Stream.map(...) 和 Collectors.mapping(...) 有什么区别?

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?

解决方案

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.