How can I convert a Java HashSet<Integer> to a primitive int array?

java arrays int hashset
jackbot picture jackbot · Mar 16, 2010 · Viewed 65.4k times · Source

I've got a HashSet<Integer> with a bunch of Integers in it. I want to turn it into an array, but calling

hashset.toArray();

returns an Object[]. Is there a better way to cast it to an array of int other than iterating through every element manually? I want to pass the array to

void doSomething(int[] arr)

which won't accept the Object[] array, even if I try casting it like 

doSomething((int[]) hashSet.toArray());

Answer

Jeffrey Bosboom picture Jeffrey Bosboom · Mar 20, 2016

You can create an int[] from any Collection<Integer> (including a HashSet<Integer>) using Java 8 streams:

int[] array = coll.stream().mapToInt(Number::intValue).toArray();

The library is still iterating over the collection (or other stream source) on your behalf, of course.

In addition to being concise and having no external library dependencies, streams also let you go parallel if you have a really big collection to copy.

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