Difference LongStream VS Stream in Collectors.toList()

Question

Why when I am getting a list from a LongStream with Collectors.toList() got an error but with Stream there is no error?

Examples :

ERROR :

Something.mapToLong(Long::parseLong).collect(Collectors.toList())

Correct :

Something.map(Long::valueOf).collect(Collectors.toList())

Answer 1

There are four distinct classes in Stream API: Stream, IntStream, LongStream and DoubleStream. The latter three are used to process the primitive values int, long and double for better performance. They are tailored for these primitive types and their methods differ much from the Stream methods. For example, there's a LongStream.sum() method, but there's no Stream.sum() method, because you cannot sum any types of objects. The primitive streams don't work with collectors as collectors are accepting objects (there are no special primitive collectors in JDK).

The Stream class can be used to process any objects including primitive type wrapper classes like Integer, Long and Double. As you want to collect to the List<Long>, then you don't need a stream of long primitives, but stream of Long objects. So you need Stream<Long> and map instead of mapToLong. The mapToLong can be useful, for example, if you need a primitive long[] array:

long[] result = Something.mapToLong(Long::valueOf).toArray();