Question

How is a HashSet different from a TreeSet in Java?

Answer 1

In Java, both HashSet and TreeSet are implementations of the Set interface and are used to store a collection of unique elements. However, there are some differences between the two:

1. Ordering: A HashSet does not maintain any order of the elements, while a TreeSet maintains the elements in sorted order. TreeSet maintains elements in ascending order by default, but you can also provide a custom Comparator to sort the elements in a different order.

2. Internal Implementation: A HashSet is implemented using a hash table, while a TreeSet is implemented using a balanced binary search tree (usually a red-black tree).

3. Performance: HashSet has a constant time complexity O(1) for add, remove, and contains operations on average, while TreeSet has a logarithmic time complexity O(log n) for these operations. However, the actual performance of HashSet and TreeSet may depend on the specific use case and the number of elements in the set.

4. Duplicates: HashSet allows null values and does not allow duplicate elements, while TreeSet does not allow null values and does not allow duplicate elements.

5. Iteration: HashSet provides faster iteration over its elements than TreeSet.

When to use HashSet vs TreeSet:

• Use HashSet when you do not need to maintain the elements in sorted order and you want faster performance for add, remove, and contains operations.
• Use TreeSet when you need to maintain the elements in sorted order or you want to use the additional methods provided by the SortedSet interface, such as subSet(), headSet(), and tailSet().