Why do the dimensions of my JPanel get messed up when I open my game from a PC with different screen resolution?

Question

My laptop's resolution is 1920x1080. I recently opened my game from another laptop with 1360x768. The JPanel changed size and half of the game was outside the screen. I understand that my game dimensions x 4 are bigger than the second laptop's resolution. But, is there a way to make it adjust to every screen? Any ideas?

package Main;

import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import java.awt.image.BufferedImage;

import javax.swing.JPanel;

import GameState.GameStateManager;
import Handlers.Keys;


@SuppressWarnings("serial")
public class GamePanel extends JPanel implements Runnable, KeyListener {

    // dimensions
    public static final int WIDTH = 320;
    public static final int HEIGHT = 240;
    public static final int SCALE = 4;

    // game thread
    private Thread thread;
    private boolean running;
    private int FPS = 60;
    private long targetTime = 1000 / FPS;

    // image
    private BufferedImage image;
    private Graphics2D g;

    // game state manager
    private GameStateManager gsm;

    // other
    private boolean recording = false;
    private int recordingCount = 0;
    private boolean screenshot;

    public GamePanel() {
        super();
        setPreferredSize(new Dimension(WIDTH * SCALE, HEIGHT * SCALE));
        setFocusable(true);
        requestFocus();
    }

    public void addNotify() {
        super.addNotify();
        if (thread == null) {
            thread = new Thread(this);
            addKeyListener(this);
            thread.start();
        }
    }

    private void init() {
        image = new BufferedImage(WIDTH, HEIGHT, BufferedImage.TYPE_INT_RGB);
        g = (Graphics2D) image.getGraphics();

        running = true;

        gsm = new GameStateManager();
    }

    public void run() {
        init();

        long start;
        long elapsed;
        long wait;

        // game loop
        while (running) {
            start = System.nanoTime();

            update();
            draw();
            drawToScreen();

            elapsed = System.nanoTime() - start;

            wait = targetTime - elapsed / 1000000;
            if (wait < 0) wait = 5;

            try {
                Thread.sleep(wait);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }

    private void update() {
        gsm.update();
        Keys.update();
    }

    private void draw() {
        gsm.draw(g);
    }

    private void drawToScreen() {
        Graphics g2 = getGraphics();
        g2.drawImage(image, 0, 0, WIDTH * SCALE, HEIGHT * SCALE, null);
        g2.dispose();
        if (screenshot) {
            screenshot = false;
            try {
                java.io.File out = new java.io.File("screenshot " + System.nanoTime() + ".gif");
                javax.imageio.ImageIO.write(image, "gif", out);
            } catch (Exception e) {
            }
        }
        if (!recording)
            return;

        try {
            java.io.File out = new java.io.File("C:\\out\\frame" + recordingCount + ".gif");
            javax.imageio.ImageIO.write(image, "gif", out);
            recordingCount++;
        } catch (Exception e) { }
    }

    public void keyTyped(KeyEvent key) { }

    public void keyPressed(KeyEvent key) {
        if (key.isControlDown()) {
            if (key.getKeyCode() == KeyEvent.VK_R) {
                recording = !recording;
                return;
            }
            if (key.getKeyCode() == KeyEvent.VK_S) {
                screenshot = true;
                return;
            }
        }
        Keys.keySet(key.getKeyCode(), true);
    }

    public void keyReleased(KeyEvent key) {
        Keys.keySet(key.getKeyCode(), false);
    }
}

Answer 1

You should be basing the size of game on the resolution of the screen.

You can easily get the resolution by using:

System.out.println(Toolkit.getDefaultToolkit().getScreenSize());

The next issue is are you using a full screen for your game or is you game displayed in a frame. If you are using a frame with decorations, then you also need to account for the size of the title bar and borders around the frame. This information can only be accessed after the frame has been packed.

frame.pack();
System.out.println("Frame Insets : " + frame.getInsets() );

So now the size of your game board would be the size of the screen less the insets of the frame decorations.