Java Collections Interview Questions

Java collections framework is one of the fundamental aspects of java programming language. In this post, we have listed some of the commonly asked java collections interview questions and answers. Hope this helps you.

  1. What is Collections Class?
  2. What is the difference between HashMap and Hashtable?
  3. What is different between Iterator and ListIterator?
  4. What is the difference between ArrayList and LinkedList?
  5. What is EnumSet?
  6. How does HashMap work in Java?
  7. What is the difference between Array and ArrayList? When will you use Array over ArrayList?
  8. What are concurrent Collection Classes?
  9. What is the importance of hashCode() and equals() methods?
  10. What are Collection related features in Java 8?
  11. What is Big-O notation? Give some examples?
  12. How can we create a synchronized collection from given collection?
  13. What are common algorithms implemented in Collections Framework?
  14. How to decide between HashMap and TreeMap?
  15. What do you understand by iterator fail-fast property?
  16. Which collection classes are thread-safe?
  17. What is an Iterator?
  18. What is the benefit of Generics in Collections Framework?
  19. What is the difference between fail-fast and fail-safe?
  20. What are different ways to iterate over a list?
  21. Why doesn’t Map interface extend Collection interface?
  22. Can we use any class as Map key?
  23. What is BlockingQueue?
  24. What is Java Collections Framework? List out some benefits of Collections framework?
  25. Why there is not a method like Iterator.add() to add elements to the collection?
  26. Why does Iterator not have a method to get next element directly without moving the cursor?
  27. What are different Collection views provided by Map interface?
  28. What is the difference between Comparable and Comparator interface?
  29. How to avoid ConcurrentModificationException while iterating a collection?
  30. While passing a Collection as an argument to a function, how can we make sure the function will not be able to modify it?
  31. What is the difference between Enumeration and Iterator interface?
  32. What are the similarities and difference between ArrayList and Vector?
  33. What is Comparable and Comparator interface?
  34. How can we sort a list of Objects?
  35. What are the basic interfaces of Java Collections Framework?
  36. What are Queue and Stack, list their differences?
  37. Which collection classes provide random access of it’s elements?
  38. Why there are no concrete implementations of Iterator interface?
  39. What is UnsupportedOperationException?
  40. Why doesn’t Collection extend Cloneable and Serializable interfaces?

These were some of the common java collections interview questions. There are more questions that can be asked. We will keep on adding more questions in the future. If you found this post useful, please share it with others. Thanks for reading!