NullPointerException with Graphics2D Translate

Question

I'm not having any problems fixing this. I'm creating a game and I constructed a camera to follow a player when they are moving but when I use the method translate from the Graphics2D class I get a null pointer.

This problem doesn't happen all the time, sometimes when I run the game the error occurs?

All objects are created for Gr

Exception in thread "Thread-3" java.lang.NullPointerException
at game.Game.render(Game.java:172)
at game.Game.run(Game.java:126)
at java.lang.Thread.run(Thread.java:745)

It's pointing at this line here.

    g2d.translate(cam.getX(), cam.getY());

In my render method.

    private void render() {
    BufferStrategy bs = this.getBufferStrategy();
    if (bs == null) {
        this.createBufferStrategy(3);
        return;
    }

    Graphics g = bs.getDrawGraphics();
    Graphics2D g2d = (Graphics2D) g; 

    g.setColor(Color.BLACK);
    g.fillRect(0, 0, WIDTH, HEIGHT);

    g2d.translate(cam.getX(), cam.getY());      

    handler.render(g);

    g2d.translate(-cam.getX(), -cam.getY());

    if (gameState == STATE.GAME) {
        hud.render(g);
    } else if (gameState == STATE.MENU || gameState == STATE.HELP ||    gameState == STATE.END) {
        menu.render(g);
    }
    g.dispose();
    bs.show();
}

Any ideas?

cam is coming from this code:

public Game() {

    tex = new Texture();

    handler = new Handler();
    hud = new HUD();
    menu = new Menu(this, handler, hud);
    this.addKeyListener(new KeyInput(handler));
    this.addMouseListener(menu);

    new Window(WIDTH, HEIGHT, "Let's Build A Game", this);

    spawner = new Spawn(handler, hud);

//      loader = new BufferedImageLoader();
//      background = loader.loadImage("/level.png");
//      
//      loadImageLevel(background);

    cam = new Camera(0, 0);

    if (gameState == STATE.MENU) {
        handler.createMenuParticle();
    } else {
        handler.clearEnemies();
    }
}

This is the camera class I use.

public class Camera {

private float x, y;

public Camera(float x, float y) {
    this.x = x;
    this.y = y;
}

public void tick(GameObject player) {
    x = -player.getX() + (Game.WIDTH / 2);
    y = -player.getY() + (Game.HEIGHT / 2);
}

public float getX() {
    return x;
}

public void setX(float x) {
    this.x = x;
}

public float getY() {
    return y;
}

public void setY(float y) {
    this.y = y;
}
}

`cam` must be `null` at that point. Where does it come from? — mastov, Jul 08 '15 at 08:26
I know what a Null Pointer is I just don't know why cam would be null when I set the coordinates to 0, 0. — , Jul 08 '15 at 08:33
@Oli: Accordings to the stacktrace it belongs to `Game`. The constructor, too. — mastov, Jul 08 '15 at 08:36
Your Window constructor is accepting a Game object as parameter. Is it possible that it might be invoking render() before the object is fully constructed? — Dakshinamurthy Karra, Jul 08 '15 at 08:36
@7Winds Are you manually calling `repaint` anywhere manually? — Oli, Jul 08 '15 at 08:37
@Oli I'm not using repaint anywhere manually. Yes my window does take Game as a parameter. I just removed the cam = new Cam(0,0) from the game constructor and placed it outside of the constructor in the fields and I haven't received the error yet. I think that's why, as KDM stated. — , Jul 08 '15 at 08:41

mastov · Accepted Answer · 2015-07-08T08:38:12.177

0

Since the stacktrace doesn't go any deeper, it can only be g2d and cam that may be the cause of the exception. Since you used g2d before (as g) and it worked, the problem must be cam, which is most likely null at that point.

EDIT: From your edit I can see that cam is initialized properly in the class' constructor you show us. So one of the following is likely happening:

There is another constructor that doesn't initialize cam.
There is some other code that overwrites cam.
Your render method is called before cam is set in the constructor. Try to make the cam initialization the first line of the constructor to check for this. It also might be a good idea to not launch GUI stuff in your Game constructor at all.

You should look for code that does either of those three things. Set breakpoints at the relevant lines to check, if and when they are executed.

edited Jul 08 '15 at 08:38

answered Jul 08 '15 at 08:28

mastov

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Or the render method is somehow called before the constructor completes... – Oli Jul 08 '15 at 08:36
Thanks yeah I placed cam higher up in the constructor and I haven't received any errors. I also placed the cam outside the constructor in a field and that works too. What's the difference if I place cam in the game constructor or in a field? – Jul 08 '15 at 08:44
1

@7Winds: Placing it in the member declaration will call its initialization code before the custom constructor code is called. It's almost the same as placing it in the first line of all constructors. – mastov Jul 08 '15 at 08:45
Instance fields that are initialized outside a constructor are initialized immediately after the implicit or explicit call to `super`. – Oli Jul 08 '15 at 08:46
@mastov **The first line after `super`. – Oli Jul 08 '15 at 08:46
@Oli: True, of course. – mastov Jul 08 '15 at 08:48
Thanks guys! It worked. :) – Jul 08 '15 at 08:48

NullPointerException with Graphics2D Translate

1 Answers1