Space Invaders

MainFrame.java

import java.awt.Dimension;

import javax.swing.JFrame;

@SuppressWarnings("serial")
public class MainFrame extends JFrame {

  Dimension size = new Dimension(850, 600);
  
  public MainFrame(String string) {
     setTitle(string);
     setSize(size);
     setMinimumSize(size);
     setMaximumSize(size);
     setPreferredSize(size);
     setResizable(false);
     this.pack();
     setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
  }

  public void run() {
    this.setVisible(true);
  }
}

Player.java

public class Player {
    int x = 0;
    int y;
    int height = 20;
    int width = 50;
}

SpaceInvaders.java

public class SpaceInvaders {

  public static void main(String[] args) {
      MainFrame mainFrame = new MainFrame("Space Invaders");
      SpaceInvadersPanel gamePanel = new SpaceInvadersPanel();
      mainFrame.add(gamePanel);
      mainFrame.run();
  }

}

SpaceInvadersPanel.java

import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;

import javax.swing.JPanel;

@SuppressWarnings("serial")
public class SpaceInvadersPanel extends JPanel implements KeyListener {

  boolean needs_repaint;
  boolean on_title_screen, on_game, on_game_over;
  Player player = new Player();
  
  int y = 0;
  
  public void paintComponent(Graphics g) {
    /* Draws the elements on the screen */
    super.paintComponent(g);

    y = this.getParent().getHeight() - player.height;
    
    g.fillRect(player.x, y, player.width, player.height);

    /* Repainting has finished */
    needs_repaint = false;
  }

  public SpaceInvadersPanel() {
    this.setFocusable(true);
    this.addKeyListener(this);
    Dimension size = new Dimension(850, 600);
    this.setSize(size);
    this.setMinimumSize(size);
    this.setMaximumSize(size);
    this.setPreferredSize(size);
  }

  public void keyPressed(KeyEvent e) {
    int key_code = e.getKeyCode();

    if (key_code == KeyEvent.VK_RIGHT) {
      player.x += player.width;
      needs_repaint = true;
    }

    if (key_code == KeyEvent.VK_LEFT) {
      player.x -= player.width;
      needs_repaint = true;
    }
    
    if (needs_repaint)
      this.repaint();
  }

  public void keyReleased(KeyEvent e) {}
  public void keyTyped(KeyEvent e) {}
}