Stream GroupingBy for an Object Inside another Different Object

Question

I have three Objects as below :

public class A {
     private int a;
     private ...
     private B b;
    //getters setters for B
}

public class B {
     private String name;
     private ... 
     private C c;
    //getters setters for C
}

public class C {
     private Long id;
     private ... 
}

I'm having a List<A> with B's Object in every A and C's Object In every B.

I want to group A list by C.id in a map Map<C,List<A>>

How do I achieve this using groupingBy method of Stream?

Answer 1

Map<Long, List<A>> resultSet = 
       myList.stream()
             .collect(Collectors.groupingBy(e -> e.getB().getC().getId()));

edit:

Since you want a Map<C, List<A>> as the receiver type of the result set then you'll need to override equals and hashcode in your C class like this:

class C {
     public Long getId() {
         return id;
     }

     @Override
     public boolean equals(Object o) {
         if (this == o) return true;
         if (o == null || getClass() != o.getClass()) return false;

         C c = (C) o;

         return id != null ? id.equals(c.id) : c.id == null;
     }

     @Override
     public int hashCode() {
         return id != null ? id.hashCode() : 0;
     }

     private Long id;
}

Now, you can do:

Map<C,List<A>> resultSet = 
     myList.stream()
           .collect(Collectors.groupingBy(e -> e.getB().getC()));

This means all A objects that have the same C.id will be in the same bucket (i.e group).