very abstract class none of its methods are defined An interface may also contain constants final variables 11 Designing interfaces Most of the time you will use Sun supplied Java interfaces Sometimes you will want to design your own You would write an interface if you want classes of various types to all have a certain set of capabilities For 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 methods 12 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 13 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 The indicates actual code that you must supply Now you can create a new MyKeyListener 14 Partially implementing an Interface 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 Since this class does not supply all the methods it has promised it is an abstract class You must label it as such with the keyword abstract You can even extend an interface to add methods interface FunkyKeyListener extends KeyListener 15 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 In writing Applets it is common to have one class implement several different listeners Example class MyApplet extends Applet implements ActionListener KeyListener Reason 2 You can write methods that work for more than one kind of class 16 How to use interfaces You can write methods that work with more than one class interface RuleSet boolean isLegal Move m Board b void makeMove Move m class CheckersRules implements RuleSet one implementation public boolean isLegal Move m Board b public void makeMove Move m class
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