Trying to reduce memory allocations in java game - for iterator

Question

Occasionally my game has hicups/dropped frames because of garbage collection. So I am trying to reduce this by removing unnessasary allocations.

Using the allocation tracker in Eclipse I see that my code below:

for (IGameObject obj : mObjects) {
  //stuff
}

Is allocating an iterator:

java.util.ArrayList$ArrayListIterator   24 bytes    java.util.ArrayList iterator

I can easily change all places where this is happening as mObjects is an ArrayList.

Just out of interest though, will proguard or jit somehow optimise this to a basic for loop? As it seems a shame I have to make my code more verbose to avoid this allocation.

Is it 24 bytes worth an optimization? even if you allocate it for 1000 times its only 23.5KB. I thinkyou should look at your graphics or other heavy objects! — ALiGOTec
– ALiGOTec, Commented Sep 30, 2012 at 9:58
@Tecigo As they say, it's not the size that counts. Garbage collection during the game causes hicups/dropped frames. Best solution is to remove unnessasary allocations, this is just one of many allocations I'm trying to remove. — weston
– weston, Commented Sep 30, 2012 at 12:39

Matt · Accepted Answer · 2012-09-30 17:19:49Z

1

Well, as people have said, don't optimize unless it's really an issue, but there's always:

int sz = mObjects.size();
IGameObject gameObject = null;
for (int i=0;i<sz;i++) {
    gameObject = mObjects.get(i);
}

In this case you're just looping through by index. For ArrayList this is pretty efficient (as efficient as an iterator probably from a speed standpoint). However, if you drop in a LinkedList at some point instead, it goes to hell for large lists, as .get(index) is terribly inneficient for LinkedList.

I would not worry about the extra 24 bytes unless you really have memory issues.

answered Sep 30, 2012 at 17:19

Matt

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ALiGOTec · Accepted Answer · 2012-10-01 06:14:05Z

The best way to control the memory consumption is to do the allocations yourself and in basic structures.

For example you could transform your ArrayList mObjects in a simple array IGameObject[] mObjects = new IGameObject[MAX_GAME_OBJECTS]; (you should know/compute what is the maximum number of objects your game can handle). In addition you should re-use the objects. You could keep an array boolean[] isObjAlive = new boolean[MAX_GAME_OBJECTS]; (same dimension as mObjects) and it will help in the first place to know which objects you no longer display and in the second, if the object is reusable:

 for(i=0;i<MAX_GAME_OBJECTS;i++){
    if(isObjAlive[i]){
        // draw the object or anything else...
    }
 }

re-using:

 for(i=0;i<MAX_GAME_OBJECTS;i++){
    if( ! isObjAlive[i]){
        // use the setters to customize your object for the new role...
    }
 }

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