Java game

Main.java

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

public class Main extends JPanel implements ActionListener, KeyListener {
    private Timer timer;
    private int playerX, playerY, playerVelocityY;
    private boolean isJumping, isMovingLeft, isMovingRight;
    private static final int PLAYER_WIDTH = 50;
    private static final int PLAYER_HEIGHT = 50;
    private static final int GROUND_Y = 300;
    private static final int GRAVITY = 1;
    private static final int PLAYER_JUMP_VELOCITY = -15;
    private static final int PLAYER_MOVE_SPEED = 5;

    public Main() {
        setPreferredSize(new Dimension(500, 400));
        setBackground(Color.WHITE);
        setFocusable(true);
        addKeyListener(this);

        playerX = 50;
        playerY = GROUND_Y - PLAYER_HEIGHT;
        playerVelocityY = 0;

        timer = new Timer(15, this);
        timer.start();
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        // Draw player
        g.setColor(Color.BLUE);
        g.fillRect(playerX, playerY, PLAYER_WIDTH, PLAYER_HEIGHT);
        // Draw ground
        g.setColor(Color.GREEN);
        g.fillRect(0, GROUND_Y, getWidth(), getHeight() - GROUND_Y);
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        // Update player position
        if (isMovingLeft && playerX > 0) {
            playerX -= PLAYER_MOVE_SPEED;
        }
        if (isMovingRight && playerX < getWidth() - PLAYER_WIDTH) {
            playerX += PLAYER_MOVE_SPEED;
        }
        if (isJumping) {
            playerVelocityY += GRAVITY;
            playerY += playerVelocityY;
            if (playerY >= GROUND_Y - PLAYER_HEIGHT) {
                playerY = GROUND_Y - PLAYER_HEIGHT;
                playerVelocityY = 0;
                isJumping = false;
            }
        }
        repaint();
    }

    @Override
    public void keyPressed(KeyEvent e) {
        int key = e.getKeyCode();
        if (key == KeyEvent.VK_LEFT) {
            isMovingLeft = true;
        }
        if (key == KeyEvent.VK_RIGHT) {
            isMovingRight = true;
        }
        if (key == KeyEvent.VK_SPACE && !isJumping) {
            isJumping = true;
            playerVelocityY = PLAYER_JUMP_VELOCITY;
        }
    }

    @Override
    public void keyReleased(KeyEvent e) {
        int key = e.getKeyCode();
        if (key == KeyEvent.VK_LEFT) {
            isMovingLeft = false;
        }
        if (key == KeyEvent.VK_RIGHT) {
            isMovingRight = false;
        }
    }

    @Override
    public void keyTyped(KeyEvent e) {}

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> {
            JFrame frame = new JFrame("Simple Platformer");
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            frame.getContentPane().add(new Main());
            frame.pack();
            frame.setLocationRelativeTo(null);
            frame.setVisible(true);
        });
    }
}