Extending Collector to handle a post-transform

Brian Goetz brian.goetz at oracle.com
Tue Jun 11 16:39:12 PDT 2013

Creating a new type that only solves half the problem has negative 
value; the user still has to puzzle about the wildcards in the 
full-blown Collector (which show up not only when R != I, but also in 
parameters of combinators like groupingBy), and now has to learn TWO 
library types, one of which's sole purpose is to unsuccessfully hide the 

On 6/11/2013 7:31 PM, Remi Forax wrote:
> On 06/12/2013 01:24 AM, Brian Goetz wrote:
>>> Still don't get why you can not do:
>>> interface SimpleCollector<T, R> extends Collector<T, R, R> {
>>>    ...
>>> }
>> Not all Collectors will have I=R.  Consider toStringBuilder, where
>> I=StringBuilder and R=String.
> yes, for the ones without post-function like toList, you can use the
> SimpleCollector, for the ones like toString you use the full blown
> Collector.
> Rémi

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