Top 40 Java Developer Interview Questions

Java remains one of the most widely used programming languages in the tech industry. Whether you’re a fresh graduate or an experienced developer, preparing for a Java developer interview requires both technical knowledge and confidence. To help you get ready, here are 40 carefully selected Java interview questions divided by skill level, plus essential skills, interview tips, and a short conclusion.

Basic Level Java Developer Interview Questions

1. What is Java?

2. Explain the difference between JDK, JRE, and JVM.

3. What are the main features of Java?

4. What is the difference between an object and a class?

5. What are data types in Java?

Intermediate Level Java Developer Interview Questions

1. What is the difference between ArrayList and LinkedList?

2. What are the different types of inheritance in Java?

3. What is an interface, and how is it different from an abstract class?

4. What is the use of the final keyword?

5. What is exception handling in Java?

Experienced Level Java Developer Interview Questions

1. What are design patterns in Java? Give examples.

2. What is the difference between HashMap and Hashtable?

3. What is garbage collection in Java?

4. What is the difference between shallow copy and deep copy?

5. How does Java achieve platform independence?

Basic Level Java Developer Interview Questions

Preparing for a Java developer interview means knowing the basic questions that cover essential Java concepts. Whether you’re new to Java or just starting your career, understanding these common topics will help you demonstrate your skills clearly and confidently.

1. What is Java?

Java is a high-level, object-oriented, and platform-independent programming language developed by Sun Microsystems (now Oracle). It allows developers to write code once and run it anywhere using the Java Virtual Machine (JVM). Java is widely used for building web, mobile, and desktop applications.

2. Explain the difference between JDK, JRE, and JVM.

The difference between JDK, JRE, and JVM is as follows:

JDK

JRE

JVM

Used for developing and running Java programs

Used only for running Java programs

Used to execute Java bytecode

Includes JRE, compiler, and development tools

Includes JVM and core libraries

Part of JRE, runs Java programs

Needed by Java developers

Needed by users who just want to run Java apps

Works behind the scenes to interpret bytecode

Platform-specific software package

Also platform-specific

Platform-independent (runs bytecode)

Example: javac (Java compiler) is part of the JDK

Cannot compile code, only runs it

Executes code line-by-line or JIT compiled

3. What are the main features of Java?

Some of the main features of Java are as follows:

Simple: Easy to learn and use.
Object-Oriented: Everything is treated as an object.
Platform-Independent: Java code runs on any system with a JVM.
Secure: Provides a secure environment for application development.
Robust: Handles errors and memory management well.
Multithreaded: Supports running multiple tasks at once.
High Performance: Uses Just-In-Time (JIT) compiler.

4. What is the difference between an object and a class?

Class: A blueprint or template that defines the structure and behaviour (variables and methods).
Object: A real-world entity created from a class. It uses the structure defined by the class.

Example:

5. What are data types in Java?

Data types in Java define the kind of value a variable can hold. Java supports two main categories of data types: primitive and non-primitive.

Primitive data types are the basic building blocks of data manipulation. They include:

byte – A very small integer (8-bit), often used when saving memory is important.
short – A 16-bit integer, used for slightly larger numbers than byte.
int – The most commonly used 32-bit integer type for whole numbers.
long – A 64-bit integer, used when int is not large enough.
float – A 32-bit decimal number with single precision.
Double – A 64-bit decimal number with double precision, suitable for most decimal calculations.
char – A single 16-bit Unicode character.
boolean – Represents true or false values.

These types are predefined by the language and store simple values.

Non-primitive data types are more complex and are created by the programmer. They include:

Strings – A sequence of characters, such as names or messages.
Arrays – A group of values of the same type stored together.
Classes – Templates for creating objects.
Interfaces – Abstract types used to define a contract for classes.

Non-primitive types can store multiple values or more complex data and support various operations and methods. Understanding these helps in choosing the right type for each variable, which improves performance and code clarity.

6. Explain the concept of OOPs in Java.

Java is an Object-Oriented Programming (OOP) language. It uses four main concepts to organise code:

Encapsulation: Wrapping data and methods inside a class. It keeps the data safe from outside interference.
Inheritance: Allows one class to use the properties and methods of another, promoting code reuse.
Polymorphism: Enables one method or function to behave differently based on the object calling it.
Abstraction: Hides complex implementation and shows only important details to the user.

These concepts help make Java programs more structured, reusable, and easier to manage.

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

== compares memory locations (reference comparison).
.equals() compares the actual content or values (logical comparison).

Example:

8. What is a constructor in Java?

A constructor is a special method used to initialize objects when they are created. It has the same name as the class and no return type. It runs automatically when an object is created.

