Top Core Java Interview Questions and Answers – A Complete Guide

Preparing for a Core Java interview can be challenging, especially when MNC companies expect clear, practical explanations, not textbook definitions. This guide covers the 25 most important Core Java interview questions, written in a clean, conversational tone similar to how strong candidates answer in real interviews.

Why Interviewers Ask These Questions?

These questions help companies evaluate whether you truly understand Java, beyond syntax. They test your fundamentals, your debugging, problem-solving ability, your understanding of the JVM memory model, and your knowledge of Java 8+. Strong answers here show you can think like a developer, write clean code, and handle real-world scenarios, and often place candidates in the top 10%.

Questions from Basics of Java

Recruiters ask these to check whether you truly understand Java's fundamentals. Companies like Infosys, TCS, Wipro, Zoho, Accenture, and IBM usually ask these in the first rounds to understand how strong you are in the basics.
 

1. What is Java? Why is it widely used?

Java is an object-oriented, platform-independent programming language. When you write Java code, it compiles into bytecode, which runs on the JVM, making it “write once, run anywhere.” Because it's strong, simple, and secure, it’s widely used in enterprise systems, Android apps, and backend frameworks.
Score: This answer shows you understand both definition and value.

2. Difference between JDK, JRE, and JVM.

 

• JDK (Java Development Kit) is used to develop Java applications.
• JRE (Java Runtime Environment) provides the libraries and JVM to run Java code.
• JVM (Java Virtual Machine) executes bytecode and ensures platform independence.

Score: This question is to check if you understand Java's architecture, not just coding.

3. Difference between == and equals().

 

• == compares the memory reference of two objects.
• equals() compares the values or content of the objects.
• For example, two String objects with the same value are equals() true but == false.

Score: Interviewers check whether you understand how objects work in memory.

4. What is a constructor? Types of constructors?

A constructor initializes an object when it’s created. 

Types:

• Default constructor
• Parametrized constructor
• Copy constructor (custom in Java)

Score: This shows you know object creation, the basics of OOPs.

5. Difference between method overloading and overriding.

 

• Overloading occurs when the same method name is used with different parameters in the same class (compile-time polymorphism).
• Overriding occurs Subclass provides a new version of a parent class method. (runtime polymorphism).

Score: Companies ask this to test your grasp of polymorphism.

6. Can we override static or private methods? Why not?

No. Static methods belong to the class, not objects, so overriding doesn’t apply. Private methods are not visible in child classes.
Score: This checks if you understand method binding and inheritance rules.

 

Questions from OOP Concepts

Companies like Amazon, Flipkart, Microsoft, Zoho, and Accenture ask these questions to see if you can model real-world problems in code. OOP is the backbone of Java development, so your answers here can strongly impact your overall impression.

7. What are the four main principles of OOP?

 

• Encapsulation: Protecting data through access modifiers.
• Inheritance: Reusing properties of another class.
• Polymorphism: Same method behaving differently.
• Abstraction: Showing only necessary details while hiding implementation.

Score: A clear explanation shows your concept understanding, not memorization.

8. Difference between an abstract class and an interface.

 

• An abstract class can have abstract and non-abstract methods, constructors, and variables.
• An interface defines a contract with abstract methods (plus default and static methods from Java 8). Interfaces support multiple inheritance, making them essential in frameworks.

Score: Companies ask this because modern frameworks depend on interfaces.

9. What is a String in Java? Why is it immutable?

 

• String is a special, immutable class.
• It’s immutable for security, performance (String pool), and thread safety.

Score: This question is common in Amazon, Microsoft, Oracle, and JP Morgan interviews.

10. Difference between String, StringBuilder, and StringBuffer.

 

• String is immutable.
• StringBuilder is mutable and faster, but not thread-safe.
• StringBuffer is mutable and thread-safe.

Score: Companies check if you know when to use one of these.

 

Questions from Collections & Core Concepts

Collections questions help interviewers evaluate your understanding of data structures, time complexities, and internal working. These are favorites in Amazon, Microsoft, Flipkart, JP Morgan, and service-based MNCs. Understanding Collections and data structures is easier if you follow a Java course that offers hands-on exercises and practical examples.

11. What is the Collection Framework? Why is it used?

 

• It is a set of classes and interfaces providing ready-made data structures like List, Set, and Map.
• It improves efficiency, reduces coding effort, and provides flexible, optimized algorithms.

 

12. How does a HashMap work internally?

 

• HashMap stores key-value pairs using hashing. The key’s hashCode() determines the bucket position.
• If multiple keys fall into the same bucket, HashMap uses a linked list or balanced tree (from Java 8) to store entries.

