Top 25 Java Interview Questions for 3 Years Experience (Expert Answers)

Are you preparing for a Java interview with 3 years of experience?
This guide covers the most asked Java interview questions for 3 years experience across Core Java, Java 8, Spring Boot, Collections, and more, with clear answers and code examples.

🔹 Core Java & OOPs Interview Questions

1. What are the main principles of Object-Oriented Programming?

The four core principles of OOP are:

Encapsulation
Inheritance
Polymorphism
Abstraction

2. What is the difference between `==` and `.equals()` in Java?

== checks reference equality
.equals() checks value equality

javaCopyEditString a = new String("Hello");
String b = new String("Hello");

System.out.println(a == b);       // false
System.out.println(a.equals(b));  // true

3. Explain method overloading vs. overriding.

Overloading: Compile-time polymorphism (same method name, different params)
Overriding: Runtime polymorphism (subclass redefines parent method)

4. Can a constructor be `final`?

No. Constructors can’t be final because they are never inherited.

🔹 Java 8 & Stream API Questions

5. What is a functional interface?

An interface with exactly one abstract method.

javaCopyEdit@FunctionalInterface
interface MyFunc {
    void run();
}

Used with lambda expressions.

6. What features were introduced in Java 8?

Lambda expressions
Stream API
Functional interfaces
Default & static methods in interfaces
Optional class
New Date & Time API

7. Stream API example to filter even numbers?

javaCopyEditList<Integer> numbers = Arrays.asList(1, 2, 3, 4);
List<Integer> evens = numbers.stream()
    .filter(n -> n % 2 == 0)
    .collect(Collectors.toList());

🔹 Exception Handling

8. Difference between `throw` and `throws`?

throw: Used to manually throw an exception
throws: Declares potential exceptions in method signature

9. Checked vs. Unchecked Exceptions?

Checked: Checked at compile time (IOException)
Unchecked: Runtime exceptions (NullPointerException)

10. What is the `finally` Block used for?

To execute code regardless of exception, often used to close resources.

🔹 Spring Boot Questions

11. What is Spring Boot?

A framework that simplifies Spring applications:

Embedded servers
Auto-configuration
Minimal setup

12. Difference between `@Component`, `@Service`, `@Repository`?

@Component: Generic bean
@Service: Business logic
@Repository: Data access layer

13. What is dependency injection?

A design pattern where the framework injects objects into classes instead of you manually instantiating them.

14. Use of `application.properties`?

To externalise configuration.

propertiesCopyEditserver.port=8080
spring.datasource.url=jdbc:mysql://localhost:3306/test

🔹 Java Collections

15. Difference between ArrayList and LinkedList?

Feature	ArrayList	LinkedList
Access time	Fast	Slower
Insertion	Slower	Faster
Storage	Less overhead	More memory

16. How does HashMap work internally?

Uses hashCode and equals
Keys go into buckets
Collisions handled via linked list (or tree from Java 8)

17. Difference between HashMap and Hashtable?

HashMap: Not synchronised
Hashtable: Synchronised, slower

🔹 Multithreading

18. Difference between synchronised method and block?

Method: Locks the entire method
Block: Locks only a section of code or an object

19. What is the `volatile` keyword?

Ensures the variable is read from main memory, not the thread-local cache.

20. Difference between `wait()`, `sleep()`, `join()`?

wait(): Waits and releases lock
sleep(): Waits without releasing lock
join(): Waits for a thread to die

🔹 Behavioural / Real-Life Questions

21. Have you used Git in projects?

Yes, for:

Version control
Feature branching
Pull request-based reviews

22. How do you debug production issues?

Log analysis
Stack trace reading
Tools like ELK, Splunk

23. Experience with REST APIs?

Built using @RestController
JSON handling
Consumed via RestTemplate or WebClient

24. Java Interview Questions for 3 Years Experience on Unit Testing

JUnit 5
Mockito
Coverage via IntelliJ or Jacoco

25. Why should we hire you?

3 years Java experience
Strong in Spring Boot, Java 8
Clean code, REST, Git, Testing
Proactive and team-driven

📘 More Helpful Resources

❓ FAQs

What topics are important for 3 years Java experience?

Core Java, Java 8, Collections, Spring Boot, REST APIs, SQL.

Is 3 years of experience enough for mid-level Java roles?

Yes. You are expected to know frameworks, APIs, OOPs, and be production-ready.

How to prepare fast for a Java interview?

Practice real Q&A, revise Java 8, do mock interviews, and study system design basics.

It is always advised to read official documents for building a better understanding.

Official Java docs:
👉 https://docs.oracle.com/javase/tutorial/

Spring docs:
👉 https://spring.io/guides

📩 Got a Question?

Drop your queries in the comments or contact us here.
We’ll help you crack your next Java interview! 🚀

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