Here, we have listed the most important top 50+ Java Iterator interview questions with the best possible answers.

These Iterator interview questions in java can be asked from the freshers and experienced.

We have also covered some important Iterator coding interview questions in Java that can be asked in technical tests and interviews.

So, you should practice to crack the interviews of different companies.

Iterator Interview Questions with Answers

1. What are Iterator or cursor in Java?

Answer: An iterator in Java is a special type of object that provides sequential (one by one) access to the elements of a Collection object. It is introduced in Java 1.2 Collections Framework.

We use iterator in the Collections Framework to retrieve elements sequentially (one by one). It is called a universal Iterator or cursor.

2. What are the types of Iterators in Java?

Answer: There are four types of iterators or cursors available in Java. They are as:

• Enumeration
• Iterator
• ListIterator
• Spilterator

3. What is Iterable Interface in Java?

Answer: The Collection interface extends Iterable interface that is present at the top of the collection hierarchy. The iterable interface is present in java.lang.Iterable package .

It provides a uniform way to retrieve the elements one by one from a collection object.

4. The iterable interface provides which method in Java?

Answer: The iterable interface provides just one method named iterator() which returns an instance of Iterator and provides access one by one to the elements in the collection object.

5. How to create Iterator object in Java?

Answer: We create Iterator object by calling iterator() method which is present in the iterable interface. The general syntax for creating Iterator object is as follows:

a) Iterator itr = c.iterator(); // c is any collection object.
b) Iterator itr = c.iterator(); // Generic type.

For example:
Iterator itr = c.iterator();
Iterator itr = c.iterator();

6. How Java Iterator works internally?

Answer: Go to this tutorial to get the best answer with diagram: Iteraor in Java

7. What are Iterator methods provided by Iterator interface in Java?

Answer: The Iterator interface provides three methods in Java. They are as:

(a) public boolean hasNext(): Return true if the iteration has more elements to traverse (iterate) in the forward direction.

(b) public Object next(): Return next element in the collection. It will throw NoSuchElementException when the iteration is complete.

(c) public void remove(): Removes the last or the most recent element returned by the iterator.

8. What are the advantages of iterating elements of collections using iterator?

Answer: Java Iterator provides the following advantages. They are:

• An iterator can be used with any collection classes.
• We can perform both read and remove operations.
• It acts as a universal cursor for collection API.

9. What are the limitation of Iterator in Java?

Answer: Iterator has the following limitations or drawbacks. They are:

• By using Iterator, we can iterate elements of collection only in the forwarding direction. We cannot iterate in the backward direction. Hence, these are called single-direction cursors.
• We can perform either read operation or remove operation.
• We cannot perform the replacement of new objects.

10. What is fail-fast iterator?

Answer: When an Iterator throws a ConcurrentModification exception while iterating a collection, it is called fail-fast iterator in Java.

It throws a ConcurrentModification exception under either of the following conditions:

(a) Fail Fast iterator throws a ConcurrentModificationException if the underlying collection is structurally modified while iterating over it. Structural modification is an operation that adds or deletes one or more elements in the collection.

(b) In a multi-threading environment, if one thread is attempting to modify a collection while another thread is iterating over it.

(c) When fail-fast iterator uses an original collection to traverse over elements of collection.

Some examples of fail-fast iterators are HashMap, ArrayList, Vector, HashSet, etc.

11. What is fail-safe Iterator?

Answer: When an Iterator doesn’t throw any exception while modifying the collection during iteration, it is called fail-safe iterator.

Fail-safe iterator makes the copy of the original collection to traverse over the elements. Thus, the original collection remains structurally unchanged.

Some examples of fail-safe iterators are ConcurrentHashMap, CopyOnWriteArrayList, etc.

12. What is the difference Fail-fast Iterator and Fail-safe Iterator in Java?

Answer: The fundamental difference between Fail-fast and Fail-safe Iterator is that the Fail-safe does not throw any ConcurrentModificationException in modifying the collection object during the iteration.

While fail-fast Iterator throws an exception in such scenarios. This is because fail-safe iterator works on a clone of original collection.

There are also several other differences between them based on different parameters. Let’s know them:

(a) Exception:

• Fail-fast iterator throws a ConcurrentModificationException in modifying the collection object during the iteration.
• While fail-safe iterator does not throw any kind of exception.

(b) Clone of collection object:

• Fail-fast iterator does not create a clone of collection object during iteration process. It works on the original collection.
• While fail-safe iterator creates a copy or clone of collection object during the iteration process.

(c) Memory utilization:

• Fail-fast requires low memory during the process.
• While fail-safe requires more memory during the process.

(d) Modification:

• Fail-fast iterator does not allow modification during iteration.
• While fail-safe iterator allows modification during the iteration process.

(e) Performance:

• Fail-fast iterator is fast.
• While fail-safe iterator is slightly slower than Fail Fast.

