Game Loop makes GUI "blinking"

Question

I have a Game Loop:

public void startGameThread() {
    gameLoop();
    gameThread = new Thread(this);
    gameThread.start();
}

public void gameLoop() {
    frame++;
    if (System.currentTimeMillis() - lastCheck >= 1000) {
        lastCheck = System.currentTimeMillis();
        System.out.println("FPS " + frame);
        frame = 0;
    }
}

@Override
public void run() {
    double timePerFrame = 1000000000.0/FPS;
    long lastFrame = System.nanoTime();
    long now = System.nanoTime();
    while (true) {
        now = System.nanoTime();
        if (System.nanoTime() - lastFrame >= timePerFrame) {
            repaint();
            update();
            gameLoop();
            lastFrame = now;
        }
    }
}

and Main Class with GUI:

package MainPackage;

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;

public class MainClass implements ActionListener {
    public static boolean Clicked = false;

    public static void main(String[] args) {
        PanelClass pClass = new PanelClass();
        JButton start = new JButton("START");
        start.setSize(120, 50);
        start.setFocusable(false);
        start.setLocation(630, 200);
        start.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                if(e.getSource() == start) {
                    pClass.startGameThread();
                    Clicked = true;
                    start.setVisible(false);
                }
            }
        });
        start.setVisible(true);
        JFrame frame = new JFrame("Tanks");
        frame.setSize(1600, 913);
        frame.setLayout(null);
        frame.pack();
        frame.setExtendedState(JFrame.MAXIMIZED_BOTH);
        frame.add(start);
        frame.add(pClass);
        frame.setVisible(true);
    }

    @Override
    public void actionPerformed(ActionEvent e) {

    }
}

After launching my application, GUI is "blinking". I think that's because of the Game loop. But how to fix it? Any help is really appreciated!

I was expecting to get the usual behavior of the GUI. Ask me to add more details; code, if needed.

Typing this, because stackoverflow need more "details". That's probably stupid, but I don't know what details to add.

Answer 1

There is nothing wrong with your game loop. The way you are creating the JFrame is wrong.

Mistake 1: Do not use JFrame.setSize() use JFrame.setPreferredSize() instead.

Mistake 2: Always call JFrame.pack() after adding all the components and before making the frame visible.

Mistake 3: Do not use null layout for JFrame.

After fixing these mistakes your game was running without any problems.

PanelClass pClass = new PanelClass();
JButton start = new JButton("START");
start.setSize(120, 50);
start.setFocusable(false);
start.setLocation(630, 200);
start.addActionListener(new ActionListener() {
    public void actionPerformed(ActionEvent e) {
        if(e.getSource() == start) {
            pClass.startGameThread();
            Clicked = true;
            start.setVisible(false);
        }
    }
});
start.setVisible(true);
JFrame frame = new JFrame("Tanks");
frame.setPreferredSize(new Dimension(1200, 913));
frame.setExtendedState(JFrame.MAXIMIZED_BOTH);
frame.add(start);
frame.add(pClass);
frame.setVisible(true);
frame.pack();

Complete code:

import java.awt.*;
import java.awt.event.*;

import javax.swing.*;

public class Game implements ActionListener {
    public static boolean Clicked = false;

    public static void main(String[] args) {
        PanelClass pClass = new PanelClass();
        JButton start = new JButton("START");
        start.setSize(120, 50);
        start.setFocusable(false);
        start.setLocation(630, 200);
        start.addActionListener(e -> {
            if(e.getSource() == start) {
                pClass.startGameThread();
                Clicked = true;
                start.setVisible(false);
            }
        });
        start.setVisible(true);
        JFrame frame = new JFrame("Tanks");
        frame.setPreferredSize(new Dimension(1200, 913));
        frame.setMinimumSize(new Dimension(300, 200));
        frame.setExtendedState(JFrame.MAXIMIZED_BOTH);
        frame.add(start);
        frame.add(pClass);
        frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        frame.setVisible(true);
        frame.pack();
    }

    @Override
    public void actionPerformed(ActionEvent e) {

    }
    static class PanelClass extends JPanel implements Runnable{
        int posX = 0;
        Thread gameThread;
        int frame;
        long lastCheck;
        int FPS = 60;
        public void startGameThread() {
            gameLoop();
            gameThread = new Thread(this);
            gameThread.start();
        }

        public void gameLoop() {
            frame++;
            if (System.currentTimeMillis() - lastCheck >= 1000) {
                lastCheck = System.currentTimeMillis();
                System.out.println("FPS " + frame);
                frame = 0;
            }
        }

        @Override
        public void run() {
            double timePerFrame = 1000000000.0/FPS;
            long lastFrame = System.nanoTime();
            long now = System.nanoTime();
            while (true) {
                now = System.nanoTime();
                if (System.nanoTime() - lastFrame >= timePerFrame) {
                    repaint();
                    gameLoop();
                    lastFrame = now;
                }
            }
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            posX++;
            g.fillRect(posX,getHeight()/2,10,10);
            Toolkit.getDefaultToolkit().sync();
        }
    }

}

The game loop works and there is no blinking/flickering on ubuntu or windows 7 compiled with JDK 17.

Bonus tip: If you are overriding the paint method of your PanelClass then use Toolkit.getDefaultToolkit().sync() in the end. This method ensures that the display is up-to-date.