What are the implementations of collections in Java?

Main implementations of collections in Java, part of the Java Collections Framework:

Interfaces:

• Collection - the basic interface for all collections.
• List - an ordered collection allowing duplicates.
• Set - an unordered collection of unique elements.
• SortedSet - an ordered Set.
• NavigableSet - a SortedSet with navigation methods.
• Queue - a collection designed for holding elements prior to processing (FIFO).
• Deque - a double-ended queue.
• Map - a collection of key-value pairs (keys are unique).
• SortedMap - an ordered Map.
• NavigableMap - a SortedMap with navigation methods.

Specific classes (implementations):

• ArrayList - array-based, fast index access, slow insertions/deletions not at the end.
• LinkedList - doubly linked list, fast insertions/deletions at beginning/end, slow index access.
• Vector - synchronized (thread-safe) version of ArrayList, deprecated (prefer CopyOnWriteArrayList).
• Stack - LIFO collection, extends Vector, deprecated (prefer Deque or ArrayDeque).
• HashSet - based on hash table (HashMap), fast search, insert, delete with good hash function. Does not preserve order.
• LinkedHashSet - extends HashSet, preserves insertion order.
• TreeSet - based on Red-Black tree, stores elements in sorted order. Fast search, insert, delete (O(log n)).
• PriorityQueue - a priority queue based on a binary heap.
• ArrayDeque - a double-ended queue implementation based on a resizable array.
• HashMap - based on hash table, fast key-value operations with good hash function. Does not preserve order.
• LinkedHashMap - extends HashMap, preserves insertion order or access order depending on configuration.
• TreeMap - based on Red-Black tree, stores key-value pairs in sorted order by keys. Fast search, insert, delete (O(log n)).
• Hashtable - synchronized (thread-safe) version of HashMap, deprecated (prefer ConcurrentHashMap).
• IdentityHashMap - compares objects by reference (==) instead of .equals().
• WeakHashMap - keys are stored as "weak" references. If no strong references exist to a key, it can be garbage collected.

Additionally, thread-safe collections in the java.util.concurrent package include:

• ConcurrentHashMap - thread-safe version of HashMap.
• CopyOnWriteArrayList - thread-safe version of ArrayList.
• CopyOnWriteArraySet - thread-safe version of HashSet, based on CopyOnWriteArrayList.
• ConcurrentLinkedQueue - thread-safe linked list-based queue.
• LinkedBlockingQueue - blocking queue based on linked list.
• ArrayBlockingQueue - blocking queue based on array.

The choice of specific implementation depends on the required operations (insertion, deletion, search, index access), the need to preserve order, element uniqueness, and thread safety.