运行结果radius的值一直是0

Question

abstract class Point
{
	double x;
	double y;
	abstract double S();
	abstract double V();
}
class Circle extends Point
{
	double x;
	double y;
	double radius;
	public Circle(double x,double y)
	{
		this.x=x;
		this.y=y;
	}
	public void setXY(double x,double y)
	{
		this.x=x;
		this.y=y;
	}
	public void setRadius()
	{
		this.radius=Math.sqrt(x*x+y*y);
	}
	public double getRadius()
	{
		return radius;
	}
	public double S()
	{
		return radius*radius*Math.PI;
	}
	public double V()
	{
		return 0;
	}
}
class Ball extends Point
{
	double x;
	double y;
	double radius;
	public Ball(double x,double y)
	{
		this.x=x;
		this.y=y;
	}
	public void setXY(double x,double y)
	{
		this.x=x;
		this.y=y;
	}
	public void setRadius()
	{
		this.radius=Math.sqrt(x*x+y*y);
	}
	public double getRadius()
	{
		return radius;
	}
	public double S()
	{
		return 4*Math.PI*radius*radius;
	}
	public double V()
	{
		return (3/4)*Math.PI*radius*radius*radius;
	}
}
class Cylinder extends Point
{
	double x;
	double y;
	double radius;
	int height;	
		public Cylinder(double x,double y)
	{
		this.x=x;
		this.y=y;
	}
		public void setXY(double x,double y)
	{
		this.x=x;
		this.y=y;
	}
	public void setRadius()
	{
		radius=Math.sqrt(this.x*this.x+this.y*this.y);
	}
	public double S()
	{
		return (Math.PI*radius*radius*2)+(2*Math.PI*radius);
	}
	public double V()
	{
		return Math.PI*radius*radius*height;
	}
}
public class a2
{
	public static void main(String[] args)
	{
		Ball a=new Ball(3,4);
		System.out.println(a.x);
		System.out.println(a.y);
		System.out.println(a.radius);
		a.getRadius();
		Cylinder b=new Cylinder(6,8);
		System.out.println(a.S());
		System.out.println(a.V());
		System.out.println(b.S());
		System.out.println(b.V());
	}
}

Answer 1

创建对象以后，你要先调用setRadius方法，再调用其他函数就正常了。