What collections exist in Java?

The Java Collections Framework provides a variety of collections for storing and managing data. The main interfaces and their implementations:

• Collection: the root interface.
  • List: an ordered collection that allows duplicates.
    • ArrayList: based on an array, quick access by index, slow insertion/deletion in the middle.
    • LinkedList: based on a doubly linked list, quick insertion/deletion, slow access by index.
    • Vector: thread-safe version of ArrayList, deprecated.
    • Stack: based on Vector, LIFO principle.
  • Set: a collection of unique elements, without a specific order (except SortedSet).
    • HashSet: based on a hash table, quick access and insertion/deletion.
    • LinkedHashSet: maintains insertion order.
    • TreeSet: stores elements in sorted order.
  • Queue: a collection of elements, often used for storing elements before processing in FIFO order.
    • PriorityQueue: elements are retrieved in order of their priority.
    • ArrayDeque: a double-ended queue.
  • Deque: a double-ended queue, elements can be added and removed from both ends.
• Map: stores key-value pairs, keys are unique.
  • HashMap: based on a hash table, quick access by key.
  • LinkedHashMap: maintains insertion or access order.
  • TreeMap: stores pairs in sorted order by keys.
  • Hashtable: thread-safe version of HashMap, deprecated.

Thread-safe collections:

• Collections.synchronized*: wrappers around existing collections.
• Concurrent*: package java.util.concurrent, collections with improved performance in concurrent environments.
  • ConcurrentHashMap
  • CopyOnWriteArrayList
  • ConcurrentLinkedQueue

And other less common collections.