29. Collection Framework in Java

 

Collection Framework in Java

• Any group of individual objects which are represented as a single unit is known as the collection of objects.
• A framework is a set of classes and interfaces which provide a ready-made architecture.
• In Java, a separate framework named the "Collection Framework" has been defined in JDK 1.2 which holds all the collection classes and interface in it.
• Before the Collection Frameworks was introduced, the standard methods for grouping Java objects were Arrays or Vectors or Hashtables.

Hierarchy of the Collection Framework

• The utility package (java.util) contains all the classes and interfaces that are required by the collection framework.
• The Iterator interface provides the facility of iterating the elements in a forward direction. It is the root interface for all the collection classes.
• The Iterator interface helps easily to retrieve the elements of a collection and perform operations on each element.
• The Collection interface is not directly implemented by any class. However, it is implemented indirectly via its sub-types or sub-interfaces like List, Queue, and Set.

Collections in Java

• List interface: It inhibits a list type data structure in which we can store the ordered collection of objects.
• ArrayList class: It uses a dynamic array internally to store the duplicate elements.
• LinkedList class: It uses a doubly linked list internally to store the duplicate elements.
• Vector class: It uses a dynamic array to store the data elements. It is similar to ArrayList.
• Stack class: It implements the Last-in-First-out data structure i.e. stack.
• Queue interface: It maintains the First-in-First-out order. It can be defined as an ordered list that is used to hold the elements which are about to be processed.
• PriorityQueue class: It holds the objects which are to be processed by their priority. It doesn't allow null values to be stored in queue.
• Deque Interface: Dequeue stands for a double-ended queue which enables us to perform the operations at both the ends.
• ArrayDeque: It also enables us to perform operations at both the ends. It is faster than ArrayList and Stack.
• Set Interface: It represents the unordered set of elements which doesn't allow us to store the duplicate items.
• HashSet class: It represents the collection of unique items that uses a hash table for storage.
• LinkedHashSet class: It is an ordered version of HashSet that maintains a doubly-linked list across all elements.
  When iterating through a HashSet the order is unpredictable, while a LinkedHashSet lets us iterate through the elements in the order in which they were inserted.
• SortedSet interface: It is a alternate of Set interface in which the elements are arranged in the ascending order.
• TreeSet class: It uses a tree for storage of unique elements in ascending order.