What is the Difference Between HashSet and TreeSet — Java Interview Question
Answering Popular Java Interview Questions
`HashSet` and `TreeSet` are two implementations of the Set interface in Java, but they have different characteristics and use cases:
HashSet and TreeSet are two implementations of the Set interface in Java, but they have different characteristics and use cases:
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Underlying Data Structure:
HashSet is backed by a hash table (actually a HashMap instance). It uses hash codes to determine the location of elements in the set.
Ordering:
Elements in a HashSet are not ordered. There is no guarantee about the order of elements when iterating over the set.
Performance:
HashSet provides constant time complexity (O(1)) for basic operations such as add, remove, and contains under ideal conditions (assuming good hash function and minimal collisions).
Null Elements:
HashSet allows one null element.
Use Case:
Use HashSet when you do not care about the order of elements and you need high performance for add, remove, and check operations.
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Underlying Data Structure:
TreeSet is backed by a TreeMap. It is implemented using a Red-Black tree, which is a self-balancing binary search tree.
Ordering:
Elements in a TreeSet are ordered according to their natural ordering or by a Comparator provided at set creation time. This means elements are sorted and iterating over the set will provide them in ascending order.
Performance:
TreeSet provides O(log n) time complexity for basic operations like add, remove, and contains because of the underlying tree structure.
Null Elements:
TreeSet does not allow null elements if the set is using natural ordering or a Comparator that does not permit null. It throws NullPointerException if you try to add null.
Use Case:
Use TreeSet when you need a sorted collection of elements and can afford the overhead of maintaining the order, or when you need range queries and sorted set operations.
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HashSet: Unordered, allows null, fast operations (O(1)), backed by a hash table.
TreeSet: Ordered, does not allow null, slower operations (O(log n)), backed by a Red-Black tree.
Choosing between HashSet and TreeSet depends on whether you need ordering and what performance characteristics are most important for your application.