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Java HashSet

A HashSet in Java is a collection that contains no duplicate elements. It implements the Set interface and is backed by a hash table. HashSet makes no guarantees as to the iteration order of the set; in particular, it does not guarantee that the order will remain constant over time.

1. Creating a HashSet

import java.util.HashSet;

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

2. Adding Elements

Use the add() method to add elements to the HashSet.

set.add("Apple");
set.add("Banana");
set.add("Cherry");
set.add("Apple"); // Duplicate element, will not be added

3. Iterating Over a HashSet

for (String item : set) {
    System.out.println(item);
}

4. Common Methods

  • contains(Object o) - Checks if the set contains the specified element.
  • remove(Object o) - Removes the specified element from the set.
  • size() - Returns the number of elements.
  • clear() - Removes all elements.

5. Key Takeaways

  • HashSet stores unique elements only (no duplicates).
  • Does not maintain any order of elements.
  • Allows one null element.
  • Not synchronized; external synchronization is required for thread safety.
  • Use when you need to prevent duplicates and don't care about order.
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