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• Using iterator() method, we can cycle over the elements of any collection class. This method is part of Collection interface and the implementations of the Collection interface(e.g., ArrayList, LinkedList, TreeSet, Vector etc) have already implemented this method.
• iterator() method returns an object that implements Iterator interface, using which we can obtain each element(one-by-one) from a collection.

Important features of Iterator

• Iterator can be used to iterate elements of a collection class.
• Iterator allows a forward direction access of a collection, starting from the first element.
• Iterator also allows us to remove an element while accessing it.

Methods in Iterator interface.

Methods	Description
boolean hasNext()	Returns true if there are more elements in a collection, otherwise it returns false.
E next()	Returns the next element in a collection.
void remove()	Removes the current element from a collection.

Using Iterator to iterate over a collection

In this code, we are populating an generic ArrayList<String> that is holding String objects and we are iterating over its element(one-by-one), by using methods provided in Iterator.

import java.util.*;

class IteratorDemo
{
public static void main(String... ar)
{
ArrayList<String> arr= new ArrayList<String>();

arr.add("C");
arr.add("A");
arr.add("4");
arr.add("F");
arr.add("x");
arr.add("1");

Iterator itr = arr.iterator();
System.out.println("Iterating over the collection using iterator() method ");

System.out.print("Contents of ArrayList are : ");
while(itr.hasNext())
{
System.out.print(itr.next() + " " );
}


System.out.println();
System.out.println("Size of arraylist = "+ arr.size());

}

}

Output is :

Iterating over the collection using iterator() method
Contents of ArrayList are : C A 4 F x 1
Size of arraylist = 6

Program Analysis

• In this code above, we have called iterator() method in ArrayList that returns an object of Iterator, pointing at the start of this ArrayList.
• Using this object, we have cycled over the elements in ArrayList by calling hasNext()method in a while loop. If hasNext() returns true then next() is called, to access the element.
• Loop stops when hasNext() returns false, i.e. there is no element left in the ArrayList to iterate.

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Removing a specific element in a collecton class, using Iterator

In this code, we are populating an generic ArrayList that is holding Integer objects and we are iterating over its element(one-by-one) to search for a specific element in it and remove it, using the methods provided in Iterator.

import java.util.*;

class IteratorDemo2
{
public static void main(String... ar)
{
ArrayList<Integer> arr= new ArrayList>Integer>();

arr.add(10);
arr.add(30);
arr.add(5);
arr.add(40);
arr.add(15);
arr.add(50);

System.out.print("Contents of ArrayList are : ");
System.out.println(arr);

//Obtaining an Iterator to the start of this collection class.
Iterator<Integer> itr = arr.iterator();

System.out.println("Removing element 40 from ArraylList");
while(itr.hasNext())
{
int element =itr.next();
if(element==40)
{
itr.remove();
}
}

System.out.print("Contents of ArrayList are : ");
System.out.println(arr);

System.out.println("Size of arraylist after removing an element = "+ arr.size());
}

}

Output is :

Contents of ArrayList are : [10, 30, 5, 40, 15, 50]
Removing element 40 from ArraylList
Contents of ArrayList are : [10, 30, 5, 15, 50]
Size of arraylist after removing an element = 5

Using the method of Iterator, we have iterating over an ArrayList to search for an element "40" in it. Once this element is found, we have removed it using the remove() method.

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