Playing with hashmap in drawing

Question

I'm developing a game in Java for Android, using LibGDX. In my render(), between the batch.begin() and batch.end() I'm running through all the game objects and draw them.

Every game object is stored in a list, which that list is stored inside a hash map. I iterate through this hash map, getting every list and going through that list, like this:

    batch.begin();
    Iterator it = objectsMap.entrySet().iterator();
    while (it.hasNext()) {
        Map.Entry pair = (Map.Entry) it.next();
        Array list = (Array) pair.getValue();
        for (int i = 0; i < list.size; i++) {
            GameObject object = (GameObject) list.get(i);
            drawObject(object);
        }
    }
    batch.end();

My drawObject() code:

private void drawObject(GameObject object, float x, float y) {
    if (object == null) {
        return;
    }
    if (object.getVisibility()) {
        TextureAtlas.AtlasRegion currentFrame = object.getCurrentFrame();
        if (currentFrame != null) {
            float objectAlpha = object.getAlpha();
            Color color = batch.getColor();
            if (objectAlpha != 1) {
                color.a = object.getAlpha();
                batch.setColor(color);
            }
            batch.draw(currentFrame, x - object.getOriginX(), y - object.getOriginY(), object.getOriginX(), object.getOriginY(), currentFrame.getRegionWidth(), currentFrame.getRegionHeight(), object.scaleX, object.scaleY, object.getSpriteDirection());
            color.a = 1;
            batch.setColor(color);
        }
    }
}

A method called setRegion() sets the current animation frames of the object:

protected void setRegion(String name, float frameDuration) {
    currentFrame = atlas.findRegion(name, 0);
    if (currentFrame == null) {
        currentFrame = atlas.findRegion(name, -1);
    }
    width = currentFrame.getRegionWidth();
    height = currentFrame.getRegionHeight();
    animationStateTime = 0;
    animation = new Animation(frameDuration, atlas.findRegions(name),      Animation.PlayMode.LOOP);
}

And in the update() of the objects this is called:

            currentFrame = (TextureAtlas.AtlasRegion) animation.getKeyFrame(animationStateTime, isLooping);

The getCurrentFrame() method only returns currentFrame variable. The alpha is a field with a value between 0 and 1. (getAlpha() returns only the field)

My question is that considered a "heavy" activity? Can this be a significant impact on the FPS values? Because I'm running my game on old devices (like Galaxy S2) and the FPS can get pretty low values. I'm trying to find the cause to it.
I used to have a reference to every list but with the time I realized I'm going to have too much lists, so I decided to do it like this. Maybe you know a better way of holding references to all game objects?

Thanks!

Answer 1

Basically, what a hashmap does is store the objects in a random location in your memory. Complexity is O(1) which means that it will always take the same amount of time to retrive an object from the hashmap. The amount of time depends on various factors. This might be quite fast for few objects, but as soon as you start drawing scene with hundreds of objects, I would consider a hashmap as very poorly performing (poor FPS as a result).
The reason for that is simple. Your memory has to go to get the object from a different location for every object. A more performant approach would be to store all objects right next to each other in memory (which in turn would be a very bad hashmap), so that you could go on where you left off in memory. Depending on the device and your application, this might as well be a major performance gain.
Also take a look at the Entity-Component-System, this article about data locality (which is the specific answer for this very question) and also all other articles on gameprogrammingpatterns.com. You might be able to improve other parts of your code as well :)