Can I transform a Stream> to a Stream?

Question

Is this possible (pseudo-Java):

someList = [
  [1,2],
  [3,4]
];
Stream<List<X>> a = someList.stream();
Stream<X> b = a.whatever(...);
assert b.collect(list).equals([1,2,3,4]);

Put more generally, is there an operation on a stream that can increase the number of elements in the stream (rather than decrease, as filter does)?

Answer 1

Yes. flatMap does that.

Stream<X> b = someList.stream().flatMap(l -> l.stream());

Assuming that someList is a List<List<X>>, flatMap would flatten all the elements of the internal lists into a single Stream.