Top 25 Java Interview Questions and Answers (2026)

Java continues to rank among the top 5 most-used programming languages globally, even in 2026, powering enterprise systems, fintech platforms, Android apps, and cloud-native backends. Yet, despite years of experience, many developers still struggle to clear Java interviews due to weak fundamentals rather than lack of coding ability. This is where java interview questions play a critical role.These java interview questions focus on real-world understanding rather than memorization, which is exactly what modern interviewers look for.

Interviewers now test conceptual clarity, JVM understanding, and real-world reasoning, not just syntax. Whether you are a fresher preparing for your first role or an experienced engineer aiming for a better opportunity, mastering Core Java Interview questions is non-negotiable. This article is designed to help you understand, not memorize, the most important Core Java questions asked in 2026 interviews, with clear explanations, updated context, and practical insights you can confidently articulate.

Overview: Why Core Java Still Matters in 2026

Core Java forms the foundation of the entire Java ecosystem, including Spring Boot, Microservices, Android, and Big Data frameworks.
Interviewers rely on Core Java questions to evaluate:

• Your understanding of object-oriented programming
• How well you grasp memory management and JVM behavior
• Your ability to write clean, maintainable, and efficient code

With modern Java versions (Java 17, 21, and beyond) emphasizing performance and readability, companies now expect developers to combine fundamentals with modern best practices.
Strong Core Java fundamentals signal that you can adapt to frameworks and scale systems, making these questions a decisive factor in interviews.

25 Java Interview Questions

Core Concepts Explained

1. What is Java and why is it platform-independent?

Java is a high-level, object-oriented programming language designed to be platform-independent and secure. It runs on the Java Virtual Machine (JVM), which acts as an abstraction layer between the compiled Java code and the underlying operating system. When Java source code is compiled, it is transformed into bytecode, a standardized intermediate representation. This bytecode can be executed on any device or operating system that has a compatible JVM, allowing developers to write code once and run it anywhere without modification. Additionally, the JVM provides features like memory management, garbage collection, and runtime optimization, which contribute to Java’s robustness and portability.

2. What are the main features of Java?

Key features include:

• Object-oriented: organizes code using classes and objects
• Platform-independent: runs on any system with JVM
• Robust and secure: handles errors and protects data
• Multithreaded: supports concurrent execution
• Automatic memory management: manages memory via garbage collection

3. Explain JVM, JRE, and JDK.

• JVM is the engine that runs Java bytecode on any platform. It interprets or compiles bytecode into machine code, managing memory and execution.
• JRE provides the necessary libraries, classes, and the JVM to run Java applications. It does not include development tools like compilers.
• JDK is a complete development kit that includes the JRE along with tools such as the Java compiler, debugger, and other utilities needed to develop Java programs.

4. What is Object-Oriented Programming?

OOP, or Object-Oriented Programming, is a style of programming that organizes code around objects rather than actions. Each object represents real-world entities with attributes and behaviors. It uses concepts like encapsulation to protect data, inheritance to reuse code, abstraction to hide complexity, and polymorphism to allow flexibility. This approach helps in building modular, maintainable, and scalable software systems.

5. What is a class and an object?

A class is a blueprint that defines the structure and behavior of objects, including fields and methods. An object is a concrete instance of a class created in memory, capable of performing the actions defined by its class.

6. Explain encapsulation and give its real world example?

Encapsulation is a fundamental object-oriented principle that restricts direct access to an object’s internal state by declaring variables as private. It provides controlled access through public methods known as getters and setters, allowing validation or modification before data is read or updated. This approach enhances security by preventing unauthorized changes and improves maintainability by centralizing control over how data is accessed and modified. It also promotes modularity, making the code easier to understand and extend.

Real-world example: Consider a banking application where a BankAccount class has a private balance variable. By using getters and setters, the class can ensure that withdrawals do not exceed the available balance and deposits are validated before updating the balance, preventing invalid operations and maintaining data integrity.

7. What is inheritance?

Inheritance is a mechanism where one class can use the fields and methods of another class. It helps in reusing existing code and establishing a hierarchical relationship between classes. In Java, this is achieved using the extends keyword.

