In the J**A collection framework, a set is an interface for storing distinct elements. There are three main implementation classes for the SET interface: HashSet, LinkedHashSet, and TreeSet. These implementation classes differ slightly in functionality and performance, and below we will compare the differences between them in detail.
1. Hashset
Hashset is the most basic implementation class in the SET interface, which uses a hash table data structure to store elements. Hashset is characterized by the time complexity of insertion, deletion, and search operations all o(1), which is very suitable for scenarios with high performance requirements. However, since hashset does not guarantee the order of the elements, it is not suitable for scenarios where the order in which elements are inserted need to be preserved. In addition, since hashsets are non-thread-safe, additional synchronization is required if used in a multi-threaded environment.
2. LinkedHashSet
LinkedHashSet is a subclass of HashSet that maintains a doubly linked list to ensure the order in which elements are inserted while keeping them non-duplicated. Therefore, the time complexity of LinkedHashSet in insert, delete, and find operations is also o(1), and the performance is comparable to that of Hashset. However, because LinkedHashSet maintains the order of elements through a linked list, it is more memory-intensive than Hashset. Similarly, LinkedHashSet is non-thread-safe.
3. Treeset
Treeset is another implementation of the SET interface, which uses a tree structure (red-black tree) to store elements. Treeset ensures the orderliness of elements and provides rich sorting functions, such as sorting in natural order or according to custom rules. Since treeset needs to maintain the sorting state of elements, the time complexity of insert, delete, and find operations is o(log n). While TreeSets are not as performant as HashSets and LinkedHashSets, they are useful in scenarios where you need to store and access elements in a specific order. In addition, treeset is thread-safe and can be used safely in a multi-threaded environment.
To sum up, HashSet, LinkedHashSet, and TreeSet each have their own characteristics and use cases. If you need high-performance insert, delete, and lookup operations and don't care about the order of the elements, you can choose HashSet or LinkedHashSet. If you need to use the set interface in a multi-threaded environment and need to ensure the orderliness of the elements, you can choose treeset. In specific use, the appropriate implementation class should be selected according to the actual needs.