Common elements in two lists

Solution 1:

Use Collection#retainAll().

listA.retainAll(listB);
// listA now contains only the elements which are also contained in listB.

If you want to avoid that changes are being affected in listA, then you need to create a new one.

List<Integer> common = new ArrayList<Integer>(listA);
common.retainAll(listB);
// common now contains only the elements which are contained in listA and listB.

Solution 2:

You can use set intersection operations with your ArrayList objects.

Something like this:

List<Integer> l1 = new ArrayList<Integer>();

l1.add(1);
l1.add(2);
l1.add(3);

List<Integer> l2= new ArrayList<Integer>();
l2.add(4);
l2.add(2);
l2.add(3);

System.out.println("l1 == "+l1);
System.out.println("l2 == "+l2);

List<Integer> l3 = new ArrayList<Integer>(l2);
l3.retainAll(l1);

    System.out.println("l3 == "+l3);

Now, l3 should have only common elements between l1 and l2.

CONSOLE OUTPUT
l1 == [1, 2, 3]
l2 == [4, 2, 3]
l3 == [2, 3]

Solution 3:

Why reinvent the wheel? Use Commons Collections:

CollectionUtils.intersection(java.util.Collection a, java.util.Collection b)

Solution 4:

Using Java 8's Stream.filter() method in combination with List.contains():

import static java.util.Arrays.asList;
import static java.util.stream.Collectors.toList;

/* ... */

List<Integer> list1 = asList(1, 2, 3, 4, 5);
List<Integer> list2 = asList(1, 3, 5, 7, 9);
    
List<Integer> common = list1.stream().filter(list2::contains).collect(toList());