Why is the combiner of the Collector interface not consistent with the overloaded collect method?

Question

There is an overload method, collect(), in interface Stream with the following signature:

 R collect(Supplier

        

Answer 1

The "inline" (3-arg) version of collect is designed for when you already have these functions "lying around". For example:

ArrayList list = stream.collect(ArrayList::new, 
                                     ArrayList::add,
                                     ArrayList::addAll);

Or

BitSet bitset = stream.collect(BitSet::new, 
                               BitSet::set,
                               BitSet::or);

While these are just motivating examples, our explorations with similar existing builder classes was that the signatures of the existing combiner candidates were more suited to conversion to BiConsumer than to BinaryOperator. Offering the "flexibility" you ask for would make this overload far less useful in the very cases it was designed to support -- which is when you've already got the functions lying around, and you don't want to have to make (or learn about making) a Collector just to collect them.

Collector, on the other hand, has a far wider range of uses, and so merits the additional flexibility.