MapStream.map* methods

[Brian Goetz]

On May 8, 2012, at 1:06 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.

The reason for this shorthand version is that you may have in hand a method that does the mapping already:

  people.mapped(Person::getBirthday)  // stream of (Person, Date)
              .mapValues(Date::toString)        // stream of (Person, String)

instead of 

              ...mapValues( (person, birthday) -> birthday.toString())

> On the other front, it would be nice to have the following method in 
> MapStream:
> 
>    MapStream<K, Iterable<V>> groupByKey();

Good catch.