hg: lambda/lambda/jdk: 3 new changesets

[Rémi Forax]

On 05/31/2012 12:11 AM, Brian Goetz wrote:
>> I don't like the class StringJoiner because despite the fact it's a
>> reduce operation,
>> it's implemented as a Fillable, so something eager which will not work
>> as is in the parallel world.
>
> That's where we started -- trying to treat string joining as a reduce. 
> But because String is immutable, doing a sequential reduce with string 
> concatenation becomes an O(n^2) operation, which is not good.
>
> (Originally we thought that the way to do joining was:
>    stream.interleaveWith(Streams.repeating(", "))
>          .reduce(String::concatenate)
> but the reduce step, which, while pretty, was inefficient.)

join is a kind reduce operation that can be easily be implemented as a 
foldLeft
on a StringBuilder.

collection.foldLeft(new StringBuilder(),
                      (element, builder) -> {
                         if (builder.length == 0)
                           return builder.append(element);
                         return builder.append(", ").append(element);
                      }).toString()

for the parallel case, the question is how to implement foldLeft ?
perhaps foldLeft should return an Iterable of StringBuilder() that you can
reduce sequentially afterward.

>
> The parallel case isn't much better.  Even if you do a number of 
> string joins at the leaves of the tree, the top-level combine still 
> has to copy all the string content, which loses most of the parallelism.
>
> But you can still do upstream ops in parallel:
>
>   collection.parallel()
>             .filter(...)
>             .map(...)
>             .sorted()
>             .sequential()
>             .into(new StringJoiner(", "));
>
> and all the upstream stuff will happen in parallel.
>
>> I think it's better to add join() on Iterable.
>
> Would like to, but we can't add methods that only apply to specific 
> parameterizations.

you don't have to because all objects have a method toString()
or said in another way StringBuilder.append() takes an Object as parameter.

cheers,
Rémi