JComponent.setbackground(Color.BLACK) is NOT working

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

public class SpaceInvaders{
     
     //variables
     public static final int WIDTH = 800;
     public static final int HEIGHT = 800;
     GamePanel panel;
     
     public SpaceInvaders()
     {
          //create frame
          JFrame demoFrame = new JFrame("Space Invaders");
          demoFrame.setBounds(0,0,WIDTH,HEIGHT);
          demoFrame.setVisible(true);
          demoFrame.setResizable(false);
          demoFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
          demoFrame.setBackground(Color.BLACK);
          
          //create panel
          GamePanel panel = new GamePanel(this);
          panel.setBackground(Color.BLACK);
          demoFrame.add(panel);
          
          //add buttons
          JButton startButton = new JButton("Start");
          startButton.setEnabled(true);
          JButton stopButton = new JButton("Stop");
          stopButton.setEnabled(true);
.....

import java.awt.*;
import javax.swing.*;
import java.awt.Image;
import java.awt.event.KeyListener;
import java.awt.event.KeyEvent;
import java.awt.image.BufferStrategy;
import javax.swing.*;
import java.awt.event.*;

public class GamePanel extends JComponent implements KeyListener, ActionListener
{
     SpaceInvaders game;
     static Player player1;
     Bullets bullet;
     private BufferStrategy strategy;
     static int alienNum = 36;
     static int alienRow = 6;
     static int alienCol = 6;
     static Aliens[][] aliens;
     static Bullets[] bullets;
     private Timer timer;
     int bulletNum=0;
     
     public GamePanel(SpaceInvaders game)
     {
          this.game = game;
          player1 = new Player(this);
          player1.start();
     
          aliens = new Aliens[alienRow][alienCol];
          for(int i=0; i<alienRow; i++)
               for(int j=0; j<alienCol; j++){
               {
                    aliens[i][j] = new Aliens(this);
                    aliens[i][j].setX(i*50);
                    aliens[i][j].setY(j*50);
               }
          }
          bullets = new Bullets[5];
          
               bullets[0]=new Bullets(this);
          
          
          
          
          timer=new Timer(500, this);
          //timer.setDelay(500);
          timer.start();
          this.setFocusable(true);
        this.addKeyListener(this);
     }
     
     public void paintComponent(Graphics g)
     {
          
          super.paintComponent(g);
          
          Image pic1 = player1.getImage();
...........