Stream operations -- current set

[Henry Jen]

On Oct 7, 2012, at 10:52 AM, Brian Goetz <brian.goetz at oracle.com> wrote:

>>>    <U> MapStream<T, U> mapped(Mapper<? super T, ? extends U> mapper);
>> From my experience, the opposite one is much more useful:
>>     <U> MapStream<U, T> mapped(Mapper<? super T, ? extends U> mapper);
>> Are we implying that in this case one should use groupBy() (create many pointless collections) and then map() to throw the collections away?
> 
> That works, but cheaper is:
> 
>  foos.mapped(mapper).swap()
> 
> which transposes keys and values (probably needs a better name.)

This is also my experience. I am guessing the concern is there is no guarantee the mapper will ensure one-to-one mapping to maintain the key uniqueness in a Map as you mentioned as implying.

Brain's suggestion is a reasonable alternative if we know the result is unique and cost is pretty much nothing as swapping happens during iteration.

Cheers,
Henry