JEP 186: Collection Literals

Brian Goetz brian.goetz at
Mon Mar 3 13:36:28 PST 2014

Just to close the loop here, we've reached the conclusion that we will 
not be pursuing collection literals as a language feature right now, but 
instead proceeding in the short term with a library-based proposal that 
is similar to what Stephen has described here, with a few ideas from 
Guava's ImmutableXxx classes.

Our motivation is largely as outlined earlier in this thread:

  - The "simple" version of this feature (sets, lists, maps only) is not 
very satisfying or popular; the "extensible" version of this feature is 
open-ended, messy, and virtually guaranteed to way overrun its design 

  - The library-based version gives us X% of the benefit for 1% of the 
cost, where X >> 1;

  - Value types are coming, and the "what would this feature look like" 
in a world with value types may well be quite different than in a world 
without, suggesting it would be questionable to try and do this work 
before value types;

  - We are better off focusing our language-design bandwidth on 
addressing more the foundational issues underlying a library-based 
version (including: more efficient varargs, array constants in the 
constant pool, immutable arrays, and support for caching (and reclaiming 
under pressure) intermediate immutable results).

Separately, I'll try and write up my notes about the various places this 
exploration led us.

On 2/19/2014 1:02 PM, Stephen Colebourne wrote:
> On 21 January 2014 19:39, Brian Goetz <brian.goetz at> wrote:
>> 3.  The real pain is in Maps, not Lists, Sets, or Arrays.  Library-based
>> solutions would be mostly acceptable for the latter, but we still lack a
>> reasonable way to describe pair literals, which is what's in the way of
>> a library-based solution for Map (other than a MapBuilder.)  Static
>> methods in interfaces make a library-based solution more practical.
>> With value types, though, library-based solutions for Map become far
>> more practical too.
> Just to note on this, I previously sent a proposal to core-libs:
> and a proof of concept patch:
> The heart of the concept was static methods on interfaces:
>   Collection.empty()
>   Collection.of(T...)
>   List.empty()
>   List.of(T...)
>   Set.empty()
>   Set.of(T...)
>   Map.empty()
>   Map.of(Entry...)
>   Map.Entry.of(K, V)
> Each of these methods would return immutable implementations. There is
> a case for extending the methods to Iterator and other collection
> types, however these are the most important. These follow the designs
> of Stream static methods IIRC.
> I think a complete solution would involve matching static methods on
> the collection classes as well, eg
>   ArrayList.of(T...)
>   TreeSet.of(T...)
> I note that the Map variant in the patch is a cunning way to beat the
> absence of a pair/tuple and generics, but it would require some
> thought before adoption in the JDK.
> Stephen

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