Outer Join on list of Objects-CodePudding

I have the following objects:

class A {
   int id;
   String propA;
}

class B {
   int id;
   String propB;
}

class C {
   int id;
   String propA;
   String propB;
}

Given Lists of Objects A and B, I would like to compute a List of Objects C, which will be an object created from joining Object A with B by id; However, if the id is present in the list of objects A and not B or B and not A, I would like to include it in a list of objects C, but with missing property. In other words, doing join on following lists:

List<A> listA = new ArrayList<>();
listA.add(new A(1, "example1"));
listA.add(new A(2, "example2"));

List<B> listB = new ArrayList<>();
listB.add(new B(2, "another example2"));
listB.add(new B(3, "another example3"));

Should be equivalent to creating the following list:

List<C> listC = new ArrayList<>();
listC.add(new C(1, "example1", null));
listC.add(new C(2, "example2", "another example2"));
listC.add(new C(3, null, "another example3"));

I have seen similar issues solved with Stream API, but it was always a one-sided Outer Join; either way, I tried to adapt it to my needs.

My attempt is following:

//creating maps of ids to list elements, so I can access it from the final stream easily
Map<Integer, A> listAbyId = listA.stream().collect(Collectors.toMap(A::getId, Function.identity()));
Map<Integer, B> listBbyId = listB.stream().collect(Collectors.toMap(B::getId, Function.identity()));

//creating set of all ids that are in both lists
Set<Integer> set = listA.stream().map(a -> a.getId()).collect(Collectors.toSet());
Set<Integer> set2 = listB.stream().map(b -> b.getId()).collect(Collectors.toSet());
HashSet<Integer> setc = new HashSet<>();
setc.addAll(set);
setc.addAll(set2);

//final stream that creates list of C objects.
List<C> listC = setc.stream().map(c -> new C(c,
                                            listAbyId.get(c) == null ? null : listAbyId.get(c).getPropA(),
                                            listBbyId.get(c) == null ? null : listBbyId.get(c).getPropB()))
                            .collect(Collectors.toList());

It works as intended, but it kind of seems like overkill. Is there an easier way to do it?

CodePudding user response：

Your start is pretty good allready but you are right the MapOfA/B is a bit overkill


public static void main(String[] args) {

    List<A> listA = new ArrayList<>();
    listA.add(new A(1, "texta"));
    listA.add(new A(2, "textA"));

    List<B> listB = new ArrayList<>();
    listB.add(new B(2, "textb"));
    listB.add(new B(3, "textB"));
    
    Set<Integer> ids = listA.stream().map(a -> a.id).collect(Collectors.toSet());
    listB.forEach(b -> ids.add(b.id));
    List<C> listC = ids.stream()
            .map(id -> new C(id, listA.stream().filter(a -> a.id == id).findAny().orElse(new A(0, null)).propA,
                    listB.stream().filter(b -> b.id == id).findAny().orElse(new B(0, null)).propB))
            .toList();

    }

CodePudding user response：

Something like this:

Map<Integer, A> map = listA.stream().collect(Collectors.toMap(A::getId, item -> new C(item.getId(), item.getPropA(), null)));
listB.stream().forEach(item -> map.containsKey(item.getId()) ? map.get(item.getId()).setPropB(item.getPropB()) : map.put(item.getId(), item);

And then convert to list.