HashSet is a collection that contains no duplicate elements. It is part of the java.util package and implements the Set interface. It is backed by a HashMap internally.

Use HashSet when you need to store unique elements and do not care about the order of insertion. Typical use cases include filtering duplicates or membership testing.

• Add: $O(1)$ (average case)
• Remove: $O(1)$ (average case)
• Contains (Search): $O(1)$ (average case)

java Standard
import java.util.HashSet;
import java.util.Set;

public class SetExample {
    public static void main(String[] args) {
        Set<Integer> uniqueIds = new HashSet<>();
        uniqueIds.add(101);
        uniqueIds.add(102);
        uniqueIds.add(101); // Duplicate, will not be added

        System.out.println("Set size: " + uniqueIds.size()); // 2
    }
}

HashSet uses an internal HashMap to store its elements.

• When an element is added to a HashSet (e.g., set.add(value)), it is actually added as a Key in the internal HashMap, with a dummy constant as the Value.
• Since keys in a HashMap must be unique, this ensures the property of the Set.

• Custom Objects: Forgetting to override equals() and hashCode() for custom objects stored in a HashSet.
• Order Assumption: Assuming that HashSet will maintain insertion order (use LinkedHashSet if order is needed).
• Modification: Modifying an object after it has been added to the set in a way that changes its hashCode (this can "lose" the object in the set).