学习内容：以点类 Point 及平面图形类 Plane 为基础设计圆类 Circle

代码示例：

import java.util.Scanner;
 
class Point2{
  private double x;
  private double y;
  public Point2(double x,double y) {
    this.x=x;
    this.y=y;
    System.out.println("Point Constructor run");
  }
  public void setX(double x) {//设置x坐标
    this.x=x;
  }
  public double getX(){//返回x坐标
    return x;
  }
  public void setY(double y) {//设置y坐标
    this.y=y;
  }
  public double getY(){//返回y坐标
    return y;
  }
  public void show() {//显示点的坐标
    System.out.println("Point(X="+x+",Y="+y+")");
  }
}
abstract class Plane2 extends Point2{
  public Plane2(double x, double y) {
    super(x, y);
  }
  double length() {
    return 0;
  }
  double area() {
    return 0;
  }
}
public class Circle extends Plane2 { 
  public Circle(double x, double y,double radius) {
    super(x,y);
    this.x=x;
    this.y=y;
    this.radius=radius;
    System.out.println("Circle Constructorrun");
  } 
  private double x;
  private double y;
  final double PI=3.14159;
  private double radius;
  public void setX(double x) {//设置x坐标
    this.x=x;
  }
  public final double getX(){//返回x坐标
    return x;
  }
  public void setY(double y) {//设置y坐标
    this.y=y;
  }
  public final double getY(){//返回y坐标
    return y;
  }
  public void setR(double r) {
    radius=r;
  }
  public final double getR() {
    return radius;
  }
  public final void show() {
    System.out.println("Circle(Point("+x+","+y+"),Radius="+radius+")");
  }
  public final double area() {
    double area=PI*radius*radius;
    System.out.println("Area="+area);
    return 0;
  }
  public final double length() {
    double length=2*PI*radius;
    System.out.println("Length="+length);
    return 0;
  } 
  public static void main(String[] args) {
    double x=0,y=0,r=0; 
    Circle c1=new Circle(0, 0, 0);
    Circle c2=c1;
    c1.show();
    System.out.println("..............................................");
    c1.area();
    c1.length();
    System.out.println("请分别输入x,y,r：");
    Scanner sc=new Scanner(System.in);
    x=sc.nextDouble();
    y=sc.nextDouble();
    r=sc.nextDouble();
    c2.setX(x);
    c2.setY(y);
    c2.setR(r);
    c2.show();
    System.out.println("..............................................");
    c2.area();
    c2.length(); 
  }
 
}

运行截图：

以点类 Point 及平面图形类 Plane 为基础设计圆类 Circle_java