畫了好久，草要么就是畫了，沒插掉原先線條，要么就是畫第二個的時候第一個也被擦掉，但其實只要調用repaint方法就好了…

import javax.swing.*;import java.awt.*;import java.awt.event.MouseEvent;import java.awt.event.MouseListener; class Rect extends JFrame{  public static void main(String[] args) {    Rect r=new Rect();  }  Draw draw=new Draw();  public Rect()  {    this.add(draw);    this.addMouseListener(draw);    this.setSize(500,500);    this.setLocation(400,400);    this.setVisible(true);    this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);  }}class Draw extends JPanel implements MouseListener {  int x1 = 0;  int y1 = 0;  int x2 = 0;  int y2 = 0;  public void paint(Graphics g) {//    super.paint(g);    g.setColor(Color.BLUE);    g.drawRect(x1, y1, x2 - x1, y2 - y1);  }  @Override  public void mouseClicked(MouseEvent e) {  }  @Override  public void mousePressed(MouseEvent e) {    x1 = e.getX();    y1 = e.getY();  }  @Override  public void mouseReleased(MouseEvent e) {    x2 = e.getX();    y2 = e.getY();    this.repaint();  }  @Override  public void mouseEntered(MouseEvent e) {  }  @Override  public void mouseExited(MouseEvent e) {  }}

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