清单项目之间的差异

Question

i have two lists ListOne and ListTwo. I have to find out all the elements of ListOne which does not exist in ListTwo. Similarly I have to find out all the elements of ListTwo which does not exist in ListOne .

My below code is working , but i am thinking there might be some better way

        List<Long> listOne=...Some valid values;
        List<Long> listTwo=...Some valid values;
        List<Long> listThree=new ArrayList<Long>();
        List<Long> listFour=new ArrayList<Long>();

    for (Long id: ListOne) {
        if(!listTwo.contains(id)){
            listThree.add(id);
        }
    }

    for (Long id: ListTwo) {
        if(!listOne.contains(id)){
            listFour.add(id);
        }
    }

Answer 1

Below should work for you

List<Long> listOne =...Some valid values;
List<Long> listTwo =...Some valid values;

List<Long> listThree = new ArrayList<>(listOne);
List<Long> listFour = new ArrayList<>(listTwo);
listThree.removeAll(listTwo);
listFour.removeAll(listOne);

From the java doc:

RemoveAll: Removes from this list all of its elements that are contained in the specified collection (optional operation).

Answer 2

As alternative to Lino answer, you can also rewrite your code in a functional way :

List<Long> listOneButListTwoElements = listOne.stream()
        .filter(l-> !listTwo.contains(l))
        .collect(Collectors.toList());


List<Long> listTwoButListOneElements = listTwo.stream()
        .filter(l-> !listOne.contains(l))
        .collect(Collectors.toList());

Answer 3

To be really efficient, you could do something like that:

Set<Long> setOne = new HashSet<>(listOne);
Set<Long> setTwo = new HashSet<>(listTwo);

List<Long> listThree = listOne.stream()
        .filter(e -> !setTwo.contains(e))
        .collect(Collectors.toList());
List<Long> listFour = listTwo.stream()
        .filter(e -> !setOne.contains(e))
        .collect(Collectors.toList());

This way you avoid the problem with m*n complexity for list lengths m and n .

---

Edit: If you prefer having no intermediary variables (as @davidxxx mentioned), you could do it like:

List<Long> listThree = listOne.stream()
        .filter(not(new HashSet(listTwo)::contains))
        .collect(Collectors.toList());

There are various ways to negate a predicate. No need to cover it here, it has been answered .