Common elements in two lists

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.

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