【说明】下面的程序先构造Point类，再顺序构造Ball类。由于在类Ball中不能直接存取类Point中的xCoordinate及yCoordinate属性值，Ball中的toString方法调用Point类中的toString方法输出中心点的值。在MovingBall类的toString方法中，super.toString调用父类Ball的toString方法输出类Ball中声明的属性值。
public class Point
private double xCoordinate;
private double yCoordinate;
public Point 0
public Point(ouble x, double y)
xCoordinate = x;
yCoordinate = y;
public String toString()
return "( + Double.toString(Coordinate)+ ","
+ Double.toString(Coordinate) + ");
//other methods
public class Ball
(1) ; //中心点
private double radius; //半径
private String colour; ///颜色
public Ball()
public Ball(double xValue, double yValue, double r)// 具有中心点及半径的构造方法
center= (2) ;//调用类Point 中的构造方法
radius = r;
public Ball(double xValue, double yValue, double r, String c)
// 具有中心点、半径及颜色的构造方法
(3) ；//调用3个参数的构造方法
colour = c;
public String toString()
return "A ball with center" + center, toString() + ", radius"
+ Double.toString(radius) + ", colour" + colour;
//other methods
public class MovingBall. (4)
private double speed;
public MovingBall()
public MovingBall(double xValue, double yValue, double r, String e, double s)
(5) ;// 调用父类Ball中具有4个参数的构造方法
speed = s;
public String toString( )
return super, toString( ) + ", speed "+ Double.toString(speed);
//other methods
public class Tester
public static void main(String args[])
MovingBall mb = new MovingBall(10,20,40,"green",25);
System.out.println(mb);
