Top 50 Java Collections + Generics Interview Questions and Answers for 1 to 3 Years Experienced

Hello guys, if you are preparing for Java interviews and looking for frequently asked Java generics and Java Collections interview questions then you have come to the right place. In the past, I have shared 130+ core java questions and best courses for Java interviews and In this article, I am going to share the best Java collection and Java Generics interview questions to crack the interview. You will not only get exposure to frequently asked questions but also learn very useful Java topics that will help you in your day job.  By the way, Java Collection and Generic are very important topics for Java Interviews. They also present some of the hardest questions to a programmer when it comes to interviews, especially Generics.
It's not easy to first understand what a particular piece of code doing with those question marks and other signs and then the pressure of interviews also makes it hard to answer complex usage of Generics. 

 But, with proper preparation and paying attention to both Java Collection and Generic, you can solve that hurdle. If you are looking for a Java job but haven't done well in the interviews you have given so far then you have come to the right place. In this blog, I have shared a lot of Java interview questions on various topics and difficulty levels.

There are Java questions for beginners as well as expert programmers. They are theoretical questions based upon Java programming concepts as well as coding and data structure algorithms questions for programmers, and this article is only going to make that collection even more valuable.

In this article, I am going to share some of the frequently asked Java Collection and Generic questions from Interviews. These are the questions you have often seen on a telephonic round of Java interviews as well as on face-to-face interviews. It's useful for both beginners having 2 to 3 years of experience as well as experienced Java programmers with 5 to 6 years of experience.

This list has a collection of questions that has both easy and tough questions in it but the most important thing is that most of the questions have already been asked in interviews. 

I am sure you might have also seen it in your interviews. knowing the answers to these questions will not only help you to crack your Java interview but also understand Java Generics and Collection topic in-depth, which will eventually help you to write better Java programmers and code.

Btw, if you are new to Java or want to solidify your Java knowledge then you should check out these Java collections and Stream courses before attempting to solve these questions. It will help you immensely by filling gaps in your knowledge and going back and forth. It's also the most up-to-date course and covers every new feature introduced between Java 8 to Java 12 releases.

50+ Java Collection and Generic Interview Questions for Beginners & 2 to 5 Years Experienced

Without wasting any more of your time, here is my list of 50+ Java interview questions on Collection and Generics. If you have done some work in Java then you should know the answer to these questions but if you don't you can always see the answer. 

Instead of writing answers here, I have linked them to relevant posts so that you can try to solve the problem by yourself here and if you need you can get an in-depth discussion on individual posts to learn the topic in depth.

1)  What is the Java Collection Framework and How do you choose different collections? (answer)
Here is the diagram which answers this question:

Top 50 Java Generics and Collection Interview Questions

2)  What is Generics in Java? (answer)
hint: Java feature to ensure type safety at compile time.

3)  Which are your favorites classes from the Java Collection Framework? (answer)
hint: Collection, List, Set, Map, ArrayList, Vector, LinkedList, HashMap, etc

4)  When do you use Set, List, and Map in Java? (answer)
hint - use set when you don't need duplicates, use List when you need order with duplicates, and use Map when you need to store key-value pair.

5)  Which Sorted Collection have you used? (answer)
hint - TreeSet is one example of a sorted Collection

6)  How HashSet works in Java? (answer)
hint - same like HashMap, using hashing and equals() and hashCode() method. HashSet is actually backed by HashMap where keys are elements you store in HashSet and values are always null.

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

8)  Can you use HashMap in a concurrent application? (answer)
hint - Yes, but only if you are reading from the HashMap and its initialized by a single thread, otherwise no.

9) What is the difference between HashMap and Hashtable in Java? (answer)
hint - HashMap is fast but not threadsafe, Hashtable is slow but thread-safe

10) What is the difference between synchronized and concurrent Collection in Java? (answer)

11) How ConcurrentHashMap works in Java? (answer)
partitions map into segments and lock them individually instead of locking the whole map.

12) What is PriorityQueue in Java? (answer)
A data structure that always keeps the highest or lowest element at the head so that you can access or remove it in constant time.

13) What is type-erasure in Generics?  (answer)
Its a part of the Java compiler which removes all type-related information after compilation from Java so that the generated code is the same as before Generics.

14) What is the difference between ArrayList and Vector in Java?  (answer)
ArrayList is not synchronized but Vector is synchronized. Vector is also legacy class and you should avoid Vector if possible. 

15) What is the difference between LinkedList and ArrayList in Java?  (answer)
While both implement List interface which means they are ordered and allow duplicates, the difference comes from underlying data structure. LinkedList is backed by a linked list while ArrayList is backed by an array data structure. This means ArrayList is good for fast search with index while LinkedList is good for frequent addition and deletion of elements. 

16) What is the difference between Hashtable and ConcurrentHashMap in Java?  (answer)
Hashtable is synchronized and legacy class. It's present since Java 1.0 while ConcurrentHashMap is a new class which provides better performance by using modern concurrency technique like segments. If you need a thread-safe cache consider using ConcurrentHashMap then Hashtable. 

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

18) Difference between extends and super in Java Generics?  (answer)

19) What do you mean by thread-safe collection? Give an example of 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)
Iterator is a relatively newer class while Enumeration is an old class. Apart from iteration, the Iterator also allows you to remove the current element from the collection which Enumeration doesn't have such functionality. 

24) When does ConcurrentModificationException occur?  (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 peek() 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<Number> in Java?  (answer)

31) How get 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) Difference between IdentityHashMap and WeakHashMap in Java?  (answer)

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

41) What is DeQueue? 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 the difference between ArrayList and ArrayList<?> in Java?  (answer)
ArrayList is a raw type which is not type safe and compile will not be able to tell that what kind of elements are stored in ArrayList while ArrayList<?> is a generic type which allows to store any kind of object but its type safe. 

45) What is the difference between PriorityQueue and TreeSet in Java?  (answer)
PriorityQueue doesn't keep all elements in sorted order, only the head of the priorityQueue is guaranteed to be highest priority but TreeSet keep them in a specific order defined by Comparable or Comparator. 

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

47) What is the "diamond" operator in Java?  (answer)

48) What is covariant method overriding in Java?  (answer)

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

That's all in this list of 50 Java Generics and Collections Interview Questions and answers. Both these topics are  very important topics from Java Interview point of view, especially collections. It's expected from a Java developers to know different Collection classes particularly the ones which are rarely used like TreeSet, TreeMap, BlockingQueue etc. Make sure you prepare them well before going for any interviews. If you need further preparation you can also check out these Java Interview books and courses:

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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 looking for the best online course to prepare for Java interviews then you can also check out these Java interview courses from Udemy and Educative. It's a great resource to prepare for Java interviews. 

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