import javax.swing.*;
import java.awt.*;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;

public class DrawTest extends JFrame {
    MyPanel mp = null;

    public static void main(String[] args) {
        new DrawTest();
    }

    public DrawTest() {
        mp = new MyPanel();
        this.addMouseListener(mp);
        this.add(mp);
        this.setSize(600, 400);
        this.setLocation(400, 300);
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setVisible(true);
    }
}

class MyPanel extends JPanel implements MouseListener {
    int x = 0, y = 0;
    Image img = null;

    public void paint(Graphics g) {
        super.paint(g);
        g.fillOval(x, y, 10, 10);
    }

    public void mouseClicked(MouseEvent e) {
        x = e.getX();
        y = e.getY();
        this.repaint();
    }

    public void mousePressed(MouseEvent e) {
    }

    public void mouseReleased(MouseEvent e) {
    }

    public void mouseEntered(MouseEvent e) {
    }

    public void mouseExited(MouseEvent e) {
    }

}

import javax.swing.*;
import java.awt.*;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;

public class DrawTest extends JFrame {
    MyPanel mp = null;

    public static void main(String[] args) {
        new DrawTest();
    }

    public DrawTest() {
        mp = new MyPanel();

        // 鼠标事件是加在frame上，而不是在mp上的，
        // 所以在mp中取得坐标时，坐标不是相对于mp的，而是相对于frame的
        // 而frame还有一个标题栏，这个是有高度的
        // 最终使用屏幕坐标转换成组件坐标可以解决这一问题
        this.addMouseListener(mp);

        this.add(mp);
        this.setSize(600, 400);
        this.setLocationRelativeTo(null);
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setVisible(true);
    }
}

class MyPanel extends JPanel implements MouseListener {
    int x = 0, y = 0;
    Image img = null;

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.fillOval(x, y, 10, 10);
    }

    public void mouseClicked(MouseEvent e) {
        Point p = e.getLocationOnScreen();
        SwingUtilities.convertPointFromScreen(p, this); // 关键在于把屏幕坐标映射成组件坐标
        x = p.x;
        y = p.y;
        this.repaint();
    }

    public void mousePressed(MouseEvent e) {
    }

    public void mouseReleased(MouseEvent e) {
    }

    public void mouseEntered(MouseEvent e) {
    }

    public void mouseExited(MouseEvent e) {
    }

}