Java8 Streams - Compare Two List's object values and add value to sub object of first list?

Question

I have two classes:

public class ClassOne {

 private String id;
 private String name;
 private String school;
 private String score; //Default score is null

 ..getters and setters..
}

public class ClassTwo {

 private String id;
 private String marks; 

 ..getters and setters..
}

And, I have two Lists of the above classes,

List<ClassOne> listOne;
List<ClassTwo> listTwo;

How can I compare two list and assign marks from listTwo to score of listOne based on the criteria if the IDs are equal. I know, we can use two for loops and do it. But I want to implement it using Java8 streams.

List<ClassOne> result = new ArrayList<>();

for(ClassOne one : listOne) {
    for(ClassTwo two : listTwo) {
        if(one.getId().equals(two.getId())) {
            one.setScore(two.getmarks());
            result.add(one);
        }
    }
}
return result;

How can I implement this using Java8 lambda and streams?

Answer 1

Following code copies marks from ObjectTwo to score in ObjectOne, if both ids are equal, it doesn't have intermediate object List<ObjectOne> result

listOne.stream()
    .forEach(one -> {listTwo.stream()
        .filter(two -> {return two.getId().equals(one.getId());})
        .limit(1)
        .forEach(two -> {one.setScore(two.getMarks());});
    });

Answer 2

Let listOne.size() is N and listTwo.size() is M. Then 2-for-loops solution has complexity of O(M*N).

We can reduce it to O(M+N) by indexing listTwo by ids.

Case 1 - assuming listTwo has no objects with the same id

// pair each id with its marks
Map<String, String> marksIndex = listTwo.stream().collect(Collectors.toMap(ObjectTwo::getId, ObjectTwo::getMarks));
// go through list of `ObjectOne`s and lookup marks in the index
listOne.forEach(o1 -> o1.setScore(marksIndex.get(o1.getId())));

Case 2 - assuming listTwo has objects with the same id

    final Map<String, List<ObjectTwo>> marksIndex = listTwo.stream()
            .collect(Collectors.groupingBy(ObjectTwo::getId, Collectors.toList()));

    final List<ObjectOne> result = listOne.stream()
            .flatMap(o1 -> marksIndex.get(o1.getId()).stream().map(o2 -> {
                // make a copy of ObjectOne instance to avoid overwriting scores
                ObjectOne copy = copy(o1);
                copy.setScore(o2.getMarks());
                return copy;
            }))
            .collect(Collectors.toList());

To implement copy method you either need to create a new object and copy fields one by one, but in such cases I prefer to follow the Builder pattern. It also results in more "functional" code.