What is the difference between a HashSet and a TreeSet in Java?

Both HashSet and TreeSet are two important classes in Java's Collection Framework that are used to store a collection of elements, but there are some key differences between them.

HashSet

HashSet is an unordered collection that uses a hashing algorithm to store elements. It does not allow duplicates and allows null values. As it does not maintain any order, it is the most efficient choice for membership testing, adding, and removing elements from the collection. It provides constant-time performance for basic operations, which makes it faster than TreeSet.

TreeSet

TreeSet is a sorted collection that is implemented using a self-balancing binary search tree called Red-Black tree. It sorts the elements in natural order or based on a custom Comparator provided at the time of creation. Unlike HashSet, it allows null values but does not allow duplicates. Because it maintains elements in a sorted order, it is a good choice for situations where you need to maintain sorted data. It has a performance of O(log n) for most of its operations, making it slower than HashSet.