问答题 【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 { {{U}} (1) {{/U}}; //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= {{U}}(2) {{/U}};//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 { {{U}} (3) {{/U}}；//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. {{U}}(4) {{/U}} { private double speed; public MovingBall() { } public MovingBall(double xValue, double yValue, double r, String e, double s) { {{U}} (5) {{/U}};// 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); } }