In this article, we will discuss Iterator interface in detail.

1. Key points about Iterator:

• This is introduced in Java 1.2 version
• Part of Collection framework
• This is applicable for every collection classes

2. Iterator interface:

• Iterator interface works on all collection classes
• This is part of collection framework introduced in Java 1.2 version
• Iterator interface allows to read objects one-by-one from collection object
• Also, we can perform remove operation, which is not possible in the legacy Enumeration interface while iterating over collection objects
• Signature:

public Iterator iterator(); // of Collection interface

• Present in java.util package

Q) How to get Iterator object ?

• We can create Iterator object using iterator() method of Collection interface
• For example,

Iterator itr = col.iterator(); // col is a any collection object

3. Advantages of Iterator over Enumeration:

• Iterator interface allows to perform both read & remove operations while iterating over collection items
• This is one of the shortcoming/limitation in legacy Enumeration interface
• Which Iterator overcomes it

4. Limitation of Iterator interface:

• It is uni-directional or single-directional cursor
• But NOT bi-directional cursor
• That is, Iterator interface is applicable for all collection classes but we can iterate over collection items only in FORWARD direction
• Iterating over collection items in backward direction is not possible
• Though both read/remove operations are allowed, but addition of new object is not allowed or
• replacing old object with new object

5. Iterator interface methods:

6. Iterator examples:

IteratorExample.java

package in.bench.resources.java.collection;

import java.util.ArrayList;
import java.util.Iterator;

public class IteratorExample {

	public static void main(String[] args) {

		// creating ArrayList object of type String
		ArrayList<String> al = new ArrayList<String>();

		// adding elements to ArrayList object
		al.add("Sundar Pichai");
		al.add("Satya Nadella");
		al.add("Shiv Nadar");
		al.add("Shantanu Narayen");
		al.add("Francisco D’Souza");

		// creating Iterator reference
		Iterator&amp;lt;String&amp;gt; ceo = al.iterator();

		// enumerating using while loop
		while (ceo.hasNext()){
			System.out.println(ceo.next());
		}
	}
}

Output:

Sundar Pichai
Satya Nadella
Shiv Nadar
Shantanu Narayen
Francisco D’Souza

Related Articles:

• Enumeration
• List Iterator
• Enumeration v/s Iterator v/s ListIterator

References:

• https://docs.oracle.com/javase/tutorial/collections/intro/
• https://docs.oracle.com/javase/tutorial/collections/interfaces/collection.html
• https://docs.oracle.com/javase/7/docs/api/java/util/Collection.html
• https://docs.oracle.com/javase/tutorial/collections/interfaces/list.html
• https://docs.oracle.com/javase/8/docs/api/java/util/List.html
• https://docs.oracle.com/javase/8/docs/api/java/util/ArrayList.html
• https://docs.oracle.com/javase/8/docs/api/java/util/Vector.html
• https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html

Happy Coding !!
Happy Learning !!