Example:

9. What are access modifiers in Java?

Access modifiers in Java define how accessible classes, variables, and methods are from other parts of the program. There are four main types:

public: The code can be accessed from anywhere in the program.
private: The code is accessible only within the same class, making it fully hidden from other classes.
protected: The code can be accessed within the same package and also by subclasses, even if they are in different packages.
default (no modifier): The code is accessible only within the same package, but not outside it.

Using access modifiers helps control where and how parts of your code can be used, improving security and structure.

10. What is method overloading and method overriding?

Both overloading and overriding deal with methods, but they work differently:

Method Overloading happens when a class has multiple methods with the same name but different parameter lists (type, number, or order). It allows a method to perform similar but slightly different tasks based on input.

Example of Overloading:

Method Overriding occurs when a subclass provides a specific implementation of a method that is already defined in its parent class. It allows a subclass to change or extend the behaviour of a method.

Example of Overriding:

Explore these helpful Interview Question Guides

Intermediate Level Java Developer Interview Questions

As you gain experience, interviewers will ask more advanced Java developer interview questions. These intermediate-level questions cover important concepts like collections, inheritance, exception handling, and multithreading to help you prepare for your next Java developer role.

11. What is the difference between ArrayList and LinkedList?

Both ArrayList and LinkedList are used to store elements, but they work differently:

ArrayList is backed by a dynamic array, which provides fast access (O(1) time) to elements using indexes but is slower when inserting or deleting elements from the middle or start.
LinkedList is backed by a doubly linked list, making it faster for insertions and deletions but slower for accessing elements, as it must traverse nodes.

Use ArrayList when you need fast access; use LinkedList when frequent add/remove operations are involved.

12. What are the different types of inheritance in Java?

Java supports the following types of inheritance:

Single Inheritance: One class inherits from another class.
Multilevel Inheritance: A class inherits from a class, which itself inherits from another class.
Hierarchical Inheritance: Multiple classes inherit from the same parent class.

Note: Java does not support multiple inheritance with classes to avoid ambiguity but allows it using interfaces.

13. What is an interface, and how is it different from an abstract class?

Interface: Contains only abstract methods (before Java 8) and can be implemented by any class. A class can implement multiple interfaces.

Abstract Class: Can have both abstract (unimplemented) and concrete (implemented) methods. A class can only extend one abstract class.

14. What is the use of the final keyword?

The final keyword is used to restrict changes:

final variable: Its value cannot be changed once assigned.
final method: Cannot be overridden by subclasses.
final class: Cannot be extended (e.g., String class is final).

It is used to create constants and enforce immutability or inheritance rules.

15. What is exception handling in Java?

Exception handling in Java is a mechanism to handle runtime errors, preventing program crashes. It uses blocks like:

try – Code that may throw an exception.
catch – Handles the exception.
Finally– Always runs, used for cleanup.
throw – Used to explicitly throw an exception.
throws – Declares the exceptions a method might throw.

It helps in writing robust and error-resistant programs.

16. What is the difference between checked and unchecked exceptions?

The difference between checked and unchecked exceptions are:

Checked exceptions are checked at compile time. If not handled, they cause a compile-time error. Example: IOException, SQLException.
Unchecked exceptions are checked at runtime. They are usually caused by logical errors. Example: NullPointerException, ArrayIndexOutOfBoundsException.

Aspect

Checked Exceptions

Unchecked Exceptions

Definition

Exceptions that are checked at compile-time

Exceptions that occur at runtime

Handling Requirement

Must be caught or declared using throws

No requirement to handle explicitly

Examples

IOException, SQLException

NullPointerException, ArrayIndexOutOfBoundsException

Hierarchy

Subclasses of Exception (excluding RuntimeException)

Subclasses of RuntimeException

Use Case

Used when the program should anticipate and recover from issues

Used for programming errors that should be fixed in code

Developers must either handle or declare checked exceptions but can choose to handle unchecked ones.

17. What are wrapper classes in Java?

Wrapper classes allow using primitive data types as objects. Java provides a wrapper class for each primitive type:

int → Integer
char → Character
boolean → Boolean, etc.

They are used in collections (like ArrayList) which store only objects, and for utility methods (like converting to strings, parsing, etc.).

18. Explain the Java Collections Framework.

The Java Collections Framework (JCF) is a set of classes and interfaces used to store and manipulate groups of data. It includes:

List (e.g., ArrayList, LinkedList)
Set (e.g., HashSet, TreeSet)
Map (e.g., HashMap, TreeMap)
Queue (e.g., PriorityQueue)

JCF helps developers use data structures efficiently with built-in methods for sorting, searching, and iteration.

19. What is multithreading?

Multithreading allows a program to perform multiple tasks at the same time, improving performance and responsiveness. Each thread runs independently but shares the same memory.

Java provides the Thread class and Runnable interface to create threads. It’s widely used in tasks like animations, network communication, and real-time systems.

