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25+ Java Collections Interview Questions for Your Tech Interview

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Java is one of the most widely used programming languages to perform various programming and software development tasks. The Java collection framework essentially offers a structure to manipulate and store objects in the language. 

Practicing Java Collections interview questions is important to ace technical interviews as a Java developer. To give you an idea of what to expect, we’ve compiled this list of Java collections interview questions that touch upon basic and advanced interview questions.

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Let’s go ahead and look at some commonly-asked collections interview questions in Java to help you prepare for your upcoming interview.

Here’s what we’ll cover in this article:

  • Common Java Collections Interview Questions and Answers
  • Java Collections Interview Questions for Beginner Programmers
  • Java Collections Interview Questions for Experienced Programmers
  • FAQs on Java Collections Interview Questions

Common Java Collections Interview Questions and Answers

A collection in Java is essentially a group of individual objects combined and represented as a single unit. Here, we’ll look at some common Java collections interview questions and answers to help you prepare for your interview.

Q1. What does the Collections framework in Java contain?

This is one of the most commonly-asked Java Collections interview questions.

The Collections framework in Java mainly comprises Classes and Interfaces used to manipulate objects. Frameworks provide a read-made architecture for sorting, searching, insertion, deletion, and manipulation.

Q2. What are the three main types of Collections in the Java framework?

The three main types of Collections in the Java framework include -

  • Ordered Lists
  • Sets
  • Directories and Maps

Q3. Which are the main types of Interfaces in the Java Collections framework?

This is one of the most popular Java Collections interview questions asked at technical interviews.

The main types of interfaces in the Java Collections framework are -

  • The Collection Interface - This is the primary interface in the collections framework. While implementing a particular collection, it is necessary to implement the collection interface. 
  • The List Interface - This interface is an ordered collection of objects and contains duplicate elements. 
  • The Set Interface - This interface is an ordered collection of objects but does not contain duplicate elements. The set interface only includes methods inherited from the collections interface. 
  • Queue Interface - This contains objects in the form of a Queue (the queue data type) and includes elements in the first-in-first-out fashion. 
  • The Dequeue Interface - The Dequeue interface allows elements to be inserted or removed from both ends and has a double-ended queue structure. This interface incorporated the properties of both queues and stacks. 
  • The Map Interface - The map interface contains unique keys with duplicate elements. This interface doesn’t need to implement the collection interface and can exist independently.

Q4. What is the function of the Properties file in Java?

The Properties file in Java helps keep updated values in the class by not having to recompile the Java class if a change in values is imputed.

Q5. What is the difference between a List and a Set in Java collections?

This, again, is a commonly-asked Java collections interview question. The fundamental differences between a List and a Set in Java collections are -

  • A list contains duplicate elements. A set does not include duplicate elements.
  • A list is an ordered collection of elements. A set is an unordered collection of elements.
  • The List interface comprises a legacy class - the Vector Class. The Set interface does not contain any legacy classes. 
  • List interfaces allow multiple null values, but Set interfaces allow only one null value.

Java Collections Interview Questions for Beginner Programmers

If you’re a junior developer or fresher, it is important to know what type of Java collections interview questions to expect at your interview. To help you prepare and practice for your interview, we’ve compiled this list of the most popular Java collections interview questions for beginner programmers.

  1. What are the differences between HashMap and TreeMap?
  2. What are the differences between HashSet and TreeSet?
  3. Explain the difference between a List and a Set in the collections framework
  4. Explain the differences between an ArrayList and a Vector
  5. Explain the difference between Iterators and ListIterators
  6. What are the differences between Iterators and Enumerators?
  7. What do you understand about Blocking Queue in Java?
  8. What is the use of the hashcode() method?
  9. What is the function of the override() method?
  10. How do you synchronize map, set, and list elements in Java?

Java Collections Interview Questions for Experienced Programmers

Experienced developers who appear for technical interviews at top firms are expected to know their way around various Java tools and frameworks, besides being able to solve tricky coding problems in core data structures and algorithms.

Let’s go ahead and look at some sample Java collections interview questions for experienced programmers.

  1. How do you synchronize array lists in Java?
  2. How do you sort collection objects in Java?
  3. How will you convert an ArrayList into an array in Java?
  4. What do you understand about Vectors in Java? How do you synchronize Vectors?
  5. Explain the different methods contained in the Queue Interface
  6. What do you understand by a Priority Queue?
  7. Explain ConcurrentHashMap in Java and its implementation.
  8. Explain the differences between failfast and failsafe
  9. What are the differences between HashSet and HashMap?
  10. Explain Map keys in the collections framework.

Practicing these Java Collections interview questions will help you ace your next Java developer interview. You can also learn Collections Sort in Java here.

FAQs on Java Collections Interview Questions

Q1. What topics should you prepare to answer Java Collections interview questions?

To effectively answer Java collections interview questions, you have to prepare the following topics - collections interfaces, map interfaces, tree interfaces, list interfaces, set interfaces, hash tables, hash maps, linked lists, iterators, and comparators. 

Q2. Are Java Collections interview questions asked at FAANG+ interviews?

Yes. As a Java developer, you must know answers to Java collections interview questions at FAANG+ interviews. These questions are typically asked during the online assessment and on-site interviews.

Q3. What important Java frameworks should you know while preparing for technical interviews?

Some important Java frameworks that you should be aware of while giving technical interviews are - Hibernate, Spring, Angular, Struts, Grails, and Vaadin, among others. 

Q4. What are some advantages of the Java collections framework?

Some notable advantages of the Java collections framework include - i) The API is consistent as it contains a predefined set of interfaces that have readily-defined functions; ii) It increases the [performance and speed of programs and applications; iii) Using the collections framework reduced programming load and effort.

Q5. What is the average Java developer salary in the US?

The average Java developer salary in the United States is $97,343 per year, according to glassdoor.com

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