Comparing and removing object from ArrayLists using Java 8

Question

My apologies if this is a simple basic info that I should be knowing. This is the first time I am trying to use Java 8 streams and other features.

I have two ArrayLists containing same type of objects. Let's say list1 and list2. Let's say the lists has Person objects with a property "employeeId".

The scenario is that I need to merge these lists. However, list2 may have some objects that are same as in list1. So I am trying to remove the objects from list2 that are same as in list1 and get a result list that then I can merge in list1.

I am trying to do this with Java 8 removeIf() and stream() features. Following is my code:

public List<PersonDto> removeDuplicates(List<PersonDto> list1, List<PersonDto> list2) {
        List<PersonDto> filteredList = list2.removeIf(list2Obj -> {
            list1.stream()
                .anyMatch( list1Obj -> (list1Obj.getEmployeeId() == list2Obj.getEmployeeId()) );
        } );
    }

The above code is giving compile error as below:

The method removeIf(Predicate) in the type Collection is not applicable for the arguments (( list2Obj) -> {})

So I changed the list2Obj at the start of "removeIf()" to (<PersonDto> list2Obj) as below:

public List<PersonDto> removeDuplicates(List<PersonDto> list1, List<PersonDto> list2) {
            List<PersonDto> filteredList = list2.removeIf((<PersonDto> list2Obj) -> {
                list1.stream()
                    .anyMatch( list1Obj -> (list1Obj.getEmployeeId() == list2Obj.getEmployeeId()) );
            } );
        }

This gives me an error as below:

Syntax error on token "<", delete this token for the '<' in (<PersonDto> list2Obj) and Syntax error on token(s), misplaced construct(s) for the part from '-> {'

I am at loss on what I really need to do to make it work.

Would appreciate if somebody can please help me resolve this issue.

Answer 1

I've simplified your function just a little bit to make it more readable:

public static List<PersonDto> removeDuplicates(List<PersonDto> left, List<PersonDto> right) {
    left.removeIf(p -> {
        return right.stream().anyMatch(x -> (p.getEmployeeId() == x.getEmployeeId()));
    });
    return left;
}

Also notice that you are modifying the left parameter, you are not creating a new List.

You could also use: left.removeAll(right), but you need equals and hashcode for that and it seems you don't have them; or they are based on something else than employeeId.

Another option would be to collect those lists to a TreeSet and use removeAll:

TreeSet<PersonDto> leftTree = left.stream()
      .collect(Collectors.toCollection(() -> new TreeSet<>(Comparator.comparing(PersonDto::getEmployeeId))));

TreeSet<PersonDto> rightTree = right.stream()
      .collect(Collectors.toCollection(() -> new TreeSet<>(Comparator.comparing(PersonDto::getEmployeeId))));

leftTree.removeAll(rightTree);