JEP 186: Collection Literals

Remi Forax forax at
Tue Jan 14 14:34:08 PST 2014

On 01/14/2014 06:02 PM, Brian Goetz wrote:
> And, we have now reached our quota of syntax discussion on this topic.
> No more, please!  Such discussions get in the way of the important
> questions like:


>    - how does this map to bytecode

We need new constant pool entries for arrays (like MSIL does), it's 
currently possible to encode arrays as a String (by example by 
uuencoding the values) but it's awkward.
Then a collection literal can be encoded using invokedynamic.
By example,
   int a = ...
   { 1, 2, a} as ArrayList (or any other syntax :)

   { "foo": 2, "bar": a } as HashMap

can be encoded like this:
   iload 'a'
   invokedynamic (int)ArrayList

   iload 'a'
   invokedynamic (int)HashMap

Because Collection and Map are two different interfaces,
we need two different bootstrap method.
For a Collection:
   - a reference to the array
   - an array of the position of the placeholder (where to insert 'a' in 
the example)
   - a methodhandle on the method used to create the builder
   - a method handle on the method used to append a value
   - a method hande to transform the builder to a Collection

and for a Map, you need a supplementary array
(one for the keys and one for the values).


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