Top 30 Java Phone Interview Questions Answers for Freshers, 1 to 2 Years Experienced

Hello guys and ladies, in this article, I am sharing 30 core Java technical questions, from screening and phone round of interviews. In telephonic interviews, questions are short, fact-based and Interviewer expects a to-the-point reply and some keywords in answers. Accordingly, I have given very short answers of all these questions, only the main points; just to make this a revision post and not the main source for preparation. For thorough preparation, my master list of 140+ Java questions is a better one to start with. I am also expecting every Java programmer to know answers to all these Java technical questions if he has more than 4 to 5 years experience. It's only freshers and junior developers who need to do a bit of research to understand topics well.

I have tried to include all classical and hugely popular, frequently asked questions from different topics like String, multi-threading, collection, design patterns, object-oriented concepts, garbage collection, generics, and advanced Java concurrency questions, but if you think those are not enough, you can see the links which are my dedicated post on each topic.

For example, on multi-threading one, you will find 50+ questions or Java thread concepts, concurrency, and multi-threading. Those are ideal for preparations, and this post is ideal for revision.

Since core Java interviewer's normally don't ask questions on JSP, Servlets, and other JEE technologies, I have not included them in this list.

The sole purpose of this list is to give freshers and less experienced developers an idea of what kind of core Java technical questions are asked on phone interviews. For curious ones, I have also included links to more detail answers and discussions.

I have not included questions based upon recent Java changes like Java 8, Java 9, Java 10, or Java 11, e.g., the introduction of the lambda expression, method reference and now the var in Java 10 but you can find a lot of such questions on this list of 200+ Interview Questions and Answers .

That is a big list with more than 200+ questions from different topics, and if you have some time in hand, you can use that to prepare well for your next Java interview.

Java Phone Interview Questions Answers

Anyway, without any further ado, here is a list of 30 frequently asked Java interview questions from the telephonic round:

1. Why is String immutable in Java? (Security, String pool implementation, see more here)

2. Can abstract class have a constructor in Java? (Yes, the detailed answer is here)

3. Which two methods are overridden by an Object, intended to be used as a key in HashMap?

(equals and hashCode, read more)

4. What is the difference between wait and sleep in Java?  (wait for release lock, sleep keeps it, for details see here)

5. Difference between List and Set in Java?

The list is ordered, allows duplicates and indexed, Set is unordered, don't allow duplicates, for a more detailed answer, see this post)

6. How do you make a class Immutable in Java?

(Make it final, final fields without a setter, the state is only set in the constructor, no leak of an internal reference, copy data for mutable members, read more)

7. Which data type you should use to represent currency in Java?

You should answer long or BigDecimal, if you say double, you need to convince them about rounding and how do you avoid floating point issues. for more detailed discussion, see this post)

8. When to use abstract class and interface in Java?

Use interface for type declaration, use an abstract class if evolution is a concern, for few more points, see this post)

9. Difference between Hashtable and HashMap in Java?

Another, common Java question, former is thread-safe and doesn't allow null, later is not thread-safe, former is also slow because of the whole locking of Map, while HashMap is fast because of no locking, read more)

10. What is the difference between ArrayList and LinkedList in Java?

former is fast, backed by an array while later is backed by linked-list, queue, former also supports index-based access at O(1), while later provides search at the cost of O(n) time, for in-depth discussion, see here)

By the way, if you are really serious about doing well on Java programming interview and don't have enough time for thorough preparation, then you should take help from a book like Java Programming Interview Exposed, one of the best ways to prepare for core Java interviews.

It covers all important topics including core Java, data structure and algorithm, frameworks like Spring and Hibernate, JDBC, JUnit, Design patterns, Android, Scala and advanced concurrency stuff for experienced programmers.

Anyway, let's move on to the next question.

11. What is the difference between Overloading and Overriding in Java?

Former take place at compile-time, later happens at runtime, the only virtual method can be overridden, the static, final, and private method can't be overridden in Java. for more in-depth discussion, see this post)

12. What kind of reference types are exists in Java? Differences?

(Strong reference, Weak references, Soft reference, and Phantom reference. Except strong, all other reference allows the object to be garbage collected. For example, if an object hash only weak reference, then it's eligible for GC if the program needs space)

13. Difference between checked and unchecked exception in Java?

(former is checked by the compiler, and it's handling is enforced by mandating try-catch or try-finally block. Later is not checked by the compiler but can be caught using try-catch or try-finally block. For example, java.io.IOException, java.sql.SQLException is checked exception, while java.lang.NullPointerException and java.lang.ArrayIndexOutOfBoundsException is an example of the unchecked exception in Java, for better answer see here)

14. Does Java array is an instance of Object?

(Yes, and this is a stark difference from the array in C/C++ though it doesn't have any method, it has an attribute called length, which denotes the size of the array, see here to know more about array in Java)

15. Does List<Number> can hold Integers?

Yes, because Integer is a subclass of Number and generics allows that,

16. Can we pass ArrayList<Number> to a method which accepts List<Number> in Java?

Yes, Again, this is a tricky question, and you need in-depth knowledge of Generics to answer this question.  If you want to know more about Generics, I suggest you join theComplete Java Masterclass course on Udemy. One of the best resource to learn Java in-depth.

