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A Guide to Iterator Implementation in Java

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11th Jun, 2020
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A Guide to Iterator Implementation in Java

In Java, there’s a utility that is used to generate consecutive elements from a series, known as the java iterator. Implemented by all the collection classes, these iterator implementations in Java return an object which then sets off the interface. Since the Java collections framework is made up of a combination of classes that runs on reusable datasets, the object returned via such an iterator interface can travel across all elements in a collection. 

There are specific characteristics of Java Iterators that you need to keep in mind before executing:

  • The collection framework in Java comes equipped with the iterator () function that can help in executing READ and REMOVE commands.
  • The iteration method uses the next() function to move through the elements, in an auto-loop, unlike other iterators.
  • Each iterator can only travel once through all elements, and once the end side of the series is reached, it’s time for you to get another iterator. 

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Executing Iterator Implementation in Java Code

Since each framework comes inherent with an object that gets returned, such methods can be used to access the elements in the following manners: 

  • Invoking the iterator() method.
  • Creation of a loop that can call the boolean hasNext(), which returns a boolean variable. Once the variable has a real value, it allows the iterator to travel through the collection. 
  • Calling E next() function to get to the next element in the collection. 
  • The default void remove() can be used to remove the underlying collection of the last element that gets returned via the iterator.
  • The default void forEachRemaining() function helps the iterator to perform any given function across every element until the entire collection completes processing. 

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Using the hasNext() and Next() functions in iterator implementation in java

As we already know, the next() function helps the iterator travel across all the collection elements one at a time; therefore, once the iterator reaches the end of the collection, next() sets off an exception. This is why using a hasnext() function before next() ensures that the iterator has the next object in line with the collection, to get rid of any exception. 

Object remove(): With the help of the object remove() function, you can delete and return the last object that was traveled by the iterator and was returned by next(). In case of any modification in the object since its previous visit, the function IllegalStateException() comes into effect.

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Here’s one simple example of how the functions take place:

ArrayList abc = new ArrayList();

Iterator xyz = xyz.iterator();;




import java.util.*;

class iteratorTrial


public static void main (String [] arg)


List trialList = new ArrayList();

String trialArray[] = {“Harry”, “Jack”, “Jacob”, “Maggie”, “George”};

for (int i=0; i<trialArray.length; i++)




Iterator xyz = trialList.iterator();

while (xyz.hasNext())




The Output generated from the above code is:






In the above example, the iterator xyz gets created for an ArrayList abc that once removes the first elements and the adjacent objects, performs a quick and easy iteration. 

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Custom Class Iterator Implementation in Java

For any user-defined custom classes, you can iterate the elements using the following methods:

  • Set and define a custom class.
  • Assign and define the collection class to the custom class.
  • In this collection class, the iterable interface needs to be implemented with the custom class as a type parameter.
  • The same collection class should be able to perform an iterator () function.
  • Once these instructions are followed in the Custom class, it is all set to run an iterator object or Enhanced For Loop() for its elements.

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Advantages and Disadvantages of Iterator Implementation in Java

  • Applicable to any Collection class
  • A universal cursor for Collection API, with secure methods and operations


  • Doesn’t support CREATE and UPDATE operations
  • Iteration only occurs in a single direction
  • ListIterator and Spliterator are better cursors in java that allow parallel iteration and bulk data iteration. 

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Enumeration and Iterator Implementation in Java: Similarities and Dissimilarities 

  • Both the enumerator and iterator are java cursors and can be used for iteration of collection class elements one at a time, in a single direction. 
  • Both the cursors support READ methods.


  • The enumeration is a legacy interface, first executed in Java 1.0, and is limited to Legacy collection classes, unlike that of Iterator. 
  • The enumerators aren’t universal cursors and therefore have tricky function names that are limited to carry out a READ function. 
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That’s it for iterator implementation in Java, folks! Hope you know now how to set off iterations in the right way. 

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Rohan Vats

Blog Author
Software Engineering Manager @ upGrad. Passionate about building large scale web apps with delightful experiences. In pursuit of transforming engineers into leaders.

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