reduceBy (original) (raw)

groupBy / reduceBy

Brian Goetz brian.goetz at oracle.com
Sat Dec 8 12:38:40 PST 2012

• Previous message: "Cancelable" streams
• Next message: groupBy / reduceBy
• Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]

So, I hate groupBy/reduceBy. Not that I hate the idea, just their current realization.

Reasons to hate them:

• They intrude Map and Collection into the Stream API, whereas otherwise there would be no connection (except Iterator) to Old Collections. This falls short of a key goal, which is for Streams to be a bridge from Old Collections to New Collections in the future. We've already severed the 32-bit size limitation; we've distanced ourselves from the pervasive mutability of Old Collections; this is the remaining connection that needs to be severed.

• They are limited. You can do one level of group-by, but you can't do two; it requires gymnastics to, for example, take a Stream and do a multi-level tabulation like grouping into a Map<Buyer, Map<Seller, Collection>. At the same time, they offer limited control over what kind of Map to use, what kind of Collection to use for the values for a given grouping, etc.

• Guava-hostile. Guava users would probably like groupBy to return a Multimap. This should be easy, but currently is not.

• The name reduceBy is completely unclear what it does.

• Too-limited control over whether to use map-merging (required if you want to preserve encounter order, but probably slower) or accumulate results directly into a single shared ConcurrentMap (probably faster, but only if you don't care about encounter order). Currently we key off of having an encounter order here, but this should be a user choice, not a framework choice.

These negatives play into the motivation for some upcoming proposals about reduce forms, which will propose a new, generalized formulation for these methods that address these negatives. Key observations:

• groupBy is really just reduceBy where the reduce seed is "new ArrayList" and the combiner function is ArrayList::add
• reduceBy is really just a reduce whose combiner function incorporates some mutable map mechanics

• Previous message: "Cancelable" streams
• Next message: groupBy / reduceBy
• Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]

More information about the lambda-libs-spec-experts mailing list