17. Can we pass ArrayList<Integer> to a method which accepts List<Number>?

(No) How to fix that? (use wildcards e.g. List<? extends Number> to know more about bounded and unbounded wildcards and other generics questions see this post)

18. What is a volatile variable in Java?

Special keyword to indicate that a variable can be read/write by more than one thread at a time. It guarantees happens-before relationship, the variable's value is read into main memory, for detail answer see here)

19. What is the difference between CountDownLatch and CyclicBarrier in Java?

Both are concurrency utility classes introduced in Java 5 and can be used to wait for other threads to finish their job before starting itself. The key difference between CountDownLatch and CyclicBarrier is that former cannot be reused once the count reaches zero while later can be reused even after the barrier is broken, for in-depth discussion, see this post)

20. Does BlockingQueue is thread-safe in Java?

(Yes, take() and put() method of this class guarantees thread-safety, no need to externally synchronize this class for adding and retrieving objects, here is an example of this class to solve the producer-consumer problem in Java)

21. Why wait and notify method should be called in a loop?

This can be really difficult to answer if you have never used wait-notify because it seems pretty logical that an if-else block can be used, but that would be wrong.

As Joshua Bloch mentioned in his classic Effective Java book, we should always call wai-notify in the loop to prevent doing the task, if a condition is not true and the thread is awake due to false alarms, checking conditions in the loop ensures that processing is only done when business logic allows.

If you haven't read that book yet, I highly recommend it. A new edition is also out, and it's available on a 50% discount at the time of writing.

22. What is the difference between "ABC".equals(unknown string) and unknown?equals("ABC")?

This is again a tricky question and common idiom to avoid NullPointerException in Java. former is safe from NullPointerException see here for more best practices andtips to avoid NPE in Java)

23. What is a marker or tag interface in Java?

An interface, which presence means an instruction for JVM or compiler, e.g. Serializable, from Java 5 onwards Annotation is better suited for this job, to learn more and answer in detail see this discussion)

24. What is the difference between the Serializable and Externalizable interface in Java?

This is another popular Java question. The Externalizable interface provides more control over the serialization process, and allow you to define a custom binary format for your object, later is also suited for the performance-sensitive application, see here for more details)

25. Can Enum types implement interface in Java? (Yes)

26. Can Enum extend a class in Java?

No, because Java allows a class to only extend one class and enum by default extends java.lang.Enum, see here for more Enum interview questions)

27. How to prevent your class from being subclassed?

(Make it final or make constructor private, for curious developers here are three technical ways to prevent your class from being subclassed.)

28. Can we override a static method in Java? Compilation error?

(No, it can only be hidden, no compilation error, see here for more details)

29. Which design pattern have you used recently?

You can answer this question based upon experience, but you should give any example except Singleton and MVC, e.g. Decorator, Strategy or Factory pattern to make a better impression. If you don't know about design patterns then you can check "From 0 to 1: Design Patterns - 24 That Matter - In Java" course by Loony Corn, An ex-Google, and Stanford.

30. What is the difference between StringBuffer and StringBuilder in Java?

Another very common questions, which is mostly asked, beginners. The short answer is that the former is synchronized, and slow, while later is not synchronized and fast, for more details see this post)

That's all on this list of 30 core Java technical questions asked in phone interviews to freshers, junior Java developers and Java programmers up-to experience ranging from 1 to 2 years. As I said before, use this list for quick revision, especially if you are in a hurry. Give short, to-the-point, and specific answers, until Interviewer asks for more or insist on elaborating by asking follow-up questions.

Since most of the phone interviews are done to screen candidates, mostly similar questions are asked of all candidates. By the way, always do a short research on the company before your phone interview, you might find some of the questions previously asked on Google itself.

Further Learning

Complete Java Masterclass

Java Fundamentals: The Java Language

Java Interview Guide: 200+ Interview Questions and Answers

20 Spring and REST Interview Questions

75 Coding Questions to Crack Any Programming Interview

100+ Data Structure and Algorithm Questions

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All the best Guys