check number of elements in list A present in list B java

user_1856744 0
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I have two lists (arraylists) and I would like to know effective way to check number of elements of List A present in List B.

List<String> listA = new ArrayList<>();
List<String> listB = new ArrayList<>();

I would like to know how many elements in listA are present in listB

I can loop over and check but I am looking for an effective and fastest way I can do using java 1.8

Thanks for help.

collections java
4 Answers

Another easy approach without looping and without creating a third list could be the following:

var listA = new ArrayList<>(list1);
var listB = new ArrayList<>(list2);
listB.retainAll(listA);
System.out.println(listB);

Note that this approach assumes that there are no duplicates in the list but should this be the case, you can simply create two sets out of the lists on hand and apply the same approach.

user_29259620

You can create third list which contain the same content of listA, then remove listB from the new list and then you can distinct how many elements in listA are present in listB like this:

List<String> listC = new ArrayList<>(listA);
listC.removeAll(listB);
int elementsPresentInListB = listA.size() - listC.size();
user_55580720

All you need to do is just to find the size of the set intersection. You can create a set from listB. Then iterate over each element in listA and check for it's containment in the set. The set contains takes O(1) time for a hash-based set. So, this takes O(n) time and space complexity. Here's how it looks.

final Set<String> setB = new HashSet<>(listB);
int c = 0;
for (String string : listA)
    if (setB.contains(string))
        c = c + 1;

A more succinct solution would be:

setB.retainAll(listA);
c = setB.size();

This operation effectively modifies this set so that its value is the intersection of the two sets.

user_41085900

Using Java 8's Stream API, you can use the following to get the intersection of your two lists:

Set<String> intersection = listA.stream()   // creates stream
                                .distinct() // filters out any duplicates
                                .filter(listB::contains) // filters the intersection
                                .collect(Collectors.toSet()); // reduces to set
int amount = intersection.size(); // your amount of duplicate elements
user_114410110
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