Java 父类和子类

 package chapter11;

 public class GeometricObject1 {
     private String color="white";
     private boolean filled;
     private java.util.Date dateCreated;

     public GeometricObject1(){
         dateCreated=new java.util.Date();
     }

     public GeometricObject1(String color, boolean filled){
         dateCreated=new java.util.Date();
         this.color=color;
         this.filled=filled;
     }

     public String getColor(){
         return color;
     }
     public void setColor(String color){
         this.color=color;
     }

     public boolean isFilled(){
         return filled;
     }
     public void setFilled(boolean filled){
         this.filled=filled;
     }

     public java.util.Date getDateCreated(){
         return dateCreated;
     }

     public String toString(){
         return "created on "+dateCreated+"\ncolor: "+color+" and filled: "+filled;
     }

 }

首先新建一个几何体类，作为父类，具有一般几何体的共性；

 package chapter11;

 public class Circle4 extends GeometricObject1 {
     private double radius;
     public Circle4(){

     }
     public Circle4(double radius){
         this.radius=radius;
     }
     public Circle4(double radius,String color,boolean filled){
         this.radius=radius;
         setColor(color);
         setFilled(filled);
     }

     public double getRadius(){
         return radius;
     }
     public void setRadius(double radius){
         this.radius=radius;
     }

     public double getArea(){
         return radius*radius*Math.PI;
     }
     public double getDiameter(){
         return 2*radius;
     }
     public double getPerimeter(){
         return 2*radius*Math.PI;
     }
     public void printCircle(){
         System.out.println("The circle is created "+getDateCreated()+
                 " and the radius is "+radius);
     }

 }

 package chapter11;

 public class Rectangle1 extends GeometricObject1 {
     private double width;
     private double height;

     public Rectangle1(){

     }
     public Rectangle1(double width,double height){
         this.width=width;
         this.height=height;
     }
     public Rectangle1(double width,double height,String color,boolean filled){
         this.width=width;
         this.height=height;
         setColor(color);
         setFilled(filled);
     }

     public double getWidth(){
         return width;
     }
     public void setWidth(double width){
         this.width=width;
     }

     public double getHeight(){
         return height;
     }
     public void setHeight(double height){
         this.height=height;
     }

     public double getArea(){
         return width*height;
     }
     public double getPerimeter(){
         return 2*(width+height);
     }

 }

然后建立了基于父类的两个子类，Circle和Rectangle类，分别具有自己的数据域和方法属性，并实现。

 package chapter11;

 public class TestCircleRectangle {

     public static void main(String[] args) {
         // TODO Auto-generated method stub
         Circle4 circle=new Circle4(1);
         System.out.println("A circle "+circle.toString());
         System.out.println("The radius is "+circle.getRadius());
         System.out.println("The area is "+circle.getArea());
         System.out.println("The diameter is "+circle.getDiameter());

         Rectangle1 rectangle=new Rectangle1(2,4);
         System.out.println("\nA rectangle "+rectangle.toString()+"\nThe area is "+
         rectangle.getArea()+"\nThe perimeter is "+rectangle.getPerimeter());

     }

 }

这是对子类的测试。

总结：

1、子类并不是父类的一个子集，实际上，一个子类通常比它的父类包含更多的信息和方法。

2、父类中的私有数据域在该类之外不可访问，同样在子类中也不能直接使用，需要使用父类中的访问器/修改器来进行访问和修改。

3、在Java中，不允许多重继承，一个Java类只能直接继承自一个父类，这种限制称为单一继承。多重继承可以通过接口来实现。