Collectors inventory (original) (raw)

Brian Goetz brian.goetz at oracle.com
Thu Feb 21 15:01:30 PST 2013

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

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

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

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

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.

Plus, all the Map-bearing ones have a concurrent and non-concurrent version.

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