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;     String date; } 

I have a List<Foo> and I want to convert it to a Map <Foo.b.id, Map<Foo.b.date, Foo>. I.e: group first by the Foo.b.id and then by Foo.b.date.

I'm struggling with the following 2-step approach, 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()                                 )                         )         ); 

Thanks in advance.

Answer 1

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

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