java.util.Collection with the lowest overhead?

Question

I'm calling a method in another API that accepts a java.util.Collection of objects. I've looked at the method and it immediately copies everything in the collection into a new ArrayList before performing its task.

This got me wondering: What is the absolute lowest overhead Java Collection that I can use to quickly assemble parameters for this method?

Answer 1

That depends on how it copies the elements, but if it creates the ArrayList-copy like this

new ArrayList<Something>(inputCollection);

or if it does

someCopy.addAll(inputCollection);

then the it will go through the inputCollection.toArray() which is probably best implemented by ArrayList.

Answer 2

It depends on your source data.

If your source data is already an array, and the array will not be used by others, the fastest way is to have a thin wrapper:

final Object[] source = ...

Collection colllection = new AbstractCollection(){
    public Object[] toArray(){ return source; }
    // other methods don't matter
}

Answer 3

If you are talking about memory footprint, take a look at this table in memory-measurer. Arrays$ArrayList is missing, but could be a good alternative to ArrayList (Arrays.asList(...)).

Update: I updated the original links to the new location of the project in github. Take into account these metrics are several years old.