6 ArrayList Interview Questions (With Example Answers)

Updated 3 March 2023

ArrayList is a class in the standard Java libraries that has the potential to change its length while a program is running. It provides users with dynamic arrays in Java that can grow automatically when users insert an element when there is no space left for an additional element. Reviewing some interview questions and their sample responses can help you understand what interviewers expect from your answers, and how to prepare responses that can impress them.

In this article, we discuss six ArrayList interview questions and their example answers, and explore a list of tips to help you prepare for your own interview.

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6 ArrayList Interview Questions

Here are six ArrayList interview questions and their example answers:

1. Can you summarise ArrayList?

When interviewers ask this question, they expect you to show your knowledge of ArrayList and mention some key features of this Java class. Explain why users prefer using ArrayList and how it helps them to create resizable arrays. You may add the varied Java ArrayList methods that allow users to work with ArrayLists efficiently to your answer. Depending on the time you get to answer this question, consider explaining some methods.

Example answer: ArrayList is a class in the standard Java libraries and an array data structure-based. When I work in Java, I employ the ArrayList class to apply the functionality of resizable arrays. This allows me to implement the List interface, which extends the Collection interface. With ArrayList, it is possible to have duplicate elements in the exact order in which users have inserted them. While accessing data is fast with ArrayList, inserting elements can be time-consuming. There are many methods in Java ArrayList.

While Java ArrayList add() inserts the element to the ArrayList, Java ArrayList addAll() puts all the elements of a collection in the ArrayList. With Java ArrayList clear(), we can remove the elements. When we use Java ArrayList clone(), we can create a copy of the ArrayList. Java ArrayList contains(), Java ArrayList get() and Java ArrayList indexOf() help verify elements in the ArrayList, return the element in the specified index and return the position of the specified element, respectively. There are many more methods, such as Java ArrayList set() and Java ArrayList forEach(), that help in working with ArrayList efficiently.

Related: 15 Java 8 Interview Questions (With Example Answers)

2. Describe the process of removing duplicates from an ArrayList

Through this question, interviewers further assess your ArrayList knowledge to see what actions you take in a certain situation. List more than one method that you may follow to remove duplicates from an ArrayList to impress recruiters with your proficiency. Once you list the methods, explain them briefly and tell interviewers the possible result of each. You can also make comparisons and use reasoning to prove how one method is better than the other.

Example answer: I can follow two ways to remove duplicates from the ArrayList. By using HashSet, I can eliminate a duplicate element, but this cannot maintain the insertion order. If I use LinkedHashSet, I can delete duplicates and also preserve the insertion order, which makes it a more effective option to remove duplicates. There are some steps involved in using LinkedHashSet. First, I can copy the ArrayList elements to LinkedHashSet, then clear the ArrayList using the Java ArrayList clear() method. After this, I can copy the elements of LinkedHashSet to ArrayList and complete the process easily.

Related: How To Prepare For A Job Interview

3. How do you describe the process of deleting redundant elements from an ArrayList object to a team member?

Recruiters can ask you to convert technical knowledge into simple information. Your answer may help them recognise your strong communication skills and how well you convey messages in simple terms. This colleague working in information technology can be a person you manage or someone who manages you. In either case, they are aware of the technical principles involved in the process. Keep your explanation short and give a response that helps recruiters identify your ability to communicate with your team coherently.

Example answer: We can follow three steps to eliminate redundant elements that are present in the ArrayList. First, we can send elements from the ArrayList to a LinkedHashSet. Next is to clear the ArrayList entirely. The last step is to copy the elements of the LinkedHashSet and send them back to the ArrayList.

Related: 40 Important Array Interview Questions (With Sample Answers)

4. Is there is a way to remove duplicates from an array in Java?

This question assesses your ArrayList knowledge indirectly. When interviewers ask this question, they wish to explore your foundational knowledge of Java. To give an impressive response, show that you possess the knowledge of constructors, such as recursions and loops, and list the methods to remove duplicates with clarity.

Example answer: There are two ways through which I can remove duplicates from an array in Java. First is by using an iterator and creating a new ArrayList. Through this, I can traverse the original ArrayList that contains the duplicate values. After this, I can put the first appearance of all the elements on the new ArrayList. I can do this by using the contains() method, which is a part of the Java String class. This way, the second ArrayList can have all the elements of the original array, but without the duplicates.

The second method involves removing the duplicates by making a LinkedHashSet from the original ArrayList. This automatically erases the duplicates. My last step can convert LinkedHashSet into an ArrayList so that I have the ArrayList without the duplicates.

Related: 60 Java Collections Interview Questions (With Answers)

5. What are some similarities and differences between ArrayList and LinkedList?

Interviewers may ask this question to assess your knowledge of how Java's data structures work. Consider keeping your answer short for an impressive response, as interviewers may have a few follow-up questions for you, such as describing the behaviour of different methods with regard to ArrayList and LinkedList. Consider starting your answer by mentioning some features that both data structures have in common and explain their dependencies to discuss the differences.

Example answer: Both ArrayList and LinkedList maintain the insertion order of the elements. They are index-based data structures that also allow duplication. These lists do not follow synchronisation, which means that it is necessary for users to synchronise them externally so that they can share them between several threads. The two are different from each other in terms of their dependencies. For instance, while an ArrayList relies on an array, the LinkedList relies on the linked list data structure. This makes methods, such as contain, add and remove, behave differently for both of them.

Related: 7 Linked List Interview Questions (With Sample Answers)

6. What is the difference between HashMap and ArrayList

Recruiters may ask you to differentiate between HashMap and ArrayList to acknowledge your understanding of these classes in the collection of Java frameworks. In your response, tell interviewers how the two differ from each other in terms of their implementation and insertion order. You may also explain in your answer how they store and process data.

Example answer: While HashMap class can implement Map interface in Java, ArrayList provides the List interface. HashMap may not maintain the insertion order and there is no guarantee that the returned key-value pairs are in a specific order. HashMap stores elements with key and value pairs, which means that it stores two objects and requires more memory for the operation. ArrayList stores the elements as values and maintains an internal index of the elements. Duplicate keys cannot be present in HashMap even though it allows duplicate values, but they can exist in ArrayList.

Related: 10 Top Hashmap Interview Questions With Sample Answers

Tips For Your Upcoming Interview

Below are some tips you may follow to make a positive impression on recruiters during a job interview:

  • Practice giving responses. Practicing how to answer questions involves exploring multiple questions that interviewers may ask. This exercise includes giving mock interviews in a mirror or to your friends and family members to seek feedback, and gain confidence.

  • Research the organisation. Find out more about the company that has called you for an interview to recognise its goals, mission and success. This may help you ask recruiters questions and determine if the company can offer you an opportunity to grow in the position.

  • Organise application materials. Read the job description thoroughly to know the required documents you can submit during the interview. This helps you bring all the necessary papers other than your resume and cover letter, and shows your organisational skills and attention to detail.

  • Ask questions from interviewers. When you have questions for interviewers, it shows them your sincerity towards the role and eagerness to join their organisation. Asking questions from interviewers also helps you to clarify your doubts regarding the position and your growth in the company.

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