RFR: 8180352: Add Stream.toList() method

[John Rose]

I agree with Stuart and Brian that streams should be null-tolerant,
including in this case.  And of course I’m delighted to see an
immutable container as the output to the utility method; I’m
a fan of reduced-copy, race-free, bug-resistant algorithms you
get from immutability.

On Nov 3, 2020, at 6:53 AM, Remi Forax <forax at univ-mlv.fr> wrote:
> 
> Also, adding a third immutable list creates a problem, it means that now when we get an immutable list it can be 3 different implementations but the VM only uses bimorphic inlining cache,
> so more callsites will fail to inline because of that. I think we have already reduced the number of implementation of immutable map from 3 to 2 for the very same reasons.

Yes, this part concerns me, with my JIT hat on.

It seems to me that the problem can be removed simply by allowing
the existing ListN object to contain nulls.  That’s not the same
thing as allowing List.of(1,2,3, null):  That factory method can
reject nulls, while the privileged factory method that makes
an array-backed ListN can simply sidestep the null check.

And if a call to Stream::toList call returns 0, 1, or 2 items,
it’s a straightforward matter to test if either is null, and then
choose to use either ListN or List12.  No new implementation
classes, at all!