What are the implementations of collections in the Java programming language?

• Interfaces:

  • Collection: The base interface for all collections.
  • List: An ordered collection (with indices).
  • Set: A collection that does not contain duplicates.
  • SortedSet: A collection that does not contain duplicates, with elements sorted.
  • Map: An associative array (key-value pairs).
  • SortedMap: An associative array sorted by keys.
  • Queue: A collection for storing elements before processing.
  • Deque: A double-ended queue.

• Classes (main implementations):

  • For List:

    • ArrayList: Dynamic array. Fast access by index, slow insert/delete in the middle.
    • LinkedList: Doubly linked list. Slow access by index, fast insert/delete in the middle.
    • Vector: Deprecated synchronized version of ArrayList.
    • Stack: Subclass of Vector, implements a stack (LIFO).

  • For Set:

    • HashSet: Hash table based implementation. Does not guarantee order.
    • LinkedHashSet: Hash table + linked list. Preserves insertion order.
    • TreeSet: Red-black tree based implementation. Elements sorted in ascending order.

  • For Map:

    • HashMap: Hash table based implementation. No guaranteed order.
    • Hashtable: Deprecated synchronized version of HashMap.
    • LinkedHashMap: Hash table + linked list. Preserves insertion order.
    • TreeMap: Red-black tree based implementation. Sorted by keys.

  • For Queue:

    • PriorityQueue: Queue with priority, elements dequeued according to natural order or comparator.
    • ArrayDeque: Double-ended queue based on array.

• Utilities:

  • Collections: Static methods for working with collections (sorting, searching, synchronization, etc.).

// Example of using ArrayList
List<String> names = new ArrayList<>();
names.add("Alice");
names.add("Bob");
System.out.println(names.get(0)); // Alice

// Example of using HashSet
Set<Integer> numbers = new HashSet<>();
numbers.add(1);
numbers.add(2);
numbers.add(1); // Duplicate will not be added
System.out.println(numbers.size()); // 2

// Example of using HashMap
Map<String, Integer> ages = new HashMap<>();
ages.put("Alice", 30);
ages.put("Bob", 25);
System.out.println(ages.get("Alice")); // 30

// Example of using PriorityQueue
Queue<Integer> priorityQueue = new PriorityQueue<>();
priorityQueue.add(3);
priorityQueue.add(1);
priorityQueue.add(2);
System.out.println(priorityQueue.poll()); // 1 (lowest priority)