What is Iterator in Java and How to use it?

If you are working on a number program and say you want to print a sequence, that’s where Iterator in Java comes into the picture. This way you can get the sequence even without adding the print statement for each and every line. So, let’s learn about Iterator in Java.

Below are the topics that I’ll be covering in this module:

• What is an iterator?
• Java Iterator Methods
• Advantages of Java Iterator
• Limitations of Java Iterator

Let’s get started!

What is an iterator?

Java mainly supports four different cursors. They are namely:

• Enumeration
• Iterator
• ListIterator
• Spliterator

Each of these Java cursors have some advantages and drawbacks. We’ll focus on Iterator in this article. 

Now, what is an iterator in Java?

Iterator is an interface that belongs to a collection framework. It allows you to traverse the collection, accesses the data element and removes the data elements of the collection. 

It is also considered as a Universal iterator as you can apply it to any Collection object. By using an Iterator, you can perform both read and remove operations. This is an improved version of Enumeration with the additional functionalities of remove-ability of an element.

Java Iterator Methods

Java iterator has 4 methods in total. Let’s understand them in detail.

Example:

class Method{ public static void main(String[] args)
ArrayList<String> list = new ArrayList<String>();
list.add("E");
list.add("D");
list.add("U");
list.add("R");
list.add("E");
list.add("K");
list.add("A");
// Iterator to traverse the list
Iterator iterator = list.iterator();
System.out.println("List elements : ");
while (iterator.hasNext())
System.out.print(iterator.next() + " ");
System.out.println();
}
}

Output: EDUREKA

Let’s take a look at ListIterator in Java.

ListIterator in Java

ListIterator in Java is an Iterator that allows users to traverse through the Collection in both directions. It contains the following methods:

Example:

public class Lists
{
public static void main(String args[])
{
// Create an array list
ArrayList al = new ArrayList();
// add elements to the array list
al.add("E");
al.add("D");
al.add("U");
al.add("R");
al.add("E");
al.add("K");
al.add("A");
// Use iterator to display contents of al
System.out.print("Original contents of al: ");
Iterator itr = al.iterator();
while(itr.hasNext())
{
Object element = itr.next();
System.out.print(element + " ");
}
System.out.println();
// Modify objects being iterated
ListIterator litr = al.listIterator();
while(litr.hasNext())
{
Object element = litr.next();
litr.set(element + "+");
}
System.out.print("Modified contents of al: ");
itr = al.iterator();
while(itr.hasNext())
{
Object element = itr.next();
System.out.print(element + " ");
}
System.out.println();
// Now, display the list backwards
System.out.print("Modified list backwards: ");
while(litr.hasPrevious())
{
Object element = litr.previous();
System.out.print(element + " ");
}
System.out.println();
}
}

Output:

Original contents of al: E D U R E K A
Modified contents of al: E+ D+ U+ R+ E+ K+ A+
Modified list backwards: A+ K+ E+ R+ U+ D+ E+

Now, let’s take a look at the advantages and limitations of this iterator interface in Java.

Advantages of Iterator in Java

Iterator in Java has the following advantages.

• You can use these iterators for any Collection class.
• Iterator in Java supports both read as well as remove operations.
• If you are using for loop you cannot update(add/remove) the Collection whereas with the help of an iterator you can easily update Collection.
• It is a Universal Cursor for the Collection API.
• The method names are very simple and are very easy to use.

Limitations of Iterator in Java

Iterators in Java has the following drawbacks:

• You can only perform forward direction iteration, that is Uni-Directional Iterator.
• Replacement and addition of a new element is not supported by the Iterator.
• ListIterator is the most powerful iterator but it is only applicable for List implemented classes. So it is not a Universal iterator.
• When you use CRUD Operations, it does not support the create and update operations.
• When you compare it with Spliterator, it does not let you iterate elements parallel. This means it supports only Sequential iteration.
• It does not support better performance to iterate a large volume of data.

This brings us to the end of this article where we have learned about how iterator in Java work. Hope you are clear with all that has been shared with you in this tutorial.

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