How to collect the results of a Stream in a primitive array?

Question

I'm trying to convert 2D list to a 2D int array. However, it seems I can only collect objects, not primitives.

When I do:

data.stream().map(l -> l.stream().toArray(int[]::new)).toArray(int[][]::new);

I get the compile-time error Cannot infer type argument(s) for <R> map(Function<? super T,? extends R>).

However, if I change int[] to Integer[], it compiles. How can I get it to just use int?

For the lambda, try `l.stream().mapToInt(Integer::intValue).toArray()` — 4castle, Jun 01 '17 at 01:51
How can I collect the results of this though? That just performs operations on the Stream without storing the results. — ack, Jun 01 '17 at 01:53
My suggestion handles the incorrect lambda you were using. It will collect properly now. — 4castle, Jun 01 '17 at 01:55

Sergey Kalinichenko · Accepted Answer · 2017-06-01T01:54:50.953

33

Use mapToInt method to produce a stream of primitive integers:

int[][] res = data.stream().map(l -> l.stream().mapToInt(v -> v).toArray()).toArray(int[][]::new);

The inner toArray call no longer needs int[]::new, because IntStream produces int[].

edited Jun 01 '17 at 01:54

answered Jun 01 '17 at 01:45

1 Answers1