On Apr 25, 2014, at 7:31 PM, Peter Levart <peter.lev...@gmail.com> wrote:
> Hi Paul,
>
> To make any sense of null return from the mergeFunction, which could mean
> that an entry with that key is absent from resulting map, such Collector
> would have to keep some more state - the collecting map (which is also
> returned at the end) and the set of removed keys. For example:
>
>
> public static <T, K, U, M extends Map<K, U>>
> Collector<T, ?, M> toMap(Function<? super T, ? extends K> keyMapper,
> Function<? super T, ? extends U> valueMapper,
> BinaryOperator<U> mergeFunction,
> Supplier<M> mapSupplier) {
>
> class State {
> final M map = mapSupplier.get();
> final Set<K> removedKeys = new HashSet<>();
> M map() { return map; }
> }
>
> BiConsumer<State, T> accumulator = (state, element) -> {
> K k = keyMapper.apply(element);
> if (!state.removedKeys.contains(k)) {
> U u = state.map.merge(k, valueMapper.apply(element),
> mergeFunction);
> if (u == null) state.removedKeys.add(k);
> }
> };
>
> BinaryOperator<State> merger = (state1, state2) -> {
> state1.map.keySet().removeAll(state2.removedKeys);
> state2.map.keySet().removeAll(state1.removedKeys);
> state1.removedKeys.addAll(state2.removedKeys);
> for (Map.Entry<K,U> e : state2.map.entrySet()) {
> U u = state1.map.merge(e.getKey(), e.getValue(),
> mergeFunction);
> if (u == null) state1.removedKeys.add(e.getKey());
> }
> return state1;
> };
>
> return new CollectorImpl<>(State::new, accumulator, merger,
> State::map, CH_ID);
> }
>
>
> ...but the concurrent variant would be tricky to implement.
>
Yes, i suspect a finishing stage would be required to clean up the map.
The simplest thing to do is throw an NPE if Map.merge returns null. It would be
rather surprising for partial map value merging to have a global subtractive
effect (esp. if that could occur non-determinisitically when encounter order of
processing values is not important).
Collectors.toMap and groupingBy are already null-hostile, Map.merge (and your
update using putIfAbsent preserving the same merge behaviour) will throw an NPE
on a null input value.
Plus I now realize that throwing an NPE after removal has occurred is OK since
that side-effect should never be observed as resulting map will not be returned
to the caller.
Paul.
