Refactor of Collector interface

Brian Goetz brian.goetz at
Fri Feb 8 08:36:05 PST 2013

Your subjective sense is accurate, which is why I brought this up.  This 
may be an example where is better to depart from the traditional approach.

To your question, it depends what you mean by "purely to do with an 
implementor."  Collector *users* are going to be burdened with the 
performance consequences of multiple layers of wrapping/conversion.

The implementation used to be full of alternation between:

interface Foo<T,U> {
     U transform(T t);

class FooAdapter<T,U> {
     FooAdapter(Function<T,U> lambda) { ... }

     U transform(T t) { return lambda.apply(t); }


Function<T,U> parentTransformer = foo::transform;

and back again, introducing layers of wrapping even when the function is 
not changing across layers.

On 2/8/2013 11:22 AM, Kevin Bourrillion wrote:
> My subjective sense of good Java API design very strongly prefers the
> "before" picture here, which I see as a lot more "Java-like", so I'm
> taking a closer look.
> I assume that the trade-offs we're weighing here are purely to do with
> what it's like to be a Collector implementor, correct?
> On Fri, Feb 8, 2013 at 7:25 AM, Brian Goetz <brian.goetz at
> <mailto:brian.goetz at>> wrote:
>     FYI: In a recent refactoring, I changed:
>     public interface Collector<T, R> {
>          R makeResult();
>          void accumulate(R result, T value);
>          R combine(R result, R other);
>     }
>     to
>     public interface Collector<T, R> {
>          Supplier<R> resultSupplier();
>          BiConsumer<R, T> accumulator();
>          BinaryOperator<R> combiner();
>     }
>     Basically, this is a refactoring from typical interface to
>     tuple-of-lambdas.  What I found was that there was a lot of
>     adaptation going on, where something would start out as a lambda,
>     we'd wrap it with a Collector whose method invoked the lambda, then
>     take a method reference to that wrapping method and then later wrap
>     that with another Collector, etc.  By keeping access to the
>     functions directly, the Collectors code got simpler and less wrappy,
>     since a lot of functions could just be passed right through without
>     wrapping.  And a lot of stupid adapter classes went away.
>     While clearly we don't want all interfaces to evolve this way, this
>     is one where *all* the many layers of manipulations are effectively
>     function composition, and exposing the function-ness made that
>     cleaner and more performant.  So while I don't feel completely
>     super-great about it, I think its enough of a win to keep.
> --
> Kevin Bourrillion | Java Librarian | Google, Inc. |kevinb at
> <mailto:kevinb at>

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