20. What is synchronization in Java?

Synchronization ensures that only one thread accesses a block of code or resource at a time. It prevents data inconsistency when multiple threads access shared data.

You can use the synchronized keyword on methods or blocks. It is especially important in multithreaded applications to avoid race conditions and ensure data integrity.

Experienced Level Java Developer Interview Questions

For seasoned professionals preparing for Java Developer Interview Questions, understanding advanced concepts is key. This section covers important topics such as design patterns, memory management, Java 8 features, and concurrency. Mastering these questions will help demonstrate your in-depth knowledge and problem-solving skills in Java development.

21. What are design patterns in Java? Give examples.

Design patterns are reusable solutions to common software design problems. They represent best practices used by experienced developers. Patterns help improve code readability, maintainability, and scalability.

Examples:

Singleton – Ensures a class has only one instance (e.g., logging, config settings).
Factory – Creates objects without exposing the instantiation logic to the client.
Observer – Used when multiple objects need to be notified of changes in another object (commonly used in event handling and MVC architecture).

22. What is the difference between HashMap and Hashtable?

Aspect

HashMap

Hashtable

Thread Safety

Not synchronized (not thread-safe)

Synchronized (thread-safe)

Performance

Faster in single-threaded applications

Slower due to synchronization

Null Keys/Values

Allows one null key and multiple null values

Doesn’t allow null keys or null values

Legacy Status

Part of the Java Collections Framework

Considered legacy, rarely used in new code

Tip: In multi-threaded environments, use ConcurrentHashMap instead of Hashtable.

23. What is garbage collection in Java?

Garbage collection (GC) in Java is an automatic process that frees up memory by deleting objects that are no longer accessible in the program. This helps avoid memory leaks and manual memory management.

The JVM uses different GC algorithms, such as:

Serial GC – For small applications.
Parallel GC – For applications requiring high throughput.
G1 GC – Splits the heap into regions and is suitable for large heaps.

Developers can influence GC using JVM flags (-Xms, -Xmx, -XX:+UseG1GC), and must be cautious with object creation in loops or large collections.

24. What is the difference between shallow copy and deep copy?

Shallow Copy: Copies only the top-level structure. References inside the object still point to the same memory.
Deep Copy: Recursively copies all objects, creating entirely independent instances.

When to use:
Use deep copy when you want to avoid side effects of shared references. Use shallow copy when working with immutable or temporary objects.

25. How does Java achieve platform independence?

Java achieves platform independence through the Java Virtual Machine (JVM).

Java source code is compiled into bytecode (.class files).
This bytecode can run on any platform with a compatible JVM, regardless of hardware or OS.

The “Write Once, Run Anywhere” principle relies on JVM implementations for different platforms interpreting the same bytecode.

26. What are functional interfaces in Java 8?

A functional interface is an interface with exactly one abstract method. It can have default and static methods. They are primarily used for lambda expressions and method references.

Examples:

Runnable – for threading.
Comparator<T> – for sorting.
Function<T, R> – takes one argument and returns a result.

Marking an interface with @FunctionalInterface ensures it meets this rule at compile time.

27. What are lambda expressions?

Lambda expressions provide a concise way to represent functional interfaces using a clean, readable syntax.

Syntax Example:
java
CopyEdit
(parameter) -> expression

Why use it?

Reduces boilerplate code.
Improves readability.
Enables functional programming features like map, filter, and reduce with streams.

Example Use Case: Replacing anonymous inner classes in event listeners or collections.

28. Explain Stream API in Java.

The Stream API, introduced in Java 8, allows processing of collections and arrays using a declarative and functional style. It doesn’t modify the original data but performs operations like:

filter() – filters elements.
map() – transforms data.
collect() – gathers the result.

Benefits:

Supports parallel processing.
More concise and readable.
Reduces the need for external iteration (like loops).

Example Use Case: Processing a list of employees to find and sort names by department.

29. How does the volatile keyword work in Java?

The volatile keyword ensures that a variable’s value is always read from the main memory, not from the thread’s local cache. It is useful in multi-threaded environments to ensure visibility of shared variables across threads.

Note: It does not guarantee atomicity (use synchronized or AtomicInteger for that).

Use case: Flags or status variables that multiple threads read and update.

30. What is the role of the transient keyword?

The transient keyword in Java is used to indicate that a variable should not be serialized. When an object is serialized, transient fields are skipped.

Why use it?

To avoid storing sensitive information like passwords.
To prevent serialization of temporary or derived data.

Example: A session token that changes often and shouldn’t be stored on disk.

Java Programming Interview Questions

This section covers practical Java programming interview questions that test your coding skills and problem-solving ability. From reversing strings and checking prime numbers to implementing recursion and exception handling, these questions help prepare you for real-world Java coding challenges commonly asked in interviews.

