Loose ends: Collection.toArray(IntFunction<A[]> generator)

Paul Sandoz paul.sandoz at oracle.com
Thu May 30 02:18:43 PDT 2013

A while ago there was some lively discussion on toArray and we finally settled on the following method in Stream:

      <A> A[] toArray(IntFunction<A[]> generator);

Due to "noise" of other stuff i think we forgot to discuss whether it is valuable to add a similar default method to Collection (as recently suggested by Peter Levart on the lambda-dev list which motivated this email).

The functionality can already be achieved with:


but is not likely to be as efficient as the existing concrete Collection.toArray implementations (e.g. see ArrayList.toArray)

This is also the way to produce an array in parallel:


where efficiencies can be obtained if the collection is large enough.

But we could also add to Collection:

    default <T> T[] toArray(IntFunction<T[]> generator) {
        return toArray(generator.apply(size()));


Seems like a reasonable thing to do. The sequentially efficiency is retained without the "stink" of using the other toArray method accepting an array instance (of or not of the right length).


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