java绘图的一个小疑点

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) {    }}