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Collection Interfaces

                         

The Collections Framework is made up of a set of interfaces  for storing and manipulating groups of data into a single unit. It consists of several interfaces, and classes that implement those interfaces, contained within the java.util package. It provides tools for maintaining a data container of objects. 

Different interfaces describe different types of functionalities.

The following diagrams shows the framework of core collection interfaces hierarchy.


Table of the ordered and unordered Collection interfaces shown as:

 Interface name  Ordered  Allows duplicates  Maps key to object
 Collection  No  Yes  No
 Set  No  No  No
 List  Yes  Yes  No
 Map  No  No  Yes
 SortedSet  Yes  No  No
 SortedMap  Yes  No  Yes

Collection Interface:

The Collection interface is the root interface for the Java collections hierarchy. It is extended by the List, Set, and the SortedSet interfaces. The Collection interface is used to represent a group of objects, or elements. Typically, it represents data items that form a natural group. Some collection allows duplicate elements while others do not. It consists of both ordered and unordered elements.

 The declaration of the Collection interface is shown as:

public interface Collection<E>.. 

The <E> syntax tells that, the declared interface is a generic interface i.e. when you declare a Collection instance you should specify the type of object contained in the collection.
This interface supports basic operations like adding and removing. When you try to remove an element, only a single instance of the element in the collection is removed, if it is available. 

Following methods can be used for adding and deleting an element respectively.

boolean add(Object element)      
boolean remove(Object element)

The list of other methods belonging to the Collection interface is shown in the table given below

Method

Uses

 add(E o)
    
  Adds the specified element to this set if it is not already present (optional operation).
         
 clear()   Removes all of the elements from this set (optional operation).
contains(Object o)  Returns true if this set contains the specified element.
 equals(Object o)   Compares the specified object with this set for equality.
 hashCode()   Returns the hash code value for this set.
  isEmpty()    Returns true if this set contains no elements.
 iterator()   Returns an iterator over the elements in this set.
 remove(Object o)   Removes the specified element from this set if it is present (optional operation).
 size()   Returns the number of elements in this set (its cardinality).

                         

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