60 Java Collections Interview Questions (With Answers)

By Indeed Editorial Team

Published 4 May 2022

The Indeed Editorial Team comprises a diverse and talented team of writers, researchers and subject matter experts equipped with Indeed's data and insights to deliver useful tips to help guide your career journey.

Java Collections is a framework that helps store and manipulate a collection of objects in Java. It provides an essential application programming interface (API) for Java developers at both junior and senior levels. Researching Java Collections interview questions can help you prepare if you are applying for positions like a software engineer, backend developer, Java developer or website developer. In this article, we list 60 questions related to Java Collections and share sample answers of six more to help you model your own during an interview.

Basic Java Collections interview questions

Here are some of the most common Java Collections interview questions you are likely to face:

1. What are the benefits of the Java Collections framework?

The interviewer may ask this question early on as it requires only a basic knowledge of the system to answer. While it may lead to more complex questions, it is important to speak from experience instead of taking a theoretical approach. Try to display your expertise and passion.

Example answer: "The benefits of Java Collections framework include highly efficient performance, reusability, ease of maintenance and opportunities to customise collection types. In my experience as a Java developer, I found the customisation to be useful.

At my previous company, Tech Solutions, we had to make an unmodifiable map with real-time data. I did this by implementing a custom collection in Java Collections where the map interface keys annotated petrol pumps state-wide, and a simple 'get' operation pulled telemetry data from sensors at each location. This allowed us to monitor gas levels used during hurricane evacuations, which we reported back to the concerned department to help decide where more fuel can be necessary to support evacuations during a crisis.”

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2. What is the technical difference between a Collection and Collections framework?

Questions that ask you to point out differences in related terms are about understanding the unique nuance of the framework. This question also helps establish the level of technical knowledge you have about the system. Be clear and concise with each definition to show the interviewer that you know the distinction between the terms.

Example answer: "A Collection is a contained object that holds the many elements required to create an application or script. The Collections framework is the system of collections that allows for the centralised management and maintenance of each one."

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3. What are the primary interfaces used in the Java Collections framework? Describe each of them.

This question requires a more detailed answer. To be thorough in your answer, respond thoughtfully and in multiple parts. In the first part of the answer, list all five primary interfaces. In the second part of the answer, define each of them. If you are researching for an interview and come across something that has multiple types, such as interfaces in a Java Collections framework, take notes and commit those types and their functions to memory. This can help you answer these types of questions with confidence.

Example answer: "The five primary interfaces of the Java Collections framework are the collection interface, set interface, queue interface, list interface and map interface. Using these collections, developers can make several applications.

The collection interface is the core of the framework and the foundation upon which we build most projects. Set interface is similar to the set command, and it does not allow for index-based searches or duplicate entries. The queue interface stores entries in a queue based on the order one enters them. List interfaces help store the ordered collection and contain duplicate entries if desired. Key-value pairing is essential to the map interface, which is a data structure that does not allow duplicates as keys, which means one key can map to a maximum of one value.”

4. What is the advantage of using the generic collection?

This is a basic question that tests your understanding of the concept. It is necessary to know the purpose and benefits of using the generic collection in Java programming before implementing it. Think of the answer in terms of bullet points and only state the most important advantages in precise terms.

Example answer: "As a programmer, there are mainly three ways using the generic collection benefits us. In using the generic class, we do not require typecasting. As it is type-safe, we can also check it at the time of compilation. Lastly, it provides great code stability by detecting bugs during compilation."

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5. Explain the process of removing duplicates from an ArrayList, as if you are talking to a board member.

This question not only tests your knowledge but also your ability to assess professional situations and behave accordingly. As the interviewer has asked you to explain a technical concept to a board member who may not have the same technical background, it is necessary to explain the concept in simple terms so that it is easily understandable. Clarity and professionalism are key in this response. Use appropriate reasoning to justify your choice.

