Java.util.Collection at the lowest cost?

Question

I am calling a method in another API that accepts java.util.Collection objects. I looked at the method and immediately copied everything in the collection to a new ArrayList before doing my task.

This made me wonder: what is the absolute lowest Java utility collection that I can use to quickly assemble parameters for this method?

Answer 1

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

 new ArrayList<Something>(inputCollection); 

or if he does

 someCopy.addAll(inputCollection); 

then it will go through 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 shell:

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

Answer 3

If you are talking about the amount of memory, look at this table in memory-measurer . Arrays$ArrayList missing, but may be a good alternative to ArrayList ( Arrays.asList(...) ).