In Java, the Java Collections Framework provides several implementations of the Set interface to cater to different use cases and performance requirements. Here are some of the commonly used set implementations:


HashSet: HashSet is the most commonly used set implementation. It uses a hash table to store elements, which allows for fast insertion, deletion, and lookup operations. However, it does not guarantee any specific order of elements.

LinkedHashSet: LinkedHashSet is similar to HashSet, but it maintains the order of elements based on their insertion. It uses a hash table along with a linked list to achieve this, providing a predictable iteration order.

TreeSet: TreeSet is an implementation based on a red-black tree data structure. It maintains elements in sorted order, making it suitable for scenarios where you need to access elements in a sorted manner.

EnumSet: EnumSet is a specialized set implementation designed to work exclusively with enum types. It offers a highly efficient and memory-optimized representation of enum values.

CopyOnWriteArraySet: CopyOnWriteArraySet is a concurrent set implementation that provides thread-safety during iteration. It achieves this by creating a new copy of the underlying array when modifying the set, ensuring that the original set remains unchanged during iteration.

HashSet (Java 9+): In Java 9, HashSet was enhanced to provide a more memory-efficient representation for small sets. It is internally backed by a compact array-based representation to reduce memory overhead for sets with a small number of elements.


Each of these set implementations has its own strengths and is suitable for different use cases. When choosing a set implementation, consider factors like performance requirements, element ordering, thread-safety, and memory efficiency to select the most appropriate one for your specific needs.