Java Collections

Collections in Java

A collection is group of objects. Java Collections framework is consist of the interfaces and classes which helps in working with different types of collections such as lists, sets, maps, stacks and queues etc.

These ready-to-use collection classes solve lots of very common problems where we need to deal with group of homogeneous as well as heterogeneous objects. The common operations in involve add, remove, update, sort, search and more complex algorithms. These collection classes provide very transparent support for all such operations using Collections APIs.

Java Collections Hierarchy:

1. List
Lists represents an ordered collection of elements. Using lists, we can access elements by their integer index (position in the list), and search for elements in the list (Because list extends Randomaccess interface). index start with 0, just like an array.

Some useful classes which implement List interface are – ArrayList, LinkedList, Stack, Vector and CopyOnWriteArrayList.

2. Set
Sets represents a collection of sorted elements. Sets do not allow the duplicate elements. Set interface does not provides guarantee to return the elements in any predictable order; though some Set implementations store elements in their natural ordering and guarantee this order.

Some useful classes which implement Set interface are – ConcurrentSkipListSetCopyOnWriteArraySetEnumSetHashSetLinkedHashSet and TreeSet.

3. Map
The Map interface enable us to store data in key-value pairs (keys should be immutable). A map cannot contain duplicate keys; each key can map to at most one value.

The Map interface provides three collection views, which allow a map’s contents to be viewed as a set of keys, collection of values, or set of key-value mappings. Some map implementations, like the TreeMap class, make specific guarantees as to their order; others, like the HashMap class, do not.

Some useful classes which implement Map interface are – ConcurrentHashMap, ConcurrentSkipListMap, EnumMap, HashMap, Hashtable, IdentityHashMap, LinkedHashMap, Properties, TreeMap and WeakHashMap.

4. Stack
The Java Stack interface represents a classical stack data structure, where elements can be pushed to last-in-first-out (LIFO) stack of objects. In Stack we push an element to the top of the stack, and popped off from the top of the stack again later.

5. Queue
A queue data structure is intended to hold the elements (put by producer threads) prior to processing by consumer thread(s). Besides basic Collection operations, queues provide additional insertion, extraction, and inspection operations.

Queues typically, but do not necessarily, order elements in a FIFO (first-in-first-out) manner. One such exception is priority queue which order elements according to a supplied Comparator, or the elements’ natural ordering.

In general, queues do not support blocking insertion or retrieval operations. Blocking queue implementations classes implement BlockingQueue interface.

Some useful classes which implement Map interface are – ArrayBlockingQueue, ArrayDeque, ConcurrentLinkedDeque, ConcurrentLinkedQueue, DelayQueue, LinkedBlockingDeque, LinkedBlockingQueue, LinkedList, LinkedTransferQueue, PriorityBlockingQueue, PriorityQueue and SynchronousQueue.

6. Deque
A double ended queue (pronounced “deck“) that supports element insertion and removal at both ends. When a deque is used as a queue, FIFO (First-In-First-Out) behavior results. When a deque is used as a stack, LIFO (Last-In-First-Out) behavior results.

This interface should be used in preference to the legacy Stack class. When a deque is used as a stack, elements are pushed and popped from the beginning of the deque.

Some common known classes implementing this interface are ArrayDeque, ConcurrentLinkedDeque, LinkedBlockingDeque and LinkedList.

Java collection examples:

  1. ArrayList
  2. LinkedList
  3. HashMap
  4. Hashtable
  5. LinkedHashMap
  6. TreeMap
  7. HashSet
  8. LinkedHashSet
  9. TreeSet
  10. Comparable
  11. Comparator
  12. Iterator
  13. ListIterator
  14. Spliterator
  15. PriorityQueue
  16. PriorityBlockingQueue
  17. ArrayBlockingQueue
  18. LinkedTransferQueue
  19. CopyOnWriteArrayList
  20. CopyOnWriteArraySet
  21. Collection Sorting
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