How do I get an IntStream from a List<Integer>?

Question

I can think of two ways:

public static IntStream foo(List<Integer> list) {     return list.stream().mapToInt(Integer::valueOf); }  public static IntStream bar(List<Integer> list) {     return list.stream().mapToInt(x -> x); } 

What is the idiomatic way? Maybe there is already a library function that does exactly what I want?

Answer 1

I guess (or at least it is an alternative) this way is more performant:

public static IntStream baz(List<Integer> list) {     return list.stream().mapToInt(Integer::intValue); } 

since the function Integer::intValue is fully compatible with ToIntFunction since it takes an Integer and it returns an int. No autoboxing is performed.

I was also looking for an equivalent of Function::identity, i hoped to write an equivalent of your bar method :

public static IntStream qux(List<Integer> list) {     return list.stream().mapToInt(IntFunction::identity); } 

but they didn't provide this identity method. Don't know why.

Answer 2

An alternate way to transform that would be using Stream.flatMapToInt and IntStream.of as:

public static IntStream foobar(List<Integer> list) {     return list.stream().flatMapToInt(IntStream::of); } 

Note: Went through few linked questions before posting here and I just couldn't find this suggested in them either.