Collectors.toConcurrentMap() strategy

[Peter Levart]

Hi Paul,

Unrelated to fixing the exception message, I realized that 
Collectors.toMap() and Collectors.toConcurrentMap() are practically 
equivalent. They use same strategy, only Map implementation used in 
collecting is different: HashMap vs. ConcurrentHashMap. This strategy 
does not require the map to be concurrent - each thread collects into 
it's own instance of the map and there is a merge step that reduces 
multiple map instances into a single map, mutating each map instance in 
a single thread at a time. Such approach is required for non-concurrent 
Map implementations, but ConcurrentHashMap could be better utilized by 
using a different approach where entries are collected concurrently into 
a shared single map instance. Merge-ing step is therefore not required. 
It's very easy to achieve that at least for the toConcurrentMap method 
that constructs a collector for unique keys, but I think it's also 
possible with other toConcurrentMap collectors that use 
ConcurrentMap.merge() - on top of the patch for exception message:


     public static <T, K, U>
     Collector<T, ?, ConcurrentMap<K,U>> toConcurrentMap(Function<? 
super T, ? extends K> keyMapper,
Function<? super T, ? extends U> valueMapper) {
         ConcurrentMap<K, U> map = new ConcurrentHashMap<>();
         return new CollectorImpl<>(() -> map,
uniqKeysMapAccumulator(keyMapper, valueMapper),
                                    (m1, m2) -> m1,
                                    CH_CONCURRENT_ID);
     }


What do you think?

Regards, Peter