Reducing reduce (original) (raw)

Brian Goetz brian.goetz at oracle.com
Mon Feb 11 08:41:33 PST 2013

Now that we've added all the shapes of map() to Stream (map to ref/int/long/double), and we've separated functional reduce (currently called reduce) from mutable reduce (currently called collect), I think that leaves room for taking out one of the reduce methods from Stream:

 <U> U reduce(U identity,
              BiFunction<U, ? super T, U> accumulator,
              BinaryOperator<U> reducer);

This is the one that confuses everyone anyway, and I don't think we need it any more.

The argument for having this form instead of discrete map+reduce are:

However, since we now have primitive-bearing map methods, and we can do filtering before and after the map, is this form really carrying its weight? Specifically because people find it counterintuitive, we should consider dropping it and guiding people towards map+reduce.

For example, "sum of pages" over a stream of Documents is better written as:

docs.map(Document::getPageCount).sum()

rather than

docs.reduce(0, (count, d) -> count + d.getPageCount(), Integer::sum)

The big place where we need three-arg reduce is when we're folding into a mutable store. But that's now handled by collect().

Have I missed any use cases that would justify keeping this form?

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