Array index out of bounds

Use Iterator which supports remove operation on it's collection. To give you an idea (all irrelevant code from OP removed):

 List<Bullet> bullets = new ArrayList<Bullet>(); List<Alien> aliens = new ArrayList<Alien>(); Iterator<Alien> alienIterator = aliens.iterator(); Iterator<Bullet> bulletIterator = bullets.iterator(); while (bulletIterator.hasNext()) { //for every player bullet while (alienIterator.hasNext()) { //for every player bullet every alien Bullet bullet = bulletIterator.next(); Alien alien = alienIterator.next(); if (bullet.getBounds().intersects(alien.getBounds())) { //player bullet vs alien collison bulletIterator.remove(); alienIterator.remove(); System.out.println("player bullet removed"); } } } 

Note that it's still susceptible to IllegalStateException if you modify aliens/bullets somewhere else concurrently.

The problem is that you are removing items from an array you are traversing. According to the Javadoc:

int size() Returns the number of elements in this collection. 

This is yielding a number which, when you remove items from the collection, will be larger than the actual collection size, hence resulting into an out of bounds.

You need to use an iterator to traverse the collections and then use the iterator.remove() to remove items from a collection while you are traversing it.

You should create iterators to iterate over the players and aliens. Then when one should be removed do that on the iterator instead of the original list. That makes the looping works.

As @Toomai suggest, instead of removing, flag them as dead, possibly delete them later, is also a nice solution.

Let's say that player 0 is killed. When you remove his bullet, all the other bullets move down one in index. So when the index increments to 1, you will be looking at what was originally bullet 2, not bullet 1.

When removing from a List by index, always go through the loops in reverse order, so that you do not affect the order of elements still to be looked at.

However, the problem you see (the kapow) happens if the last bullet kills anything. While you are still in the for (j) loop, you remove the last bullet. The next call to getBullet(i) will fail because there is no longer a bullet i, there are now only i-1. When a bullet kills something, you should break out of the inner loop. (Assuming a bullet can;t kill 2 aliens at once)

That said, you would be much better served by

1. Using the new style for looping, for (Bullet bullet : player.getBullets() {} It will really really simplify your code!!!

2. Consider not deleting on the fly. Collect everything to be deleted into two temporary lists, deadAliens and usedBullets. At the end of your loop call

   aliens.getAliens().removeAll(deadAliens ); player.getBullets().removeAll(deadBullets);

In your code, you are removing items from what i guess is a list

 aliens.getAliens().remove(j); // alien dies player.getBullets().remove(i) 

What might happen is that you are still looping in the inner loop while the condition of the outer loop is not valid.

Looks to me that player.getBullets().remove(i); removes something but you continue to loop. You have to break out of the inner for loop after System.out.println...