The x and y instance variables indicate the position of the center of the shape, and the color instance variable indicates the color of the shape. The color of the shape is computed randomly in the constructor of the shape class and never changes after that, like this: color = new Color((float)Math.random(), (float)Math.random(), (float)Math.random()); The isVisible method is abstract, and indicates whether the shape is currently visible or not inside a window of width w and of height h. The isIn method is abstract, and indicates whether the point at coordinates (x, y) is currently inside the shape or not. The draw method simply changes the color of the graphics object g to be the correct color for the shape. Also add to your program a Start class to test your Shape class.帮我写出以上Java代码

以下是一个可能的实现：

import java.awt.Color;
import java.awt.Graphics;

public abstract class Shape {
    protected int x, y;
    protected Color color;

    public Shape(int x, int y) {
        this.x = x;
        this.y = y;
        color = new Color((float)Math.random(), (float)Math.random(), (float)Math.random());
    }

    public abstract boolean isVisible(int w, int h);

    public abstract boolean isIn(int x, int y);

    public void draw(Graphics g) {
        g.setColor(color);
    }
}

public class Start {
    public static void main(String[] args) {
        Shape shape = new Circle(50, 50, 20);
        System.out.println("Is shape visible? " + shape.isVisible(100, 100));
        System.out.println("Is (60, 60) inside shape? " + shape.isIn(60, 60));
    }
}

注意，上面的代码中 Shape 是一个抽象类，不能被直接实例化。你需要创建特定的子类来实现 isVisible 和 isIn 方法。例如，下面是一个可能的 Circle 类的实现：

public class Circle extends Shape {
    private int radius;

    public Circle(int x, int y, int radius) {
        super(x, y);
        this.radius = radius;
    }

    @Override
    public boolean isVisible(int w, int h) {
        return x - radius > 0 && x + radius < w && y - radius > 0 && y + radius < h;
    }

    @Override
    public boolean isIn(int x, int y) {
        return Math.sqrt((x - this.x) * (x - this.x) + (y - this.y) * (y - this.y)) <= radius;
    }

    public void draw(Graphics g) {
        super.draw(g);
        g.fillOval(x - radius, y - radius, 2 * radius, 2 * radius);
    }
}

注意，上面的 Circle 类重写了 Shape 类的 draw 方法，实现了在图形上绘制圆的功能。你可以创建其他的子类来实现不同的形状。