写一个画板程序 鼠标轨迹导致画板闪烁的问题

[月球上的_人]

如果在已有的画面上再画东西，比如画直线，拖拽的直线轨迹会导致画面闪烁，试了网上说的双缓冲，本来直线的轨迹不会闪，只有画面闪，现在变成直线的轨迹也会闪烁了，请问要怎么解决？

import java.awt.*;
import java.awt.event.*;
import java.util.LinkedList;
import javax.swing.*;

public class DrawFrame extends JFrame {
	private static final long serialVersionUID = 1L;
	JToolBar ToolBar=new JToolBar();
	MyCanvas canvas=new MyCanvas();
	LinkedList<Shape> ShapeList=new LinkedList<>();
	int btNum=0;
	
	public DrawFrame() {
		setTitle("画板");
		setBounds(100, 100, 500, 400);
		setDefaultCloseOperation(EXIT_ON_CLOSE);
		setVisible(true);
		
		add(ToolBar, BorderLayout.NORTH);
		add(canvas, BorderLayout.CENTER);
		MakeButton("直线", 1);
		MakeButton("矩形", 2);
		MakeButton("圆形", 3);
		MakeButton("椭圆", 4);
		MakeButton("擦除", 5);
		MakeButton("清除", 6);
		MakeButton("选颜色", 7);
	}
	
	private void MakeButton(String text, int num) {
		JButton bt=new JButton(text);
		ToolBar.add(bt);
		bt.addActionListener(event->btNum=num);
	}
	
	
	class MyCanvas extends Canvas{	
		private static final long serialVersionUID = 1L;
		private double x1, x2, y1, y2;
		private Color paintColor=Color.BLACK; 
		private Graphics g;
		
		public MyCanvas() {
			setBackground(Color.WHITE);
			addMouseListener(new MouseAdapter() {
				public void mousePressed(MouseEvent e)
				{
					x1=e.getX();
					y1=e.getY();
		
				}
				public void mouseReleased(MouseEvent e) {
					x2=e.getX();
					y2=e.getY();
					g=getGraphics();
					if(btNum==1) {
						Shape list=new Line(x1, y1, x2, y2, paintColor);
						list.draw(g);
						ShapeList.add(list);
					}
							
				}
			});
			addMouseMotionListener(new MouseMotionAdapter() {
				public void mouseDragged(MouseEvent e) {
					x2=e.getX();
					y2=e.getY();
					g=getGraphics();
					if(btNum==0) {
						Shape list=new Line(x1, y1, x2, y2, paintColor);
						list.draw(g);
						ShapeList.add(list);
						x1=x2;  
				        y1=y2;
					}
					else if(btNum==1) {
						Shape list=new Line(x1, y1, x2, y2, paintColor);
						list.draw(g);
					}
					if(btNum!=0) {
						repaint();
					}			
				}
			});
		}
		
		public void paint(Graphics g){ 
			for(Shape sh:ShapeList)
				sh.draw(g);
		}   
		
	}
	
	
	public static void main(String[] args) {
		new DrawFrame();
	}
}