Java streams compare object from two streams and create new Object

Question

I have two Lists:

List<Object1> listOne = provider.getObjects().stream().collect(Collectors.toList());
List<Object2> listTwo = provider2.getObjects().stream().collect(Collectors.toList());

Now I want create List containg all possible Object1-Object2 combinations: List<ObjectCombinations> result;

class ObjectCombinations { 
    Object1 object1; 
    Object2 object2;

    public ObjectCombinations(Object1 object1, Object2 object2)  { 
        this.object1 = object1;
        this.object2 = object2;
    }
}

How is that possible with java 8 streams?

score 2 · Accepted Answer · answered Oct 14 '20 at 05:10

2

You can use flatMap to get all the combinations:

List<ObjectCombinations> result =
    listOne.stream()
           .flatMap(o1 -> listTwo.stream()
                                 .map(o2 -> new ObjectCombinations(o1,o2)))
           .collect(Collectors.toList());

First you create a Stream<Object1>, then you use flatMap to combine each element of that stream with all the elements of listTwo, and create the ObjectCombinations instances, which you collect to a List.

answered Oct 14 '20 at 05:10

Eran

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What if we use map instead of flatmap? – Jeet Kumar Oct 14 '20 at 05:23
@JeetKumar you will get a different result (a `Stream>` instead of `Stream`, which will be collected to a `List>` instead of `List`). – Eran Oct 14 '20 at 05:28

Eklavya · Answer 2 · 2020-10-14T05:22:33.527

1

You can use flatMap where you can stream over 2nd list and create ObjectCombinations and flatten the list.

List<ObjectCombinations> res = 
       listOne.stream()
              .flatMap(a -> listTwo.stream().map(b -> new ObjectCombinations(a,b)))
              .collect(Collectors.toList());

edited Oct 14 '20 at 05:22

answered Oct 14 '20 at 05:12

Eklavya

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Java streams compare object from two streams and create new Object

2 Answers2