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Wednesday, May 10, 2023

50+ Java Collections and Generics Interview Questions and Answers

Hello guys, If you are preparing for Java interviews then you may know that Java Generics and Collections are two of the most important topics, particularly Generics where some of the trickiest Java questions can be asked. They are also used to gauge your experience and depth of your Java knowledge, hence it's imperative for both beginners and experienced Java programmers to prepare this topic well. While the Java Collection framework provides an implementation of some command data structures like ListSetArray, Hash table, Queue, Stack, PriorityQueue, etc, Generics provides Type safety.

Generics was not part of the original Java release, in fact, it was introduced later at Java 5 release but since then it has become an integral part of Java code. I haven't written a single line of code where I have used Collections but not Generics. It's natural because Java has been promoting the use of generics and provides compile-time safety with common errors like storing string into an ArrayList of Integer or vice-versa.

If you have coded in the pre-generics era then you know that how dreadful those ClassCastException can be which doesn't come when you store elements but comes at the later part of your code when you access those elements. That makes your code brittle and debugging hard because the source and the manifestation of error were at two different places.

Anyway, those eras have gone now but the knowledge of this fundamental concept is important while answering Java Generics and Collection interview questions on the real interviews. I have also seen people with experience also struggle to answer Generics questions, particularly the code ones and that's why I have listed down all frequently asked Java generics and Collections interview questions from the last couple of years.  Y

You can use this list to prepare for this topic well, btw, if you are not able to answer any questions then you can check the link or join a course like The Complete Java Masterclass to fill the gaps in your learning. It's one of the best and most up-to-date courses and I highly recommend it to all my readers.

50+ Java Collection and Generics Interview Questions Answers

Without wasting any more of your time, here is my list of frequently asked Java Collections and Generics Interview questions for Java programmers. This list is useful for both Java beginners as well as experienced Java programmers ranging from 2 to 5 years of experience.

1)  What is the Java Collection Framework? (answer)

2)  What are Generics in Java? (answer)

3)  Which are your favorite classes from Java Collection Framework? How do you choose the right collections class for the job? (answer)
Here is a nice diagram to choose different Java collection classes based upon different requirements and scenarios. 

50+ Java Collection and Generics Interview Questions with Answers

4)  When do you use Set, List, and Map in Java? (answer)

5)  Which Sorted Collection have you used? (answer)

6)  How HashSet works in Java? (answer)

7)  Which two methods you should override for an Object to be used as a key in Hash-based Collections? (answer)

8)  Can you use HashMap in a concurrent application? (answer)

9)  What is the difference between HashMap and Hashtable in Java? (answer)

10) What is the difference between Synchronized and Concurrent Collection in Java? (answer)

11) How ConcurrentHashMap works in Java? (answer)

12) What is PriorityQueue in Java? (answer)

13) What is type-erasure in Generics? (answer)

14) What is the difference between ArrayList and Vector in Java? (answer)

15) What is the difference between LinkedList and ArrayList in Java? (answer)

16) What is the difference between Hashtable and ConcurrentHashMap in Java? (answer)

17) What is the difference between LinkedHashSet and TreeSet in Java? (answer)

18) What is the difference between extends and super in Java Generics? (answer)

19) What do you mean by thread-safe collection? Give an example of a 2 thread-safe Collection in Java? (answer)

20) What is the relationship between equals and compareTo in Java? (answer)

21) What is the default size of ArrayList and HashMap in Java? (answer)

22) What is the load factor, capacity, and size of the Collection in Java? (answer)

23) What is the difference between Iterator and Enumeration in Java? (answer)

24) When does ConcurrentModificationException occur in Java? (answer)

25) What is the difference between fail-safe and fail-fast Iterator in Java? (answer)

26) What is CopyOnWriteArrayList in Java? (answer)

27) When do you use BlockingQueue in Java? (answer)

28) What is the difference between the peak() and poll() method of the Queue interface? (answer)

29) How do you find if an ArrayList contains an Object or not? (answer)

30) Can we store Integer in an ArrayList in Java? (answer)

31) How get the method of HashMap works in Java? (answer)

32) How do you sort a Collection in Java? (answer)

33) What is the difference between ListIterator and Iterator in Java? (answer)

34) What is the difference between HashSet and LinkedHashSet in Java? (answer)

35) When do you use EnumSet in Java? (answer)

36) List down 4 ways to iterate over Map in Java? (answer)

37) How to create a read-only Collection in Java? (answer)

38) What is IdentityHashMap in Java? (answer)

39) What is WeakHashMap in Java? (answer)

40) What is the difference between Comparator and Comparable in Java? (answer)

41) What is Deque? When do you use it? (answer)

42) How do you remove an Object from the Collection? (answer)

43) What is the difference between the remove() method of Collection and Iterator in Java? (answer)

44) What is an "unchecked" warning? (answer)

45) What is a bridge method? (answer)

46) How can I avoid "unchecked cast" warnings? (answer)

47) What is the "diamond" operator? (answer)
Its a new operator introduced in Java 7 and used for type inference. This means you don't need to declare type required, compiler can infer correct type from context. For example when you declare 

List<Employee> listOfEmployee = new ArrayList<>()

then compile know that ArrayList holds object of type Employee, hence no need to explicitly declare that. 

48) What are covariant return types? (answer)

50) What is the difference between bounded and unbounded wildcards in Java generics? (answer)

51) Difference between ArrayList and ArrayList in Java? (answer)

That's all about frequently asked Java Generics and Collections Interview questions for both beginners and experienced programmers. If you see most of the questions have linked to articles to my blogs because I have been discussing them in detail for a long time. You can read them to learn the underlying concepts and fill gaps in your learning.

 But, If you feel that you are looking for the answers too many times it means you may need to go through a book or a comprehensive course like The Complete Java Masterclass to learn those concepts better and in a more systematic and structured way. This will save a lot of your time instead of going back and forth between questions and answers.

Other Interview Questions Articles you may like to explore
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Thanks for reading this article so far. If you like these Java Generics and Collections interview questions then please share them with your friends and colleagues. If you have any questions or feedback then please drop a note.

P.S. - If you are new to the Java world and want to learn Java but looking for some free courses then you can also check out the list of my favorite free Java Courses. These are some of the best online courses from Udemy, Coursera, Pluralsight, Educative and other places and completely free. 

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