Computer Science A 14: 31/3

Inheritence You can extend a class or change behaviour. Inheritance is a method to reuse code. Concept. Extending classes with fields and methods Late binding Casting objects

Example class Point{ int x; int y; Point(int x,int y){ this.x=x; this.y=y; } public String toString(){ return "("+x+","+y+")"; } toString is called when an object is used as a string

Example class Point3D extends Point{ int z; Point3D(int x,int y,int z){ super(x,y); this.z=z; } public String toString(){ return "("+x+","+y+","+z+")";} }

Example Point a=new Point(2,4); System.out.println(a.toString()); Point3D b=new Point3D(7,9,13); System.out.println(b.toString()); A Point3D ”is-a” Point object and may be used as a Point object

Object-oriented design When analysing a problem area you can often identify a hierarchy in the structure and use inheritance in the description. Model the world and find “is-a” relations: A circle is a shape A textfield is a component

Polymorphism Point x = new Point3D(1,2,3); System.out.println(x.toString()); Depends on the actual object. If x refers to a Point3D object it print 3 values If x refers to a Point object it prints 2 values Polymorphism (many shapes): Behavior can vary depending on the actual type of an object Called late binding: resolved at runtime Different from overloading; overloading is resolved by the compiler (early binding)

Polymorphism JSlider extends JComponent JSpinner extends JComponent Swing will call a method to drw the oject but the methods are different

interface MyShape interface MyShape{ void drawOn(JCanvas canvas); }

class Tree class Tree implements MyShape{ Tree(int x,int y){this.x=x; this.y=y;} int x,y; public void drawOn(JCanvas canvas){ canvas.drawArc(x,y,200,200,290,320); canvas.drawLine(x+80,y+300,x+80,y+180); canvas.drawLine(x+120,y+300,x+120,y+180); }

class Car class Car implements MyShape{ Car(int x,int y){this.x=x; this.y=y;} int x,y; public void drawOn(JCanvas canvas){ canvas.drawRect(x,y+50,300,50); canvas.drawLine(x+50,y+50,x+100,y); canvas.drawLine(x+100,y,x+200,y); canvas.drawLine(x+200,y,x+250,y+50); canvas.drawOval(x+50,y+100,50,50); canvas.drawOval(x+200,y+100,50,50); }

class MyShapeTest port javax.swing.*; public class MyShapeTest{ public static void main(String args[]){ JFrame frame=new JFrame("MyShapes"); frame.setSize(600,600); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); JCanvas canvas=new JCanvas(); frame.add(canvas); MyShape s1=new Car(10,300); MyShape s2=new Car(270,400); MyShape s3=new Tree(330,40); s1.drawOn(canvas); s2.drawOn(canvas); s3.drawOn(canvas); frame.setVisible(true); }

Example

And more… An interface is a placeholder for a number of classes. A class can be an extension of only one class but it may implement several interfaces