8. What is polymorphism and give its real world example?

Polymorphism allows a method or an object to take many forms, enabling the same interface or method to behave differently depending on the context or the object that invokes it. This concept is fundamental in object-oriented programming because it promotes flexibility, code reuse, and scalability. Polymorphism is achieved mainly through method overloading, where multiple methods share the same name but differ in parameters, and method overriding, where a subclass provides a specific implementation of a method already defined in its superclass.

Real-world example: Consider a payment processing system. You might have a Payment class with a method processPayment(). Different subclasses like CreditCardPayment, PayPalPayment, and BankTransferPayment can override this method to implement their specific payment logic. When the system calls processPayment() on a Payment reference, the correct subclass method executes based on the actual object type, allowing the system to handle multiple payment methods seamlessly without changing the calling code.

9. Difference between method overloading and overriding.

• Overloading occurs at compile time when multiple methods in the same class have the same name but different parameter lists. It allows a class to have multiple behaviors for a method depending on the input parameters.
• Overriding occurs at runtime when a subclass provides a specific implementation of a method that is already defined in its superclass with the same method signature. It enables runtime polymorphism, allowing the program to decide which method to execute based on the object’s actual type.

Feature	Method Overloading	Method Overriding
Definition	Same method name, different parameters in the same class	Subclass provides a new implementation of a superclass method
Compile/Runtime	Resolved at compile time	Resolved at runtime
Parameters	Must differ (number, type, or order)	Must be the same
Return Type	Can be different (if parameters differ)	Must be the same or covariant type
Access Modifier	Can vary	Cannot be more restrictive than superclass method
Inheritance	Not required	Required
Purpose	To increase method readability and flexibility	To provide specific behavior in subclass

10. What is abstraction?

Abstraction is a fundamental object-oriented programming concept that allows developers to hide the complex implementation details of a class or method while exposing only the necessary functionality to the user. In Java, abstraction can be achieved using abstract classes and interfaces. An abstract class can contain both abstract methods (without implementation) and concrete methods (with implementation), allowing partial implementation that subclasses must complete. Interfaces, on the other hand, define a contract of methods that implementing classes must provide, ensuring a consistent API without revealing the underlying logic. This approach simplifies code usage, enhances maintainability, and promotes modular design by separating what an object does from how it does it.

Memory, Strings, and Exception Handling

These java interview questions frequently test a candidate’s understanding of memory behavior, immutability, and error handling.

11. What is the difference between == and .equals()?

== checks if two object references point to the same memory location, whereas .equals() checks if the objects have the same content.
For example,
String a = new String("Java");
String b = new String("Java");
a == b is false, but a.equals(b) is true.

12. Why are Strings immutable in Java?

Immutability in Java means that once a String object is created, its value cannot be changed. This property ensures thread safety because multiple threads can access the same String instance without causing data inconsistency. It also enhances security by preventing unauthorized modifications to sensitive data. Additionally, immutability allows efficient memory usage through the String Constant Pool, where identical string literals are shared rather than duplicated, reducing memory overhead.

13. Explain the String Constant Pool.

It is a special memory area in the heap that stores unique string literals to optimize memory usage.

14. What is exception handling?

Exception handling is a mechanism in Java to handle runtime errors, ensuring the normal flow of the program. The key components are:

• try: Defines a block of code to test for exceptions.
• catch: Catches and handles the exception thrown in the try block.
• finally: Contains code that always executes after try/catch, regardless of exceptions.
• throw: Used to explicitly throw an exception from a method or block.
• throws: Declares the exceptions a method might throw, informing the caller to handle them.
  Example:
  try {
  int result = 10 / 0;
  } catch (ArithmeticException e) {
  System.out.println(“Cannot divide by zero”);
  } finally {
  System.out.println(“Execution completed”);
  }

15.Difference between checked and unchecked exceptions.

Feature	Checked Exception	Unchecked Exception
Validation	Validated at compile time	Occur at runtime
Handling Requirement	Must be caught or declared using throws	Optional to handle
Use Case	Represents recoverable conditions	Represents programming errors or bugs

Collections, Multithreading, and Performance

In most technical rounds, java interview questions around collections and multithreading are used to assess performance awareness.

16. What is the Java Collections Framework?

It provides a comprehensive set of standardized data structures and algorithms, including List, Set, Queue, Map, and their various implementations, along with utility classes for sorting, searching, and manipulating collections, enabling developers to handle data efficiently, reduce boilerplate code, and improve performance in real-world applications.

