Collectors update

[Brian Goetz]

>> 4.  Rejigger Partition to return an array again, with an explicit
>> lambda (which will likely be an array ctor ref) to make the array.
>> Eliminated the silly Partition class.
>
> Please don't do that, it's pure evil.

The silly Partition class was also evil.  We're in the "lesser of evils" 
business here.

>    public static<T> Collector<T, Collection<T>[]>
> partitioningBy(Predicate<T> predicate, IntFunction<Collection<T>[]>
> arraySupplier) {
>
> in order to call this method users have to send a lambda that creates an
> array of parametrized type something which will be unsafe (the only way
> to get type safety is to create a class that inherits from Collection
> and to provide a type argument something like class Foo extends
> ArrayList<String> {}) so while the code of the library is typesafe, no
> user code will never be typesafe.

Yes, let's have this discussion.  The above is true, but is this really 
a problem?  The array is simply a box for carrying the result back.  The 
typical use case will be:
  - call partition
  - scoop out the two collections / reductions / whatever
  - throw away the box

So the box is intended to be a short-lived container (essentially a 
workaround for the lack of tuples.)  This is not the same case as 
Stream.toArray.

> and BTW, the IntFunction will be always called with 2 as argument.

Yes.  Is that bad?