Efficiency of method call in for loop condition

Question

I am writing a game engine, in which a set of objects held in a ArrayList are iterated over using a for loop. Obviously, efficiency is rather important, and so I was wondering about the efficiency of the loop.

for (String extension : assetLoader.getSupportedExtensions()) {
    // do stuff with the extension here
}

Where getSupportedExtension() returns an ArrayList of Strings. What I'm wondering is if the method is called every time the loop iterates over a new extension. If so, would it be more efficient to do something like:

ArrayList<String> supportedExtensions = ((IAssetLoader<?>) loader).getSupportedExtensions();

for (String extension : supportedExtensions) {
    // stuff
}

? Thanks in advance.

Answer 1

By specification, the idiom

for (String extension : assetLoader.getSupportedExtensions()) {
  ...
}

expands into

for (Iterator<String> it = assetLoader.getSupportedExtensions().iterator(); it.hasNext();)
{
    String extension = it.next();
    ...
}

Therefore the call you ask about occurs only once, at loop init time. It is the iterator object whose methods are being called repeatedly.

However, if you are honestly interested about the performance of your application, then you should make sure you're focusing on the big wins and not small potatoes like this. It is almost impossible to make a getter call stand out as a bottleneck in any piece of code. This goes double for applications running on HotSpot, which will inline that getter call and turn it into a direct field access.