The Collection in Java is a framework that provides an architecture to store and manipulate the group of objects.


All the operations that you perform on a data such as searching, sorting, insertion, manipulation, deletion, etc. can be achieved by Java Collections.
Java Collection means a single unit of objects. Java Collection framework provides many interfaces (Set, List, Queue, Deque, etc.) and classes (ArrayList, Vector, LinkedList, PriorityQueue, HashSet, LinkedHashSet, TreeSet, etc.).


A Java collection framework includes the following:

• Interfaces
• Classes
• Algorithm

Iterator interface:

Iterator interface provides the facility of iterating the elements in a forward direction only and modify the elements.
Iterator interface has three methods which are mentioned below:

1. public boolean hasNext() – This method returns true if iterator has more elements.
2. public object next() – It returns the element and moves the cursor pointer to the next element.
3. public void remove() – This method removes the last elements returned by the iterator.

Collection Interface:

The Collection interface is the interface which is implemented by all the classes in the collection framework. It declares the methods that every collection will have.
Some of the methods of Collection interface are Boolean add ( Object obj), Boolean addAll ( Collection c), void clear(), etc. which are implemented by all the subclasses of Collection interface.

List Interface:

A List is an ordered Collection of elements which may contain duplicates. It is an interface that extents the Collection interface. Lists are further classified into the following:

1. ArrayList
2. LinkedList
3. Vectors

List interface is implemented by the classes ArrayList, LinkedList, Vector, and Stack.
To instantiate the List interface, we must use :

• List  list1= new ArrayList();
• List  list2 = new LinkedList();
• List  list3 = new Vector();
• List  list4 = new Stack();

ArrayList:

ArrayList is the implementation of List Interface where the elements can be dynamically added or removed from the list. Also, the size of the list is increased dynamically if the elements are added more than the initial size.
The ArrayList class maintains the insertion order and is non-synchronized.

Example1:

package com.dev.praticecollection; import java.util.ArrayList; import java.util.Iterator; public class TestArrayList {                public static void main(String[] args) {                               ArrayList list = new ArrayList();                               list.add("RAM");                               list.add("Ganesh");                               list.add("Ganesh");                               list.add("Urvashi");                               System.out.println("Output:");                               // traversing list                               Iterator itr = list.iterator();                               while (itr.hasNext()) {                                              System.out.println(itr.next());                               }                } } Output: RAM Ganesh Ganesh Urvashi