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
{
(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);
}
}
【参考答案】
[解析] (1)private Point center Ball类以Point类的center对象作为私有成员。 (......
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