How to calculate the intersection of two sets?

java

set-operations

java-8

collections

byAlex Kataev·Oct 13, 2024

To quickly find the intersection of two sets in Java, leverage the power of the retainAll() method, which keeps only common elements.

Set<Integer> setA = new HashSet<>(Arrays.asList(1, 2, 3));
Set<Integer> setB = new HashSet<>(Arrays.asList(2, 3, 4));
setA.retainAll(setB); // Where magic happens! Intersection is [2, 3]

Avoid altering your original sets by creating a new set for the result:

Set<Integer> result = new HashSet<>(setA);
result.retainAll(setB); // Presto! Result is [2, 3], setA and setB remain at peace

What's happening under the hood?

Remember, retainAll() is a bit of a vandal. It modifies the set it's called upon, only leaving elements that also exist in the other set. It's like a crowd controller at a party, allowing only those on the guest list (setB) to stay! So if you wish to keep the original sets intact, use a copy constructor to clone a new set.

Choosing the right tool for the job

In the quest of performance, choosing the right Set implementation is vital. The retainAll method can be a marathon runner or a sprinter based on which set you use. HashSet seems to have been enjoying a protein-rich diet, offering constant-time performance for basic operations.

Third-party libraries: Stepping up the game

Sometimes, vanilla Java Collections may fall short. Welcome aboard, third party solutions! Libraries like Guava or Apache Commons Collections can offer sophisticated set operations, and bonus marks for immutability.

Alternatives worth considering

Beyond retainAll, Java offers an array of methods in its toolbox. Let's discover some of them:

Stream API is Love

Streams in Java 8 provide a functional approach:

Set<Integer> intersection = setA.stream()
                                .filter(setB::contains)
                                .collect(Collectors.toSet()); // Oh, the beauty of Java 8

With Apache Commons, Life is easy

Apache Commons Collections helps us with utility methods:

Set<Integer> intersection = SetUtils.intersection(setA, setB); // Isn't this simple?

Guava for Set Operations? Piece of cake

Guava, a widely used library, also has its arsenal for set operations:

Set<Integer> intersection = Sets.intersection(setA, setB); // Guava to the rescue!

These methods not only increase readability but also add flexibility.

Large sets: A bigger battlefield

For large sets, the game changes. Optimization becomes a key, prioritizing the iteration over the smaller set and checking against the larger one. Parallel streams and concurrency frameworks might serve as your knights in shining armor here, helping distribute the operation across multiple threads.

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