Score: Understanding this shows your depth of Java internals, important for product companies.

13. Difference between ArrayList and LinkedList.

 

• ArrayList offers fast access but slow insert/delete inside the list.
• LinkedList offers fast insert/delete but slower access.

Score: Interviewers expect you to know performance trade-offs.

14. Difference between HashMap and Hashtable.

 

• HashMap is not synchronized and allows one null key and multiple null values.
• Hashtable is synchronized, thread-safe, and does not allow null keys or values.

Score: This evaluates your understanding of thread safety.

15. What are fail-fast and fail-safe iterators?

 

• Fail-fast iterators throw ConcurrentModificationException when the structure is modified (e.g., ArrayList, HashMap).
• Fail-safe iterators operate on a copy, preventing exceptions (e.g., ConcurrentHashMap, CopyOnWriteArrayList).

Score: This checks concurrency basics.

16. What is Exception Handling? Checked vs Unchecked exceptions.

 

• Exception handling allows a program to handle runtime errors smoothly.
• Checked exceptions occur at compile time (IOException, SQLException).
• Unchecked exceptions occur at runtime (NullPointerException, ArithmeticException).

Score: Companies check if you know how to write stable programs.

 

17. Difference between throw and throws.

 

• throw manually throws an exception.
• throws lists exceptions that a method might pass to its caller.

Score: This tests your understanding of exception flow.

18. Will finally always execute?

Yes, the finally block almost always executes and is used for cleanup. It only skips execution in rare cases like when System.exit() is called, the JVM crashes, or the program stops unexpectedly.

 

Questions from Multithreading & Concurrency

These questions test whether you understand thread safety, synchronization, and how to avoid performance issues or deadlocks. Product companies like Amazon, Netflix, Microsoft, JP Morgan, and Mastercard focus heavily on concurrency because real-world systems depend on performance and safe multithreaded code.

19. What is a thread? How to create one?

 

• A thread is a lightweight unit of execution.
• You can create it by extending Thread or implementing Runnable.

Score: This is the first step to checking your concurrency understanding.

20. Difference between run() and start().

 

• start() creates a new thread and calls run() internally.
• Calling run() directly executes code in the same thread.

Score: Interviewers check if you understand the thread lifecycle.

21. What is synchronization? Why is it needed?

Synchronization prevents multiple threads from accessing shared resources simultaneously to avoid data inconsistency.
Score: This is to evaluate your ability to write safe, concurrent code.

22. What is a deadlock? How to avoid it?

Deadlock occurs when two threads wait for each other’s locks. Avoid by proper lock ordering, using synchronized blocks carefully, or using higher-level concurrency APIs.
Score: This shows if you can handle real-world problems.

 

Questions from Advanced Java Concepts

Advanced Java questions can show how deep your understanding goes beyond the basics. They help recruiters judge your ability to show maturity, logical thinking, and readiness for senior or intermediate roles.These are asked in Amazon, Oracle, Microsoft, Visa, JP Morgan, and Zoho. 

23. What is Serialization? What does the transient keyword do?

Serialization converts objects into byte streams. Transient prevents specific fields from being serialized, often used for passwords or temporary values.
Score: This is to test knowledge of object persistence.

24. What is Optional in Java 8? Why is it used?

Optional helps avoid NullPointerException by providing a container that may or may not hold a value.
Score: This checks whether you understand modern Java practices.

25. Explain JVM Architecture and Garbage Collection.

 

• JVM includes a class loader, memory areas, and an execution engine. 
• Garbage Collection automatically removes unused objects to free memory.

Score: This question evaluates understanding of memory handling and performance

Things to Keep in Mind Before Your Interview

 

1. Interviewers can focus on some emphasis Collections, others prefer Multithreading or Java 8–17 features.
2. Freshers are usually tested on OOP, Strings, Collections, and Exceptions, while experienced candidates face JVM internals, GC, concurrency, and performance topics.
3. Stay updated with modern Java Updates and features to perform confidently.

Conclusion

Mastering these 25 Core Java interview questions will help you confidently face interviews at top MNCs and product companies. Your goal isn’t to memorize definitions but to explain concepts the way developers think. 

And if you want structured guidance, Osiz Labs is one of the leading software training institutes in Madurai. The Java training program includes hands-on projects, industry-level coding practice, interview preparation, career guidance, and professional, industry-recognized certification. With the right mentorship and consistent practice, you can have strong Java basics and confidently handle technical discussions in any company interview.