java8 - groupingby from a property inside an object

Question

I want to create a Map<String,List<A>> where A contains B and the groupby is made from a property from B.

public class A{
        private int a;
        private ...
        private B b;
}

public class B{
           private String name;
           private ... 
}

and I have a

List<A> alist;

and I want to do something like that :

Map<String, List<A>> resultmap = alist.stream().collect(Collectors.groupingby(B::getName()));

except this is not possible to access B.

Answer 1

You just need to write a lambda expression to access b:

Map<String, List<A>> resultMap = alist.stream()
                                      .collect(Collectors.groupingBy(a -> a.getB().getName()));

Assuming there is a getter getB() to retrieve the b property, you can call that inside the lambda expression.

---

As a side-note, your current method-reference is invalid: you don't need the parentheses inside B::getName().

Answer 2

You can do this with a lambda expression instead of a method reference :

Map<String, List<A>> resultmap =     
    alist.stream().collect(Collectors.groupingby(a->a.getB().getName()));