Should I use Java 8 Streams Api to combine two Collections?

Question

I have this situation where it seems that the Java 8 Streams API would be helpful, but I'm not fully sure how it could be.

From two Collections with distinct element types, I want to build a third collection whose elements are all the possible Pairs of elements from both collections. Basically:

The two distinct element types...

public class A {}
public class B {}

A "pair" of As and Bs.

public class Pair {
   private A a;
   private B b;

   public Pair(A a, B b){
     this a = a;
     this b = b;
   }
}

The "combination" made using old-style java.util.Collection API:

 public Collection<Pair> combine(Collection<A> as, Collection<B> bs){
    Collection<Pair> pairs = new ArrayList();
    foreach(A a: as){
      foreach(B b: bs){
          Pair pair = new Pair(a,b);
          pairs.add(pair);
      }
    }
    return pairs;
 }

The ordering in the resulting pairs collection is not important. So, every instance of Pair could be created and added to the resulting collection in parallel. How could I achieve this?

The best I could figure out by myself was to use the Streams version of foreach:

as.foreach(
  a -> {
    bs.foreach(
      b -> {
          Pair pair = new Pair(a,b);
          pairs.add(pair);
      }
  }
);

This example was made trivial for the sake of simplification. The class Pair is an example of processing two elements into a third one (that is, a java.util.function.BiFunction), and adding them to a Collection is just an example of a mutable reduction.

Is there a more elegant way to do that? Or preferable, in a more profitable way in regards to efficiency? Something like

BiFunction<A,B,Pair> combinator = Pair::new; //or any other function f(a,b)=c;

Stream<Pair> pairStream = 
  Streams.unknownElegantMethod(as.stream(), bs.stream(), combinator);

Answer 1

I hope I don't have any silly typos, but basically what you can do is :

List<Pair> list = as
                  .stream()
                  .flatMap(a -> bs.stream().map (b -> new Pair(a,b)))
                  .collect (Collectors.toList());

1. First you create a Stream<A> from as.
2. For each a instance
   2.1 Create a Stream<B> of bs
   2.2 Map each b to a pair of (a,b)
3. Flatten all the pairs to a single Stream.
4. Finally I collected them to a List, though you can choose other collections.