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

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.