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

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

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.

Answer 2

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 3

This should work.
Map<String, String> collect = listTwo.stream().collect(Collectors.toMap(ObjectTwo::getId, ObjectTwo::getMarks));
listOne
.stream()
.filter(item -> collect.containsKey(item.getId()))
.forEach(item -> item.setScore(collect.get(item.getId())));