Java 8 grouping using custom collector?

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When grouping a Stream with Collectors.groupingBy, you can specify a reduction operation on the values with a custom Collector. Here, we need to use Collectors.mapping, which takes a function (what the mapping is) and a collector (how to collect the mapped values). In this case the mapping is Person::getName, i.e. a method reference that returns the name of the Person, and we collect that into a List.

Map<Integer, List<String>> collect = 
    members.stream()
           .collect(Collectors.groupingBy(
               Person::getAge,
               Collectors.mapping(Person::getName, Collectors.toList()))
           );

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