Java Streams: group a List into a Map of Maps

Question

How could I do the following with Java Streams?

Let\'s say I have the following classes:

class Foo {
    Bar b;
}

class Bar {
    String id;
    Str         


        

Answer 1

You can group your data in one go assuming there are only distinct Foo:

Map> map = list.stream()
        .collect(Collectors.groupingBy(f -> f.b.id, 
                 Collectors.toMap(f -> f.b.date, Function.identity())));

Saving some characters by using static imports:

Map> map = list.stream()
        .collect(groupingBy(f -> f.b.id, toMap(f -> f.b.date, identity())));