Java HashSet Class

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The HashSet class belongs to the java.util package where it serves as an implementation of the Set interface. The HashSet stores only unique items by using a hash table which provides constant-time performance for fundamental operations including add, remove, and contains (average case).

Key Features

  • Stores unique elements only
  • Unordered: does not guarantee the order of elements
  • Allows null elements (only one null allowed)
  • No indexing: can't access elements by position
  • Not synchronized

Syntax

HashSet set = new HashSet<>();

Methods

Adding Elements

 fruits.add("Apple");
fruits.add("Banana");
fruits.add("Orange");
fruits.add("Banana");

Example

 import java.util.HashSet;

public class HashSetExample {
    public static void main(String[] args) {
        HashSet colors = new HashSet<>();

        // Add elements
        colors.add("Red");
        colors.add("Green");
        colors.add("Blue");
        colors.add("Green");  // Duplicate

        // Display the set
        System.out.println("HashSet: " + colors);

        // Check existence
        if (colors.contains("Blue")) {
            System.out.println("Blue is in the set.");
        }

        // Remove an item
        colors.remove("Red");

        // Print after removal
        System.out.println("Updated HashSet: " + colors);

        // Clear the set
        colors.clear();

        // Check if empty
        System.out.println("Is the set empty? " + colors.isEmpty());
    }
}
Run ›

Note

  • If you need to store items without duplicates and order doesn't matter then HashSet is the appropriate choice.
  • To maintain element order you should choose LinkedHashSet or TreeSet over this structure.
  • This implementation lacks thread safety but offers Collections.synchronizedSet() and CopyOnWriteArraySet as concurrent alternatives.
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