Advanced Collections and Collectors - Java 8 Lambdas - page 73

already in the core library and is called counting . So, we can rewrite the example into
Example 5-14 .

Example 5-14. Using collectors to count the number of albums for each artist
public public Map < Artist , Long > numberOfAlbums ( Stream < Album > albums ) {

return

return albums . collect ( groupingBy ( album -> album . getMainMusician (),
counting ()));

}

This form of groupingBy divides elements into buckets . Each bucket gets associated with
the key provided by the classifier function: getMainMusician . The groupingBy operation
then uses the downstream collector to collect each bucket and makes a map of the results.

Let's consider another example, in which instead of building up a grouping of albums, we
just want their names. Again, one approach is to take our original collector and then fix up
the resulting values in the Map . Example 5-15 shows how we might do that.

Example 5-15. A naive approach to finding the names of every album that an artist has pro-
duced
public public Map < Artist , List < String >> nameOfAlbumsDumb ( Stream < Album > albums ) {

Map < Artist , List < Album >> albumsByArtist =
albums . collect ( groupingBy ( album -> album . getMainMusician ()));

Map < Artist , List < String >> nameOfAlbums = new new HashMap <>();

for for ( Entry < Artist , List < Album >> entry : albumsByArtist . entrySet ()) {

nameOfAlbums . put ( entry . getKey (), entry . getValue ()
. stream ()
. map ( Album: : getName )
. collect ( toList ()));

}
return return nameOfAlbums ;

}

Again, we can produce nicer, faster, and easier-to-parallelize code using another collector.
We already know that we can group our albums by the main artist using the groupingBy col-
lector, but that would output a Map<Artist, List<Album>> . Instead of associating a list of
albums with each Artist , we want to associate a list of strings, each of which is the name of
an album.

In this case, what we're really trying to do is perform a map operation on the list from the
Artist to the album name. We can't just use the map method on streams because this list is