(f) Examples:

• The fail-fast iterator returned by classes are HashMap, ArrayList, Vector, HashSet, etc.
• The fail-safe iterator returned by classes are CopyOnWriteArrayList, ConcurrentHashMap, etc.

13. Explain different ways to iterate over collection in Java.

Answer: There are many ways to iterate over elements of a collection in Java. Following are the most common methods.

(a) Using enhanced For loop: The following code is an example.

Collection collection = Arrays.asList("AA", "BB", "CC", "DD", "EE");
for(String s : collection) {
     System.out.println(s);
}

(b) Using Iterator method: The following code is an example.

Collection collection = Arrays.asList("One", "Two", "Three", "Four", "Five");
Iterator itr = collection.iterator();
while(itr.hasNext()) {
    System.out.println(itr.next());
}

(c) Using Simple For loop: A sample code is as:

List list = Arrays.asList("One", "Two", "Three", "Four");
for( int i = 0; i 
(d) Using forEach method: An example code is as:
Collection collection = Arrays.asList("Apple", "Orange", "Banana", "Guava");
collection.forEach(s -> System.out.println(s));

This is recently introduced in Java 8. It can be invoked on any Iterable and takes one argument implementing the functional interface java.util.function.Consumer.
14. Suppose that you are iterating over a collection using Iterator. Can you remove an element from the collection using its Iterator? If yes, how to?
Answer: Iterator interface provides a method named remove() that removes the last element that was returned by the next method.
The following example code will remove “Two” from the List using Iterator.
List list = new ArrayList();
 list.add("One");
 list.add("Two");
 list.add("Three");

Iterator iterator = list.iterator();
while(iterator.hasNext())
{
  String str = iterator.next();
  if(str.equals("Two"))
    iterator.remove();
}

15. Difference between Enumeration and Iterator.
Answer: Both are useful in retrieving elements from a collection. But the major difference between Enumeration and Iterator is regarding functionality.
(a) By using an enumeration, we can perform only read access, but using an Iterator, we can perform both read and remove operation.
(b) Iterator is fail-fast while enumeration is not fail-fast.
(c) Iterator is slower than enumeration.
16. What is ListIterator in Java?
Answer: ListIterator is the most powerful iterator or cursor. It is a bi-directional cursor that retrieve the elements from a collection object in both forward and reverse directions. It is an interface.
17. How to create ListIterator object in Java?
Answer: We can create a ListIterator object by calling listIterator() method of the List interface. The general syntax for creating ListIterator object is as:
ListIterator litr = l.listIterator(); // l is any list object and Type is type of objects being iterated.
For example:
ListIterator litr = l.listIterator();

18. How Java ListIterator works internally?
Answer: Refer to this tutorial for better answer: ListIterator in Java
19. What are the advantages of ListIterator in Java?
Answer: ListIterator provides several advantages. They are:
ListIterator supports many operations, such as read, remove, replacement, and the addition of new objects.
Using ListIterator, we can perform Iteration in both forward and backward directions.
Methods of ListIterator are easy to use.
20. What are the limitations of ListIterator in Java?
Answer: ListIterator is the most powerful cursor, but it still has some limitations. They are as:
ListIterator is applicable only for the list implemented class objects. Therefore, it is not a universal Java cursor.
It is not applicable to whole collection API.
21. What is the fundamental difference between Iterator and ListIterator in Java?
Answer: The major difference between Iterator vs ListIterator in Java is as:
(a) Iterator is applicable to the whole Collection API. While ListIterator is only applicable for List implemented classes such as ArrayList, CopyOnWriteArrayList, LinkedList, Stack, Vector, etc.
(b) It is a Universal Iterator. While ListIterator is not a Universal Iterator.
(c) Iterator supports only forward direction iteration. While ListIterator supports both forward and backward direction iterations.
(d) It supports only read and delete operations. While ListIterator supports all the operations such as read, remove, replacement, and the addition of the new elements.
(e) It is known as a uni-directional iterator. While ListIterator is also known as bi-directional iterator.
22. Can we modify the collection structure while iterating using for-each loop?
Answer: While executing a for-each loop, we cannot modify the collection.
23. Why is Iterator considered more thread safe?
Answer: Iterator is considered being more thread safe because it throws exception if we modify the collection while iterating.
24. Can we modify the collection structure while iterating using an Iterator?
Answer: It is not permissible for one thread to modify the underlying collection structure while another thread is iterating over it.
25. How to avoid ConcurrentModificationException while iterating a collection?
Answer: To avoid ConcurrentModificationException while iterating a collection, use concurrent collection class such as CopyOnWriteArrayList instead of ArrayList.
In this tutorial, we have listed the top 25 Iterator interview questions in Java with the best possible answers. Hope that you will have understood all the answers of the above Iterator questions and practiced them.

All the best!!!
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