HashSet and TreeSet are two commonly used implementations of the Set interface in Java. Here are the key differences between them:

1. Implementation

• HashSet: Uses a hash table for storage.
• TreeSet: Implements a balanced tree structure (Red-Black tree).

2. Ordering

• HashSet: Does not maintain any order of elements.
• TreeSet: Maintains a natural ordering of elements or a custom comparator.

3. Performance

• HashSet: Offers constant time performance for basic operations (add, remove, contains).
• TreeSet: Provides logarithmic time performance for these operations due to its tree structure.

4. Null Elements

• HashSet: Allows one null element.
• TreeSet: Does not allow null elements if it uses natural ordering.

5. Use Cases

• HashSet: Best suited for fast lookups and when order is not important.
• TreeSet: Ideal when you need sorted data or range queries.

Conclusion

The choice between HashSet and TreeSet depends on your specific requirements regarding ordering, performance, and null handling. HashSet is preferable for speed and efficiency, while TreeSet is suitable when sorted order is necessary.