Question

I’m using AndEngine to make a game for the Android platform. I need to detect if a circular body contacts a circular sprite. To do this, I’m calculating the centers and radii of both and using pythag to determine if they are indeed touching. However, I keep getting a nullpointerexception when I try to get the x coordinate of my body. spriteNum increases every time the screen it touched and a sprite is created. Any ideas why? My code:

public class grow implements Runnable{
    Sprite sp;
    Body body[] = new Body[100];
    public grow(Sprite sprite){
        sp = sprite;
    }
    @Override
    public void run() {
        float radf, rads; //fill radius/stationary radius
        float[] fill; //fillx/stationaryx
        float fx=0, fy=0, sx, sy;

        while(down){
            //Log.e("Tag","Running thread. Down = "+Boolean.toString(down));
             yourSprite[spriteNum].setScale(scale += 0.1);
             fill = yourSprite[spriteNum].getSceneCenterCoordinates();
             fill[0]=fx;
             fill[1]=fy;
             radf=yourSprite[spriteNum].getHeightScaled()/2;    

             Log.e("Fill X before for",Float.toString(fx));

            if(spriteNum>0)
                for(int x=0;x<spriteNum;x++){               
                    rads=yourSprite[x].getHeightScaled()/2;
                    sx = body[x].getWorldCenter().x; //Null pointer exception
                    sy = body[x].getWorldCenter().y;

                    if(Math.sqrt(Math.pow((fx-sx),2)+Math.pow((fy-sy),2))<(radf+rads))
                            down = false;
                    Log.e("Fill x",Float.toString(fx));
                    Log.e("Stat x",Float.toString(sy));
                }
             try {
                Thread.sleep(75);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }

        }
        body[spriteNum] = PhysicsFactory.createCircleBody(mPhysicsWorld, yourSprite[spriteNum], BodyType.DynamicBody, FIXTURE_DEF);
        mPhysicsWorld.registerPhysicsConnector(new PhysicsConnector(yourSprite[spriteNum], body[spriteNum], true, true));
        if(body[0]!=null)
            Log.e("Body created","not null"); //This says it's not null, but when I did the same thing just inside of my for() loop and it did say it was null
    }

Answer 1

I guess it’s because you have initialized body array Body body[] = new Body[100] but not individual Body elements in it. So when you access body[x] it returns null. You should also initialize body elements in array. Something like:

for(int i=0; i<body.length; i++) {
   // probably some cod here..
   body[i] = new Body(..);
   // probably some cod here..
}

---

Another cause that came into my mind for your problem is, you said:

spriteNum increases every time the screen it touched and a sprite is
created.

So it can be possible that you have initialized some body elements on startup and then when sprite count increases you do not create new elements in body array, so when you try body[0] it is not null but for new sprites for which body elements may not have been initialized you get null and thus NPE.