17. Difference between ArrayList and LinkedList.

• ArrayList provides constant-time positional access because it uses a dynamic array internally, making it faster when retrieving elements by index. However, inserting or deleting elements in the middle requires shifting subsequent elements, which can be costly.
• LinkedList uses a doubly-linked list structure, so accessing elements by index requires traversal, making it slower for random access. Its strength lies in efficient insertion and deletion at any position, as it only involves updating node references without shifting other elements.

18. What is a HashMap?

HashMap is a collection that stores data in key-value pairs. It uses a hashing mechanism for fast access and permits one null key and multiple null values.

19. What is multithreading?

Multithreading allows multiple threads to run concurrently within a single program, enabling tasks like background processing, parallel computations, and responsive user interfaces. It improves CPU utilization, reduces execution time for independent tasks, and allows programs to handle multiple operations simultaneously without blocking the main thread.

20. Difference between Thread and Runnable.

• A Thread represents a separate path of execution in a program and can be created by extending the Thread class.
• Runnable is a functional interface that defines a single run() method, allowing a class to execute code in a thread without extending Thread, promoting better design flexibility and supporting multiple inheritance.

JVM, Keywords, and Advanced Basics

21. What is garbage collection?

Garbage Collection automatically removes unused objects from memory, improving performance and preventing memory leaks.

22. What are access modifiers?

• public: accessible from anywhere
• private: accessible only within the class
• protected: accessible within the package and subclasses
• default: accessible only within the package
  They control visibility and encapsulation.

23. Difference between final, finally, and finalize.

• final: constant or non-inheritable
• finally: executes regardless of exception
• finalize: called before garbage collection

24. What is the static keyword?

Static members are associated with the class itself rather than any individual object, meaning all instances of the class share the same static variables and methods. They can be accessed without creating an object of the class.

25. What is serialization?

Serialization is the process of converting an object into a byte stream so that it can be saved to a file, sent over a network, or stored in a database, and later reconstructed using de serialization. This is typically achieved by implementing the Serializable interface in Java.

Trends and Updates in Core Java Interviews Questions (2026)

In 2026, interviews increasingly emphasize:

• JVM internals and memory tuning
• Immutable objects and thread safety
• Java 17+ language features
• Clean code and SOLID principles
• Practical reasoning over theoretical answers

Candidates who can connect concepts to real-world usage stand out significantly.

Common Mistakes Candidates Make

• Memorizing answers without understanding
• Ignoring JVM and memory topics
• Weak explanation of OOP concepts
• Confusing similar keywords (final vs finally)
• Overlooking practical examples

Avoiding these mistakes dramatically improves interview performance.

Expert Tips & Best Practices

• Always explain concepts with real examples
• Revise fundamentals before advanced topics
• Practice explaining answers aloud
• Focus on “why” and not just “what”
• Keep answers structured and concise

Interviewers value clarity and confidence over lengthy explanations.

Complete Your Interview Preparation

While mastering OOPs Interview Questions and Answers builds a strong conceptual foundation, cracking real interviews requires combining OOPs with programming and database skills.

To prepare holistically for fresher interviews in 2026, continue with:

• SQL Interview Questions and Answers – Critical for database-driven applications and backend roles.
• Python Interview Questions and Answers – Perfect for scripting, backend, and automation roles.
• OOPs Interview Questions and Answers – Fundamental for mastering object-oriented design, problem modeling, and technical interview evaluations.
• System Design Interview Questions and Answers – Essential for understanding scalability, performance, databases, APIs, caching, load balancing, and designing real-world applications asked in modern technical interviews.

Conclusion

Core Java remains the backbone of Java interviews in 2026, regardless of role or experience level. Strong fundamentals signal that you can write reliable code, understand frameworks, and scale with evolving technologies. By mastering these Top 25 Core Java Interview Questions, you build not just interview confidence but long-term technical strength.
Instead of rote learning, focus on conceptual clarity, real-world application, and structured explanations. This approach not only helps you crack interviews but also makes you a better Java developer overall. Keep revising, keep practicing, and treat every interview question as an opportunity to demonstrate how well you truly understand Java.

Frequently Asked Questions