works

package com.company;

import javax.imageio.ImageIO;
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.awt.image.BufferedImage;
import java.net.URL;

import java.awt.*;
import javax.swing.*;

public class WindowApp extends JFrame {
    private JPanel panel;
    private JLabel label;

    private final static int WINDOW_SIZE_X=500;
    private final static int WINDOW_SIZE_Y=500;

    private int prevMouseCoordX;
    private int prevMouseCoordY;
    private int currMouseCoordX;
    private int currMouseCoordY;

    private Image dbImage=null;

    WindowApp(){
        //label
        label=new JLabel();
        this.add(label,BorderLayout.SOUTH);
        //panel
        panel = new JPanel(){
//            @Override
//            public void paint(Graphics g){
////                dbImage=createImage(WINDOW_SIZE_X,WINDOW_SIZE_Y);
////                Graphics Graphics=dbImage.getGraphics();
////                paintComponent(Graphics);
////                g.drawImage(dbImage,0,0,this);
//            }
            @Override
            public void paintComponent(Graphics g){
                g.drawImage(dbImage,0,0,this);
            }
        };
        panel.setDoubleBuffered(true);
        this.add(panel, BorderLayout.CENTER);
        panel.setFocusable(true);
        //window
        setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        setSize(new Dimension(WINDOW_SIZE_X,WINDOW_SIZE_Y));
        setResizable(false);
        //listeners
        panel.addMouseListener(new MouseAdapter() {
            @Override
            public void mousePressed(MouseEvent e) {
                super.mousePressed(e);
                prevMouseCoordX=e.getX();
                prevMouseCoordY=e.getY();
            }
        });
        panel.addMouseMotionListener(new MouseMotionAdapter() {
            @Override
            public void mouseDragged(MouseEvent e) {
                Graphics graphics=panel.getGraphics();
                if(dbImage==null) {
                    dbImage = createImage(WINDOW_SIZE_X, WINDOW_SIZE_Y);
                }
                Graphics dbGraphics=dbImage.getGraphics();
                graphics.drawLine(prevMouseCoordX,prevMouseCoordY,e.getX(),e.getY());
                dbGraphics.drawLine(prevMouseCoordX,prevMouseCoordY,e.getX(),e.getY());
                prevMouseCoordX=e.getX();
                prevMouseCoordY=e.getY();
            }
        });
    }
}

class PaintPanel extends JPanel{


    protected void paintComponent(Graphics g){
        super.paintComponent(g);
        g.drawLine(100,100,200,200);
    }
}