Advanced Collections and Collectors - Java 8 Lambdas - page 68

do that using the toCollection collector, which takes a function to build the collection as its
argument (see Example 5-5 ) .

Example 5-5. Collecting into a custom collection using toCollection
stream . collect ( toCollection ( TreeSet: : new new ));

To Values

It's also possible to collect into a single value using a collector. There are maxBy and minBy
collectors that let you obtain a single value according to some ordering. Example 5-6 shows
how to find the band with the most members. It defines a lambda expression that can map an
artist to the number of members. This is then used to define a comparator that is passed into
the maxBy collector.

Example 5-6. Finding the band with the most members
public public Optional < Artist > biggestGroup ( Stream < Artist > artists ) {

Function < Artist , Long > getCount = artist -> artist . getMembers (). count ();
return return artists . collect ( maxBy ( comparing ( getCount )));

}

There's also a minBy , which does what it says on the tin.

There are also collectors that implement common numerical operations. Let's take a look at
these by writing a collector to find the average number of tracks on an album, as in
Example 5-7 .

Example 5-7. Finding the average number of tracks for a list of albums
public public double double averageNumberOfTracks ( List < Album > albums ) {

return

return albums . stream ()
. collect ( averagingInt ( album -> album . getTrackList (). size ()));

}

As usual, we kick off our pipeline with the stream method and collect the results. We then
call the averagingInt method, which takes a lambda expression in order to convert each
element in the Stream into an int before averaging the values. There are also overloaded op-
erations for the double and long types, which let you convert your element into these type
of values.

Back in Primitives , we talked about how the primitive specialized variants of streams, such
as IntStream , had additional functionality for numerical operations. In fact, there are also a
group of collectors that offer similar functionality, in the vein of averagingInt . You can add