Collectors inventory (original) (raw)

Remi Forax forax at univ-mlv.fr
Sat Feb 23 01:40:37 PST 2013

On 02/22/2013 12:01 AM, Brian Goetz wrote:

As I promised a long time ago, here's an overview of what's in Collectors currently.

I think there are too many methods in Collectors, we should restrain ourselves to 2 forms (3 max).

There are 12 basic forms: - toCollection(ctor) - toList() - toSet() - toStringBuilder() - toStringJoiner(delimiter) - to{Long,Double}Statistics - groupingBy(classifier, mapFactory, downstream collector) - groupingReduce(classifier, mapFactory, mapper, reducer) - mapping(mappingFn, downstream collector) - joiningWith(mappingFunction, mergeFunction, mapFactory) - partitioningBy(predicate, downstream collector) - partitioningReduce(predicate, mapper, reducer) The toXxx forms should be obvious. Mapping has four versions, analogous to Stream.map: - mapping(T -> U, Collector<U, R>) - mapping(T -> int, Collector.OfInt) - mapping(T -> long, Collector.OfLong) - mapping(T -> double, Collector.OfDouble) GroupingBy has four forms: - groupingBy(T->K) -- standard groupBy, values of resulting Map are Collection - Same, but with explicit constructors for map and for rows (so you can produce, say, a TreeMap<K, TreeSet> and not just a Map<K,Collection>) - groupingBy(T->K, Collector<T,D>) -- multi-level groupBy, where downstream is another Collector - Same, but with explicit ctor for map

You can remove the third one give, you have the one with an explicit constructor.

GroupingReduce has four forms: - groupingReduce(T->K, BinaryOperator) // simple reduce - groupingReduce(T->K, Function<T,U>, BinaryOperator) // map-reduce - above two with explicit map ctors

keep only the ones with explicit constructors.

JoiningWith has four forms: - joiningWith(T->U) - same, but with explicit Map ctor - same, but with merge function for handling duplicates - same, with both explicit map ctor and merge function

remove the third one.

PartitioningBy has three forms: - partitioningBy(Predicate) - Same, but with explicit constructor for Collection (so you can get a Map<Boolean, TreeSet>) - partitioningBy(Predicate, Collector) // multi-level PartitioningReduce has two forms: - predicate + reducer - predicate + mapper + reducer Impl note: in any category, all but one are one-liners that delegate to the general form.

Rémi

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