Collectors inventory

[Remi Forax]

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<R>)
>  - mapping(T -> long, Collector.OfLong<R>)
>  - mapping(T -> double, Collector.OfDouble<R>)
>
> GroupingBy has four forms:
>  - groupingBy(T->K) -- standard groupBy, values of resulting Map are 
> Collection<T>
>  - Same, but with explicit constructors for map and for rows (so you 
> can produce, say, a TreeMap<K, TreeSet<T>> and not just a 
> Map<K,Collection<T>>)
>  - 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<T>) // simple reduce
>  - groupingReduce(T->K, Function<T,U>, BinaryOperator<U>) // 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<T>>)
>  - 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