在java 8中对不同的对象流执行flatmap时是否可以合并列表属性？

Question

I have a stream operation which is not quite doing what I want it to

Given this data structure

CatOwner {
    id, 
    name, 
    List <CatType>
}           

This code

    List<CatOwner> catOwner = owners.stream()
               .map(CatOwnerAccount:getCatOwnerAccountDetails)
               .filter(Objects::nonNull)
               .flatMap(catOwnerFunction)
               .distinct()
               .collect(Collectors.toList());

Where

Function<CatOwnerAccountDetails, Stream<CatOwner>> catOwnerFunction = catOwnerDetails -> getCatOwners(catOwnerDetails);

I would like this to give me a merged list of unique owners, combining all their cats from different sources, so rather than producing this (as it is flatmapping and then doing a distinct on objects wioth different lists) it would merge the lists

so instead of producing this

{ id : 1, name: Steve, cattype : [tabby, ginger] }
{ id : 1, name: Steve, cattype : [black] }
{ id : 2, name: Mandy, cattype : [tortashelle] }
{ id : 2, name: Mandy, cattype : [manx, grey] }

it would produce this

{ id : 1, name: Steve, cattype : [tabby, ginger, black] }
{ id : 2, name: Mandy, cattype : [tortashelle, manx, grey]  }

Can anyone help ?

Answer 1

You can do so by creating an intermediate map, but it's not very pretty.

Collection<CatOwner> values = owners.stream()
    // blah blah, the other stuff you had
    .collect(
        Collectors.toMap(
            owner -> new Pair<>(owner.getId(), owner.getName()),
            Function.identity(),
            (a, b) -> {
                a.getTypes().addAll(b.getTypes());
                return a;
            }
        )
    )
    .values();

I used a javafx.util.Pair as the key, though any tuple class that implements equality as the combined equality of the two elements will do.

owner -> new Pair<>(owner.getId(), owner.getName())

This says that the values of the map are the CatOwner items themselves (that is, the map is Pair<Id, Name> to CatOwner ).

Function.identity()

This says that if there is an collision (two items with the same ID and name), the way to resolve that is to add all of the types from one CatOwner to the other.

(a, b) -> {
    a.getTypes().addAll(b.getTypes());
    return a;
}

The result is a Collection , since you take all of the values of the map. If you really want/need a list, you can add the values into a new list.

new ArrayList<>(values);