java top 50 interview questions

Java Collection and Generic are a very important topic 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 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 article, 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 a comprehensive course like The Complete Java Masterclass 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 in new Java releases

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:

2) What are Generics in Java? (answer) hint: Java feature to ensure type safety at compile time. 3) Which are your favorite classes from 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 work in Java? (answer) hint — same as 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 work 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)

hint — ArrayList is not synchronized hence fast, Vector is synchronized hence slow 15) What is the difference between LinkedList and ArrayList in Java? (answer)

hint — ArrayList is backed by array while LinkedList is backed by a linked list which means search with index is only possible in ArrayList. 16) What is the difference between Hashtable and ConcurrentHashMap in Java? (answer)

hint — ConcurrentHashMap is a new concurrent class with better scalability as only a portion of the map called segment is locked while Hashtable is an old class where the whole map is Locke for synchronization. Seet Java Collections: Fundamentals course for more details.

By the way, you would need a Pluralsight membership to join this course which costs around $29 per month and $299 per annum (14% discount) but its completely worth it. Alternative. you can also use their 10-day-free-trial to watch this course FREE.

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

hint — TreeSet is a sorted set where elements are stored in their natural or custom sorting order depending upon comparable and comparator while LinkedHashSet is just an ordered collection that maintains insertion order. 18) 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? (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 in Java? (answer) 31) How get method of HashMap works in Java? (answer)

hint — hashing, hashcode method is used to find bucket location for putting the mapping and equals is used for retrieval. 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)

You can further see The Complete Java Masterclass course for more details. It’s also the most up-to-date course and covers every new feature introduced in new Java releases

37) How to create 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) 45) What is the difference between PriorityQueue and TreeSet in Java? (answer) 46) How can I avoid “unchecked cast” warnings? (answer) 47) What is the “diamond” operator in Java? (answer) 48) What is the 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. They are a very important topic from the Java Interview point of view, especially collections. 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:

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 serious about mastering Java Collections — one of the most important Java API then I also suggest you check out the Java Collections: Fundamentals course by @Richrad Warburton, a Java Champion on Pluralsight. It’s a great course to learn why and which collections Java programmers should use.

P. P. S — Quick Update,Pluralsight free weekend is here and you can access all 7000+ Pluralsight courses and projects for FREE this weekend. Make this count and learn a new skill or level up the existing one. Don’t miss this out, it’s only for this weekend. And here is the link again:

I am Java programmer, blogger, working on Java, J2EE, UNIX, FIX Protocol. I share Java tips on and

Java Interview Questions and Answers | Edureka

As a Java professional, it is essential to know the right buzzwords, learn the right technologies and prepare the right answers to commonly asked Java Interview Questions. Here’s a definitive list of top Java Interview Questions that will guarantee a breeze-through to the next level.

In case you attended any Java interview recently, or have additional questions beyond what we covered, we encourage you to post them in our QnA Forum. Our expert team will get back to you at the earliest.

So let’s get started with the first set of basic Java Interview Questions.

Basic Java Interview Questions for Freshers


It stands for Java Development Kit. It stands for Java Runtime Environment. It stands for Java Virtual Machine.
It is the tool necessary to compile, document and package Java programs. JRE refers to a runtime environment in which Java bytecode can be executed. It is an abstract machine. It is a specification that provides a run-time environment in which Java bytecode can be executed.
It contains JRE + development tools. It’s an implementation of the JVM which physically exists. JVM follows three notations: Specification, Implementation, and Runtime Instance.

Before you go! Take this “Java Interview Questions” interview guide with you

Do you have what it takes to ace a Java Interview? We are here to help you in consolidating your knowledge and concepts in Java. Before we begin, lets understand what Java is all about.

Java is the high-level programming language that was developed by James Gosling in the year 1982. It is based on the principles of object-oriented programming and can be used to develop large-scale applications. Learn More.

The following article will cover all the popular Core Java interview questions, String Handling interview questions, java 8 interview questions, java multithreading interview questions, java OOPs interview questions, java exception handling interview questions, collections interview questions, and some frequently asked java coding interview questions.

Go through all the questions to enhance your chances of performing well in the interviews. The questions will revolve around the basic, core & advanced fundamentals of Java.

So, let’s dive deep into the plethora of useful Java Interview questions and answers for freshers and experienced candidates in depth. Crack your next tech interview with confidence! Take a free mock interview, get instant⚡️ feedback and recommendation

What’s the difference between String, StringBuffer, and StringBuilder?

String is an immutable class. In older JDKs the recommendation when programmatically building a String was to use StringBuffer since this was optimized to concatenate multiple Strings together.

However, the methods StringBuffer were marked as synchronized, which meant that there was a performance penalty, hence StringBuilder was introduced to provide a non-synchronized way to efficiently concatenate and modify Strings.


What are the most common Java interview questions?

Most Popular Java Interview Questions and Answers
  • Q #1) What is JAVA? …
  • Q #2) What are the features of JAVA? …
  • Q #3) How does Java enable high performance? …
  • Q #4) Name the Java IDE’s? …
  • Q #5) What do you mean by Constructor? …
  • Q #6) What is meant by the Local variable and the Instance variable? …
  • Q #7) What is a Class?

What are the basic Java interview questions?

Java Basic Interview Questions
  • Why is Java a platform independent language? …
  • Why is Java not a pure object oriented language? …
  • Difference between Heap and Stack Memory in java. …
  • Can java be said to be the complete object-oriented programming language? …
  • How is Java different from C++? …
  • Pointers are used in C/ C++.

Which is the best site for Java interview questions?

Which is the best site for Java interview questions?
  • Tutorialspoint.
  • StackOverflow.
  • DZone.
  • Wikipedia.
  • IBM Developer Works.

What should I prepare for Java interview?

Which topics to prepare for Java interviews?
  1. Java Fundamentals.
  2. Data Structure and Algorithms.
  3. Object-Oriented Concepts.
  4. Multithreading, concurrency, and thread basics.
  5. Java Collections Framework.
  6. Date type conversion and fundamentals.
  7. Array.
  8. Garbage Collection.

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