HashSet and TreeSet are both implementations of the Set interface in Java, but they have some key differences:

1. HashSet:
   • Implemented using a hash table data structure.
   • Does not guarantee any specific order of elements.
   • Allows null elements.
   • Offers faster performance for adding, removing, and checking element presence compared to TreeSet.
   • Time complexity: O(1) on average for basic operations, but O(n) in the worst case.
2. TreeSet:
   • Implemented using a self-balancing binary search tree (Red-Black tree).
   • Stores elements in a sorted order based on natural ordering or a custom Comparator.
   • Does not allow null elements.
   • Has slower performance for basic operations compared to HashSet, with a time complexity of O(log n).
   • Provides additional operations for range searches and retrieving first/last elements.

A common feature between HashSet and TreeSet is that both implementations do not allow duplicate elements.

When choosing between HashSet and TreeSet, consider the following:

• If element order is unimportant and you require fast operations, go for HashSet.
• If you need elements sorted or require additional operations, TreeSet is a better choice.

Remember, both implementations ensure uniqueness of elements.

I hope this clarifies the differences and helps you choose the appropriate implementation for your specific needs. Let me know if you have any further questions!