Collections in Java

Collection Framework:

  • Collection framework contains a set of classes and interfaces which are used for representing and manipulating collections.
  • Collection is an object representing a group of objects.Collection Frame Work
  • Collection interface is the root interface from which the interfaces List, Set,Queue are extended.

List interface:

  • List interface is an ordered collection in which duplicate elements are also allowed.
  • Elements are stored in a sequential way and hence its elements can be accessed using the index value.
  1. ArrayList
  2. LinkedList
  3. Vector

Set interface:

  • Set is an unordered collection.
  • It does not maintain any order while storing the elements.
  • It does not allow duplicate elements.

Thus if one requires to store a group of unique elements, set can be used.

  1. HashSet
  2. LinkedHashSet
  3. TreeSet

Map interface:

Map is an interface which maps keys to values in which each key has to be unique.

  1. HashMap
  2. Hashtable
  3. TreeMap

Retrieving elements from collection:

Iterator interface:
Iterator interface is used to iterate over a collection object and retrieve the elements one after the other.

ListIterator interface:
ListIterator is similar to Iterator interface except that the elements of the collection can be retrieved from both forward and backward direction.

Enumeration interface:
Enumeration interface is similar to Iterator interface except that it cannot remove elements and can only iterate over the collection object.

Iterating using enhanced for loop:
Enhanced for loop can also be used to iterate over the collection object.

Example:

 

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