Java streams: grouping lists into maps

How do I use Java streams to do the following?

Suppose I have the following classes:

class Foo {
    Bar b;
}

class Bar {
    String id;
    String date;
}

I have a list < foo > and I want to convert it to map < foo b.id,Map< Foo. b.date,Foo> ;. 1. E: first by foo b. ID combination, and then by foo b. Date combination I'm trying to use the following two steps, but the second one doesn't even compile:

Map<String,List<Foo>> groupById =
        myList
                .stream()
                .collect(
                        Collectors.groupingBy(
                                foo -> foo.getBar().getId()
                        )
                );

Map<String,Map<String,Foo>> output = groupById.entrySet()
        .stream()
        .map(
                entry -> entry.getKey(),entry -> entry.getValue()
                        .stream()
                        .collect(
                                Collectors.groupingBy(
                                        bar -> bar.getDate()
                                )
                        )
        );

Thank you in advance

Solution

You can group your data at once, assuming that there are only different foos:

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

Save some characters using static import:

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