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<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())));

Answer 2

Suppose (b.id, b.date) pairs are distinct. If so, in second step you don't need grouping, just collecting to Map where key is foo.b.date and value is foo itself:

Map<String, Map<String, Foo>> map = 
       myList.stream()
             .collect(Collectors.groupingBy(f -> f.b.id))    // map {Foo.b.id -> List<Foo>}
             .entrySet().stream()
             .collect(Collectors.toMap(e -> e.getKey(),                 // id
                                       e -> e.getValue().stream()       // stream of foos
                                             .collect(Collectors.toMap(f -> f.b.date, 
                                                                       f -> f))));

Or even more simple:

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

Answer 3

An alternative is to support the equality contract on your key, Bar:

class Bar {
    String id;
    String date;

    public boolean equals(Object o){
       if (o == null) return false;
       if (!o.getClass().equals(getClass())) return false;
       Bar other = (Bar)o;
       return Objects.equals(o.id, id) && Objects.equals(o.date, date);
    }

    public int hashCode(){
       return id.hashCode*31 + date.hashCode;
    }    
}

Now you can just have a Map<Bar, Foo>.