Extending Collector to handle a post-transform

[Brian Goetz]

>     1.  Reducing boxing during reducing.  Currently, the reducing
>     collectors will unbox, compute a new result, and rebox, on every
>     element.  That ugly and slow.  With a post-transform, we can use a
>     hidden mutable holder as our internal representation, and still only
>     expose "Collector from T to Long".
>
> Nice, indeed, but won't the internal type still appear as a type
> parameter of Collector?

Yes, it's not yet "all gain, no pain".  Where I am now, the existence of 
an internal type still appears; Collector takes type arguments <T, I, R> 
(input, intermediate, result), but all the occurrences of Collector in 
the API specify ? as the second parameter:

     public static Collector<String, ?, String> toStringBuilder()