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+"
color: "+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("
A rectangle "+rectangle.toString()+"
The area is "+
        rectangle.getArea()+"
The 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。