java - What's the difference between Stream.map(...) and Collectors.mapping(...)?

Question

I've noticed many functionalities exposed in Stream are apparently duplicated in Collectors, such as Stream.map(Foo::bar) versus Collectors.mapping(Foo::bar, ...), or Stream.count() versus Collectors.counting(). What's the difference between these approaches? Is there a performance difference? Are they implemented differently in some way that affects how well they can be parallelized?

Answer 1

The collectors that appear to duplicate functionality in Stream exist so they can be used as downstream collectors for collector combinators like groupingBy().

As a concrete example, suppose you want to compute "count of transactions by seller". You could do:

Map<Seller, Long> salesBySeller = 
    txns.stream()
        .collect(groupingBy(Txn::getSeller, counting()));

Without collectors like counting() or mapping(), these kinds of queries would be much more difficult.