Example answer: "I can remove duplicates from the ArrayList in one of two ways. I can either use HashSet or LinkedHashSet. If I use HashSet, it can remove the duplicate element but it cannot preserve the insertion order. But if I use LinkedHashSet I can also maintain the insertion order along with duplicate removal. So LinkedHashSet is the better option in this case. To use it, I am going to copy the ArrayList elements to LinkedHashSet and empty the ArrayList using the clear() method. Then, I can copy all the elements of LinkedHashset to ArrayList to finish the process."

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6. How do you explain removing redundant elements from an ArrayList object to a member of your team?

This question also aims to test how you can explain a technical process in simple terms. But the core difference is that you are explaining it to a colleague in information technology. This can be someone you manage or someone who supervises you, but, ultimately, it is someone on your team who knows technical principles. In this case, it is crucial to display your ability to communicate clearly and effectively with a team. Using more technical terms is acceptable in this answer.

Example answer: "To remove the redundant element from the ArrayList, I can follow three steps. First, I can offload elements in the ArrayList to a LinkedHashSet. Next, I can make sure the ArrayList is empty, and finally, I can copy all elements of the LinkedHashSet back to an ArrayList.”

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Technical Java Collections interview questions

These are some of the more nuanced questions you may come across in a Java Collections interview:

  1. Mention some of the best practices while using Java Collections.

  2. Describe the hierarchy of the Collection Framework in Java.

  3. Differentiate between Array and collection.

  4. How do you reverse an ArrayList?

  5. Name the differences between collection and Collections.

  6. Differentiate between concurrent collection and synchronized collection.

  7. List the differences between Array and ArrayList.

  8. How do you reverse an ArrayList?

  9. Explain the differences Iterator and ListIterator.

  10. What is the default size of the load factor in a hashing based collection?

  11. Differentiate between iterator and enumeration.

  12. How do you make an ArrayList read-only in Java?

  13. What are the methods that Java queue interface provide?

  14. Name the differences between ArrayList and Vector.

  15. What are the differences between ArrayList and LinkedList?

  16. Differentiate between set and map.

  17. How do you sort an ArrayList in descending order?

  18. What are the differences between HashSet and TreeSet.

  19. List the differences between comparable and comparator.

  20. Name the differences between queue and stack.

  21. Differentiate between FailFast and FailSafe.

  22. How do you synchronise List, Set and Map elements?

  23. What are the methods that Java Stack class provide?

  24. What are the uses of Properties class and the advantages of Properties file?

  25. Explain the differences between remove() and poll() method of queue interface.

  26. What is Equals() method? Explain with an example.

  27. What are the differences between the respective remove() methods of Iterator and collection?

  28. How do you handle Hash-Collision in HashTable in Java?

  29. Tell me the differences between a list and a set.

  30. How do you convert an ArrayList to an Array and vice versa?

  31. Why does not the Collection Interface extend Serializable and Cloneable Interfaces?

  32. How do you reverse an ArrayList?

  33. Differentiate between HashMap and Hashtable.

  34. How do you synchronise an ArrayList?

  35. What is the priority queue?

  36. How do you implement HashSet in Java and how does it use hashing?

  37. What is the list interface's use in Java?

  38. Explain the differences between HashMap and TreeMap.

  39. HashTable does not allow null, whereas HashMap does. Why is it so?

  40. How do you use a custom object as a key in map, set or other collection classes?

  41. What is HashCode() method?

  42. Is there any need for synchronised ArrayList when we have synchronised Vectors in Java?

  43. What is the dictionary class?

  44. Why does not the Collection Interface extend the Map interface or vice-versa?

  45. What is CopyOnWriteArrayList?

  46. How many types of LinkedList does Java support? What are they?

  47. What is Hash-Collision in HashTable?

  48. Differentiate between Array length and ArrayList size.

  49. What is a blocking queue?

  50. How do you sort the collection objects in Java?

  51. What is NavigableMap? How is it different from Map?

  52. Why does ConcurrentModificationException occur on iteration in Java?

  53. What is the purpose of RandomAccess interface and where is it implemented?

  54. How do you use ConcurrentHashMap?

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