Upendra 
HashSet vs TreeSet
HashSet class implements the Set interface, backed by a hash table (actually a HashMap instance). It 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. This class permits the null element.
This class offers constant time performance for the basic operations (add, remove, contains and size), assuming the hash function disperses the elements properly among the buckets. Iterating over this set requires time proportional to the sum of the HashSet instance’s size (the number of elements) plus the “capacity” of the backing HashMap instance (the number of buckets).
TreeSet class implements the Set interface that uses a tree for storage. It inherits AbstractSet class and implements the NavigableSet interface. The elements are ordered using their natural ordering, or by a Comparator provided at set creation time, depending on which constructor is used.
TreeSet is basically an implementation of a self-balancing binary search tree like a Red-Black Tree. Therefore operations like add, remove, and search take O(log(n)) time. The reason is that in a self-balancing tree, it is made sure that the height of the tree is always O(log(n)) for all the operations. Therefore, this is considered as one of the most efficient data structure in order to store the huge sorted data and perform operations on it. However, operations like printing N elements in the sorted order takes O(n) time.
| Property | HashSet |
TreeSet |
|---|---|---|
| Ordering | Does not maintain any order | Maintains an object in sorted order |
| Comprasion | Uses equals() method |
Uses compareTo() method |
| Data structure | Implemented using HashTable | Implemented using a tree structure |
| Null element | Allowed | Not allowed |
| Performance | Time complexity average - O(1) |
Guaranteed O(log(n)) time cost for the basic operations |
Links
https://docs.oracle.com/javase/7/docs/api/java/util/TreeSet.html
https://docs.oracle.com/javase/7/docs/api/java/util/HashSet.html
https://www.geeksforgeeks.org/hashset-in-java/
https://www.geeksforgeeks.org/treeset-in-java-with-examples/
https://www.geeksforgeeks.org/hashset-vs-treeset-in-java/
https://www.tutorialspoint.com/difference-between-tree-set-and-hash-set-in-java