MapStream.map* methods

[Rémi Forax]

On 05/08/2012 10:06 AM, Peter Levart wrote:
> Hi Mike and others,
>
> I'm trying to understand the purpose of the 3 MapStream.map* methods that have
> the following signatures:
>
> public interface MapStream<K, V>  {
>
>      <W>  MapStream<K, W>  map(BiMapper<K, V, W>  mapper) default ...
>
>      <W>  MapStream<K, W>  mapValues(Mapper<V, W>  mapper) default {
>          return map( (k, v) ->  mapper.map(v));
>      }
>
>      <W>  MapStream<K, Iterable<W>>  mapValuesMulti(
>          BiMapper<? super K, ? super V, Iterable<W>>  mapper
>      ) default ...
>
> ... the first is obvious. It replaces the values in a key/value pairs with
> values computed by BiMapper from key/value pairs. This is the most common
> method and for the less common case (to replace keys instead of values) one
> can use the following idiom:
>
>      mapStream.swap().map().swap()...
>
> ... the second (mapValues) is just a shorthand for the first in the common case
> when keys are not needed in the computation of values.

I agree.

>
> Now I think that both could have a more forgiving generic signature:
>
>      <W>  MapStream<K, W>  map(
>          BiMapper<? super K, ? super V, ? extends W>  mapper
>      );
>
>      <W>  MapStream<K, W>  mapValues(Mapper<? super V, ? extends W>  mapper);

yes !

>
> ... the third (mapValuesMulti) is the one that I don't see the point in. It is
> just a more constrained version of plain map(). Even the default
> implementation bodies are the same. Are those API choices only to help
> inferencing engine or I'm missing something?

no, map..Multi is here because at some point in the past, there were
an interface MultiMap. This should be cleanup.

>
> What I'm missing is the following method:
>
>      <W>  MapStream<K, W>  flatMap(
>          BiMapper<? super K, ? super V, ? extends Iterable<W>>
>      );
>
> By analogy to the similar method in Iterable it could also be used to flatten
> (Multi)MapStreams:
>
>      MapStream<X, Iterable<Y>>  multiMapStream = ...;
>
>      MapStream<X, Y>  flattened = multiMapStream.flatMap((x, ys) ->  ys);
>
>
> On the other front, it would be nice to have the following method in
> MapStream:
>
>      MapStream<K, Iterable<V>>  groupByKey();
>
>
>
> Regards,
>
> Peter
>
>

regards,
Rémi