How Can I Efficiently Find Common Elements Between Multiple Lists in Java?

Determining Common Elements from Multiple Lists

In Java, you may encounter the need to find common elements shared by multiple lists. This can be a useful operation for tasks such as comparing data sets or identifying intersections. This article explores an efficient approach to retrieve the common elements between two lists using Java's collections framework.

To begin, consider the scenario where you have two ArrayList objects, each containing three integers. Your goal is to find and return the elements that are present in both lists.

Solution: Collection#retainAll()

One of the most straightforward ways to achieve this is by using the Collection#retainAll() method. This method modifies the invoking collection by removing all elements that are not also present in the specified collection parameter.

listA.retainAll(listB);

After executing the above code, listA will contain only the elements that are common to both listA and listB.

Alternative Approach: Creating a New Collection

If you need to preserve the original contents of listA, you can create a new collection that contains the common elements:

List<integer> common = new ArrayList(listA);
common.retainAll(listB);</integer>

In this case, the common list will contain only the elements that exist in both listA and listB, while the original contents of listA remain intact.

Stream-Based Solution

Java 8 introduced a powerful stream API that provides a declarative way of processing and manipulating collections. You can leverage the stream#filter() method to filter elements based on a specific predicate:

List<integer> common = listA.stream().filter(listB::contains).toList();</integer>

This stream-based approach performs a filter operation on each element in listA, checking whether it exists in listB using the contains() method. The resulting list will contain only the common elements between listA and listB.

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