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

public class Main{
    public static void main(String[] args){
        Win win=new Win();
        win.Init();
    }
}
class Win extends JFrame implements MouseListener{
    JFrame jFrame=new JFrame("绘图");
 public void Init(){
     jFrame.addMouseListener(this);
     jFrame.setSize(500,500);
     jFrame.setLocation(500,500);
     jFrame.setVisible(true);
 }

    @Override
    public void mouseClicked(MouseEvent e) {
        Graphics graphics=jFrame.getGraphics();
        graphics.drawRect(e.getX(),e.getY(),50,50);
    }

    @Override
    public void mousePressed(MouseEvent e) {

    }

    @Override
    public void mouseReleased(MouseEvent e) {

    }

    @Override
    public void mouseEntered(MouseEvent e) {

    }

    @Override
    public void mouseExited(MouseEvent e) {

    }
}

之前一直以为必须重写Jpanel面板里面的paint方法才能够画，但后来发现好像没那个必要