Question

I have an ArrayList which holds Planes (enemies) on my android game. These planes move from one side of the screen to the other and the user has to dodge them. When a plane’s x value becomes less than -50 it is removed from the ArrayList. When this happens, it causes all the planes on the screen currently to ‘jump’ slightly. They disappear for a few milliseconds and then are re-drawn but 2px behind where they are supposed to be.

Here is the paint method, where planes is the ArrayList

public void onDraw(){
        bg1.onDraw(c);
    bg2.onDraw(c);
    chopper.onDraw(c);

    score.onDraw(c);

    // PAINTS THE PLANE OR DELETES IF OFF SCREEN
    for (int i = 0; i < planes.size(); i++) {
        Plane p = planes.get(i);
                    if(p.getX()<-50){
                            planes.remove(p);
                    }else{
                            p.onDraw(c);

                            if (p.getX() < 170) {
                                    detectPlaneCollision(p, c);
                            }
                    }

           }
}

Is there a way of fixing this? Should I use a different data structure?

Thanks

Tom

Answer 1

I think your problem is nothing to do with your choice of data structure, but instead because you are modifying your List while you’re looping over it.

Imagine you have three planes in your list – [P1,P2,P3].

• On the first iteration of your loop i is 0, you process P1
• P1 has getX() < -50 so you remove it, making the list now [P2,P3]
• On the next iteration of the loop i is now 1 so you process P3.
• This means P2 never gets processed and so will never be drawn, making it disappear briefly.

Try using an Iterator which will allow you to safely remove items while looping over the List.

Iterator<Plane> i = planes.iterator();

while (i.hasNext()) {
  Plane p = i.next();

  if (p.getX() < -50) {
    i.remove();
  } else {
    p.onDraw(c);

    if (p.getX() < 170) {
      detectPlaneCollision(p, c);
    }
  }
}

Although, since you don’t care about the order in which you process your Plane obejcts you could consider storing them in a Set rather than a List. A Set doesn’t have to worry about maintaining an order of items as you add and remove them.