31. Write a program to reverse a string in Java.

32. How do you check if a number is prime in Java?

33. Write a Java program to check if a string is a palindrome.

34. Write a Java program to sort an array.

35. How do you remove duplicates from a list using Java?

36. How to find the second-largest number in an array?

37. Write a program to find factorial of a number using recursion.

38. How do you handle exceptions in a try-catch block?

39. Write a program to implement a basic calculator.

40. Create a simple Java class with encapsulation principles.

Skills a Java Developer Should Have

To thrive as a Java Developer, you need a mix of technical knowledge and soft skills. Here are the key skills employers look for:

Core Java Knowledge

Strong understanding of OOP principles.
Familiarity with core libraries and collections.

Frameworks & Tools

Experience with Spring/Spring Boot, Hibernate.
Build tools like Maven or Gradle.

Database & SQL

Ability to write and optimise queries.
Knowledge of relational and NoSQL databases.

Version Control

Hands-on experience with Git and GitHub.

Problem-Solving Skills

Logical thinking and debugging skills.
Writing clean and efficient code.

Soft Skills

Communication, teamwork, and time management.

Tips to Be Successful in a Java Job Interview

Master the Core Java Concepts

Understand object-oriented programming, data structures, exception handling, collections, and multithreading. Interviewers often begin with these basics to assess your foundation.

Practice Coding Regularly

Solve Java problems on platforms like LeetCode, Codeforces, or HackerRank. Focus on algorithms, data structures, and optimisation techniques to build strong logic and speed.

Understand Frameworks and Tools

Familiarise yourself with Spring Boot, Hibernate, Maven, and version control tools like Git. These are often discussed in real-world scenario-based questions.

Study Past Projects and Experiences

Be ready to talk about your contributions in previous jobs or projects. Explain your decisions, tools used, and outcomes. Keep your answers structured and clear.

Use the AI Interview Answer Generator

AI Interview Answer Generator is a smart tool that helps you prepare personalised, polished responses to common and advanced Java interview questions. It uses natural language and context-based generation to simulate real interview answers.

Conclusion

Landing a Java Developer role requires more than just knowing how to write code; you need to communicate your thought process clearly, understand core concepts deeply, and be ready for both technical and practical questions.

As you prepare, don’t hesitate to use AI Interview Assistant. It’s designed to support your practice sessions by generating smart, tailored answers and simulating interview-like experiences. The more you practise, the more confident and natural you’ll feel during the real interview.

Keep learning, keep coding, and good luck with your next opportunity!

Top 40 Java Developer Interview Questions

Basic Level Java Developer Interview Questions

1. What is Java?

2. Explain the difference between JDK, JRE, and JVM.

3. What are the main features of Java?

4. What is the difference between an object and a class?

5. What are data types in Java?

6. Explain the concept of OOPs in Java.

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

8. What is a constructor in Java?

9. What are access modifiers in Java?

10. What is method overloading and method overriding?

Explore these helpful Interview Question Guides

Intermediate Level Java Developer Interview Questions

11. What is the difference between ArrayList and LinkedList?

12. What are the different types of inheritance in Java?

13. What is an interface, and how is it different from an abstract class?

14. What is the use of the final keyword?

15. What is exception handling in Java?

16. What is the difference between checked and unchecked exceptions?

17. What are wrapper classes in Java?

18. Explain the Java Collections Framework.

19. What is multithreading?

20. What is synchronization in Java?

Experienced Level Java Developer Interview Questions

21. What are design patterns in Java? Give examples.

22. What is the difference between HashMap and Hashtable?

23. What is garbage collection in Java?

24. What is the difference between shallow copy and deep copy?

25. How does Java achieve platform independence?

26. What are functional interfaces in Java 8?

27. What are lambda expressions?

28. Explain Stream API in Java.

29. How does the volatile keyword work in Java?

30. What is the role of the transient keyword?

Java Programming Interview Questions

31. Write a program to reverse a string in Java.

32. How do you check if a number is prime in Java?

33. Write a Java program to check if a string is a palindrome.

34. Write a Java program to sort an array.

35. How do you remove duplicates from a list using Java?

36. How to find the second-largest number in an array?

37. Write a program to find factorial of a number using recursion.

38. How do you handle exceptions in a try-catch block?

39. Write a program to implement a basic calculator.

40. Create a simple Java class with encapsulation principles.

Skills a Java Developer Should Have

Core Java Knowledge

Frameworks & Tools

Database & SQL

Version Control

Problem-Solving Skills

Soft Skills

Tips to Be Successful in a Java Job Interview

Master the Core Java Concepts

Practice Coding Regularly

Understand Frameworks and Tools

Study Past Projects and Experiences

Use the AI Interview Answer Generator

Conclusion

Top 40 Java Developer Interview Questions

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