Trouble passing the graphics class

Question

I have a very simple Game Engine that I am trying to set up and I am clueless as to why it is throwing an exception but I'm not very experienced so I understand I'm probably doing something wrong.

I want to draw directly to a JFrame with a buffer strategy using the Graphics class. Everything works until I try to pass the Graphics g in to another class and then I get a thread error. Here is what I have:

import java.awt.Dimension;
import java.awt.Toolkit;
import javax.swing.JFrame;

@SuppressWarnings("serial")
public class GameFrame extends JFrame implements Runnable {

    public static final int WIDTH = 1280;
    public static final int HEIGHT = WIDTH * 9 / 16;
    private final int FPS = 60;

    private Thread thread;
    private long lastTime;

    private Game game;

    public GameFrame() {
        game = new Game(this);
        setLocation((int) ((Toolkit.getDefaultToolkit().getScreenSize().getWidth() - WIDTH) / 2), (int) ((Toolkit.getDefaultToolkit().getScreenSize().getHeight() - HEIGHT) / 2));
        setSize(new Dimension(WIDTH, HEIGHT));
        setResizable(false);
        thread = new Thread(this);
        setVisible(true);
        createBufferStrategy(3);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        thread.start();
    }

    public void run() {
        lastTime = System.currentTimeMillis();

        while (true) {
            long time = System.currentTimeMillis();
            if ((time - lastTime) * FPS > 1000) {
                lastTime = time;
                tick();
                draw();
                getBufferStrategy().show();
            }
        }
    }

    private void tick() {
        game.tick();
    }

    private void draw() {
        game.draw();
    }

    public static void main(String[] args) {
        new GameFrame();
    }

}

and

import java.awt.Graphics2D;

public class Game {

    private GameFrame frame;
    private Map map;

    public Game(GameFrame frame) {
        this.frame = frame;
    }

    public void tick() {

    }

    public void draw() {
        Graphics2D g = (Graphics2D) frame.getBufferStrategy().getDrawGraphics();
        g.fillRect(0, 0, 50, 50);
        map.draw(g);
        g.dispose();
    }

}

The rectangle from the draw method inside of Game will work but if I pass g to my Map class (which will be drawing the same thing) it causes a thread NullPointerException.

Thanks so much in advance for any help with this or any additional comments you can provide!

1) Always copy/paste error and exception output! 2) See [What is a stack trace, and how can I use it to debug my application errors?](http://stackoverflow.com/q/3988788/418556) & [What is a Null Pointer Exception, and how do I fix it?](http://stackoverflow.com/q/218384/418556) — Andrew Thompson, Dec 27 '14 at 04:24

Madhawa Priyashantha · Answer 1 · 2014-12-27T04:25:10.420

1

because you haven't initialize map object so it's null[default value]

to initialize :you need to create a instance of map class and assign it to variable map.

you can initialize it inside constructor like follows.

public Game(GameFrame frame) {
    this.frame = frame;
    map=new Map();//initialize map nullpointer will go away
}

edited Dec 27 '14 at 04:25

answered Dec 27 '14 at 04:19

Madhawa Priyashantha

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Trouble passing the graphics class

1 Answers1