接口。

1.接口的作用：方法声明和方法实现相分离，每实现接口的类根据自身的实际情况，给出实际实现方法；

 

//平面图形接口
public interface PlaneGraphics2
{
    public abstract double area();  //计算面积
	public abstract double perimeter();  //计算周长
	public abstract void print();   //显示
}

 

 

//长方形类
public class Rectangle2 implements PlaneGraphics2
{
	protected double length;
	protected double width;

	public Rectangle2(double length,double width){
	     this.length=length;
		 this.width=width;
	}
	public Rectangle2(double width){  //正方形
	     this.length=width;
		 this.width=width;
	}
	public Rectangle2(){
	     this(0,0);   //根据参数调用本类的构造方法
	}
	public Rectangle2(Rectangle2 r1){
		this(r1.length,r1.width);
	}

	public double area(){  //计算面积
		return this.width*this.length;
    }
	public double perimeter(){
	    return (this.width+this.length)*2; //计算周长
	}
	public void print(){
		if(this.length==this.width)
			System.out.print("一个正方形,边长为:"+this.length);
		else
			System.out.print("一个长方形，长度为"+this.length+"宽度为："+this.width);
		System.out.println(",面积为："+this.area()+",周长为"+this.perimeter());
	}
	public static void main(String args[]){
	   Rectangle2 r1=new Rectangle2(10,20);
	   r1.print();
	   r1=new Rectangle2(10);
	   r1.print();
	}
}

 

一个类可以 继承一个父类 并 实现多个接口：

 

//立体图形接口
public interface SolidGraphics1
{
	public abstract double volume(); //计算体积
}

 

 

//长方体类
public class Cuboid1 extends Rectangle2 implements SolidGraphics1
{
	protected double height;
	public Cuboid1(double length,double width,double height){
		super(length,width);   //调用父类的构造方法
		this.height=height;
	}
	public Cuboid1(Rectangle2 r1,double height){
	    this(r1.length,r1.width,height); //调用本类构造函数
	}
	public Cuboid1(double width){
		this(width,width,width);
	}
	public Cuboid1(){
		this(0,0,0);
	}

	public double area(){
		return super.perimeter()*this.height+2*super.area(); //super引用父类的同名成员
	}
	public double volume(){
		return super.area()*this.height;//super引用父类的同名成员
	}
	public void print(){
		System.out.print("一个长方体，长度为："+this.length+",宽度为："+this.width+
			",高度为："+this.height);
		System.out.print(",表面积为："+this.area()+",体积为："+this.volume());
	}
	public static void main(String args[]){
		Cuboid1 c1=new Cuboid1(10,20,30);
		c1.print();
	}
}

 

球类实现多个接口：

 

 

//实现多个接口，实现平面图形接口和立体图形接口
public class Globe1 implements PlaneGraphics2,SolidGraphics1
{
	private double radius;
	public Globe1(double radius){
		this.radius=radius;
	}
	public Globe1(){
		this(0);
	}

	public double area(){  //计算表面积，覆盖PlaneGraphics2接口中的抽象方法
		return 4*Math.PI*this.radius*this.radius;
	}
	public double perimeter(){ //虽然球没有周长的概念，也必须覆盖接口中的抽象方法
		return 0;   
 	}
	public double volume(){
		return Math.PI*this.radius*this.radius*this.radius*4/3;
	}
	public void print(){
		System.out.println("一个球，半径为："+this.radius+"，表面积为："+this.area()+
			"，体积为："+this.volume());
	}
	public static void main(String args[]){
		Globe1 g1=new Globe1(10);
		g1.print();
	}
}