Arrays and apple not visible in the game window --Snake Game

Question

I was following a java tutorial on yt about a snake game using java swing gui and I managed to make the game window opening with the borders and background I wanted, but the arrays and the apple I created are not visible in the game window.

I tried to change colours to see if they are invisible somehow but they still do not appear. Can you guys suggest anything?? I tried to @Overrite the paintComponent method but still doesn't work. I'M using IntelliJ if that is important.

SnakeGame.java class

public class SnakeGame {
        public static void main(String[] args) {

            new GameFrame();
        }
    }

GameFrame.java class

import javax.swing.*;

    public class GameFrame extends JFrame {

        GameFrame(){
            GamePanel panel = new GamePanel();
            this.add(panel);
            this.setTitle("Snake");
            this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            this.setResizable(false);
            this.pack();
            this.setVisible(true);
            this.setLocationRelativeTo(null);
        }
    }

GamePanel.java class

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

import java.util.Random;
import javax.swing.JPanel;

public class GamePanel extends JPanel implements ActionListener {

    //screen width and height
    static final int SCREEN_WIDTH = 600;
    static final int SCREEN_HEIGHT = 600;
    static final int UNIT_SIZE = 25;

    //set game units in the panel
    static final int GAME_UNITS = (SCREEN_WIDTH*SCREEN_HEIGHT)/UNIT_SIZE;

    //create delay for timer
    static final int DELAY = 75; //the higher the number of the delay, the slower the game will be
                                 //the lower the number of the delay, the faster the game will be

    //create arrays(x,y) to hold the coordinates of the snake
    final int x[] = new int[GAME_UNITS];
    final int y[] = new int[GAME_UNITS];

    //set initial number of body parts for the snake
    int bodyParts = 6;
    int applesEaten;
    int appleX;
    int appleY;
    char direction = 'R'; //snake begin by going to the right
    boolean running = false;
    Timer timer;
    Random random;


    GamePanel(){
        random = new Random();
        this.setPreferredSize(new Dimension(SCREEN_WIDTH, SCREEN_HEIGHT));
        this.setBackground(Color.white);
        this.setFocusable(true);
        this.addKeyListener(new MyKeyAdapter());
        startGame();
    }
//Creating methods for the game
    public void startGame(){
        newApple();
        running = true;
        timer = new Timer(DELAY, this);
        timer.start();
    }

    public void paintComponent(Graphics g){
        super.paintComponent(g);
    }

    public void draw(Graphics g){

        //turn the background into a grid to help visualize
        for(int i=0;i<SCREEN_HEIGHT/UNIT_SIZE;i++){
            g.setColor(Color.GRAY);
            g.drawLine(i*UNIT_SIZE, 0, i*UNIT_SIZE, SCREEN_HEIGHT);
            g.drawLine(0, i*UNIT_SIZE, SCREEN_WIDTH, i*UNIT_SIZE);
        }
        g.setColor(Color.red);
        g.fillOval(appleX, appleY, UNIT_SIZE, UNIT_SIZE);

        for(int i = 0; i<bodyParts;i++){
            if(i == 0){
                g.setColor(Color.green);
                g.fillRect(x[i], y[i], UNIT_SIZE, UNIT_SIZE);
            }
            else{
                g.setColor(new Color(45,180,0));
                g.fillRect(x[i], y[i], UNIT_SIZE, UNIT_SIZE);
            }
        }
    }

    public void newApple(){
        appleX = random.nextInt((int)(SCREEN_WIDTH/UNIT_SIZE))*UNIT_SIZE;
        appleY = random.nextInt((int)(SCREEN_HEIGHT/UNIT_SIZE))*UNIT_SIZE;
    }