题目：

·编程实现以下功能:

        界面如下图所示;

        当点击不同的按钮时,圆的填充颜色会随之改变;

        用鼠标点击圆内部时,圆的 填充颜色会依照”面板背景色-红色-绿色-蓝色”循环改变;

        鼠标移到圆内时,光标变成十字形;

代码如下：

import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
import java.awt.geom.Ellipse2D;
import java.awt.geom.Point2D;

public class Circle extends JFrame {
    public static void main(String[] args) {
        JFrame jFrame = new Circle();
        jFrame.setDefaultCloseOperation(3);
        jFrame.setVisible(true);
    }

    public Circle() {
        this.setTitle("change color");
        this.setSize(500, 500);
        add(new jPanel());
    }
}

class jPanel extends JPanel {
    private Ellipse2D rec;
    private int num = 0;
    JButton B01 = new JButton("Green");
    JButton B02 = new JButton("Blue");
    JButton B03 = new JButton("Red");

    public jPanel() {
        MouseClickListener mcl = new MouseClickListener();
        MouseCursorListener mcll = new MouseCursorListener();
        this.addMouseListener(mcl);
        this.addMouseMotionListener(mcll);
        ButtonActionListener bal = new ButtonActionListener();
        B01.addActionListener(bal);
        B02.addActionListener(bal);
        B03.addActionListener(bal);
        this.add(B01);
        this.add(B02);
        this.add(B03);
    }

    public void paintComponent(Graphics g) {
        Graphics2D g2 = (Graphics2D) g;
        super.paintComponent(g2);
        rec = new Ellipse2D.Double(140, 140, 200, 200);
        if (num % 4 == 0) {
            g2.setPaint(Color.BLACK);
        }
        if (num % 4 == 1) {
            g2.setPaint(Color.GREEN);
            g2.fill(rec);
        }
        if (num % 4 == 2) {
            g2.setPaint(Color.BLUE);
            g2.fill(rec);
        }
        if (num % 4 == 3) {
            g2.setPaint(Color.RED);
            g2.fill(rec);
        }
        g2.draw(rec);
        g2.setPaint(Color.BLACK);
        g2.draw(rec);
        repaint();
    }

    public Boolean exist(Point2D p) {
        if (rec.contains(p)) {
            return true;
        }
        return false;
    }

    class MouseClickListener implements MouseListener {

        @Override
        public void mouseClicked(MouseEvent e) {
        }

        @Override
        public void mousePressed(MouseEvent e) {
            Point2D p = e.getPoint();
            if (exist(p)) {
                num++;
            }
        }

        @Override
        public void mouseReleased(MouseEvent e) {

        }

        @Override
        public void mouseEntered(MouseEvent e) {

        }

        @Override
        public void mouseExited(MouseEvent e) {

        }

    }

    class MouseCursorListener implements MouseMotionListener {

        @Override
        public void mouseDragged(MouseEvent e) {

        }

        public void mouseMoved(MouseEvent me) {
            Point2D p = me.getPoint();
            if (exist(p)) {
                setCursor(Cursor.getPredefinedCursor(
                        Cursor.CROSSHAIR_CURSOR));
            } else
                setCursor(Cursor.getDefaultCursor());
        }

    }

    class ButtonActionListener implements ActionListener {

        @Override
        public void actionPerformed(ActionEvent e) {
            Object obj = e.getSource();
            if (obj == B01) {
                num = 1;
            } else if (obj == B02) {
                num = 2;
            } else if (obj == B03) {
                num = 3;
            }
        }

    }
}