Extend an existing stream collector instance

Question

I need a Collector that's nearly identical to Collectors.toSet(), but with a custom finisher. I'd love to be able to do something like:

myCollector = Collectors.toSet();
myCollector.setFinisher(myCustomFinisher);

and be done, but that doesn't seem possible. The only alternative I can see is it essentially recreate Collectors.toSet() using Collector.of(), which is not very DRY.

Is there a way to take an existing Collector and amend it as described above?

EDIT

A few of the answers have recommended something like this:

  Collector<T, ?, Set<T>> toSet = Collectors.toSet();
  return Collector.of(
     toSet.supplier(),
     toSet.accumulator(),
     toSet.combiner(),
     yourFinisher,
     toSet.characteristics());

However my custom finisher isn't actually returning a Set; it's using the accumulated set to return something else. The fact that it's doing that is putting me into Generics hell which I'm still rummaging through..

Answer 1

That’s exactly what collectingAndThen(collector,finisher) does.

The custom finisher does not replace the old one, if there is one, but gets combined with it (like oldFinisher.andThen(newFinisher)), but the current implementation of toSet() has no finisher (an identity finisher) anyway.

So all you have to do, is collectingAndThen(toSet(),myCustomFinisher)…