Java Interview Questions (2 to 3 Years Experience)

Java Interview Questions (2 to 3 Years Experience)

Java Interview Questions (3 Years Experience)

🧠 1. Core Java (Most Important)

  • What are the main features of Java?
  • Difference between JDK, JRE, and JVM?
  • What is OOPs? Explain its principles.
  • Difference between method overloading and overriding?
  • What is String vs StringBuffer vs StringBuilder?
  • Why is Java called platform-independent?
  • What is final, finally, and finalize?

🧩 2. Object-Oriented Programming

  • Explain encapsulation with example.
  • What is abstraction in Java?
  • Difference between abstraction and encapsulation?
  • What is inheritance and its types?
  • Can Java support multiple inheritance? Why not?

📦 3. Collections Framework

  • Difference between ArrayList and LinkedList?
  • HashMap vs Hashtable?
  • How does HashMap work internally?
  • What is the difference between Set and List?
  • What is ConcurrentHashMap?
  • What is fail-fast vs fail-safe iterator?

⚡ 4. Multithreading & Concurrency

  • What is multithreading?
  • Difference between process and thread?
  • How to create a thread in Java?
  • What is synchronization?
  • What is deadlock?
  • What is volatile keyword?
  • Difference between wait(), notify(), notifyAll()?

🗄️ 5. Exception Handling

  • What is exception handling?
  • Checked vs unchecked exceptions?
  • Difference between throw and throws?
  • What is custom exception?
  • What is try-catch-finally?

🧱 6. Java 8 Features

  • What are Java 8 features?
  • What is lambda expression?
  • What is Stream API?
  • Difference between map() and flatMap()?
  • What is Optional class?
  • What are functional interfaces?

🌐 7. Spring / Spring Boot (Very Important for 3 Years)

  • What is Spring Boot?
  • What is dependency injection?
  • Difference between Spring and Spring Boot?
  • What is @Autowired?
  • What is REST API in Spring Boot?
  • What is microservices architecture?
  • What is application.properties used for?

🗄️ 8. Database & Hibernate

  • What is JDBC?
  • What is Hibernate?
  • Difference between JDBC and Hibernate?
  • What is ORM?
  • What is lazy loading and eager loading?
  • What is HQL?

💻 9. Coding Questions (Very Common)

  • Reverse a string without using built-in function
  • Find duplicate elements in array
  • Check palindrome number/string
  • Find second highest number in array
  • Fibonacci series
  • Factorial program
  • Remove duplicates from ArrayList

🧠 10. Scenario-Based Questions (Important for Experience)

  • How did you handle production issues in your project?
  • Explain your current project architecture.
  • How do you optimize performance in Java applications?
  • How do you handle memory leaks?
  • How do you debug application failures?
  • What challenges did you face in microservices?

⚡ Interview Tip (3 Years Experience)

Interviewers expect:

  • Strong Core Java + Collections + Multithreading
  • Basic Spring Boot + REST API knowledge
  • Real project explanation (very important)
  • Some coding ability (not very hard, but logic-based)

Java Mock Interview (3 Years Experience)

👨‍💼 Round 1: HR + Introduction

Interviewer: Tell me about yourself.

👉 What you should cover:

  • Current role
  • Tech stack (Java, Spring Boot, REST APIs, DB)
  • 1–2 key projects
  • Your responsibilities

👉 Sample answer:
“I am a Java developer with 3 years of experience working on backend development using Java, Spring Boot, and REST APIs. I have worked on building scalable microservices and integrating databases like MySQL. In my current project, I am responsible for developing APIs, optimizing performance, and handling production issues.”

Interviewer: Why are you looking for a change?

👉 Good answer approach:

  • Growth
  • Better opportunities
  • Learning new technologies

💻 Round 2: Core Java

Interviewer: What is the difference between HashMap and Hashtable?

👉 Answer:

  • HashMap is not synchronized, Hashtable is synchronized
  • HashMap allows null keys/values, Hashtable does not
  • HashMap is faster

Interviewer: How does HashMap work internally?

👉 Answer:

  • Uses hashing
  • Stores data in buckets
  • Uses equals() and hashCode()
  • Handles collisions using linked list / tree (Java 8+)

Interviewer: What is the difference between String, StringBuilder, StringBuffer?

👉 Answer:

  • String → immutable
  • StringBuilder → mutable, not thread-safe
  • StringBuffer → mutable, thread-safe

🧩 Round 3: OOPs Concepts

Interviewer: Explain polymorphism.

👉 Answer:
Polymorphism means “many forms”. It allows method overloading (compile-time) and overriding (runtime).

Interviewer: Why Java does not support multiple inheritance?

👉 Answer:
To avoid ambiguity and complexity caused by diamond problem.

⚡ Round 4: Multithreading

Interviewer: What is synchronization?

👉 Answer:
It ensures that only one thread can access a resource at a time to avoid data inconsistency.

Interviewer: What is deadlock?

👉 Answer:
A situation where two or more threads are waiting for each other to release resources, causing infinite blocking.

🗄️ Round 5: Spring Boot

Interviewer: What is dependency injection?

👉 Answer:
It is a design pattern where objects are provided their dependencies by Spring container instead of creating them manually.

Interviewer: What is REST API?

👉 Answer:
REST API is a web service that uses HTTP methods (GET, POST, PUT, DELETE) for communication.

Interviewer: Difference between @Component, @Service, @Repository?

👉 Answer:

  • @Component → generic bean
  • @Service → business logic layer
  • @Repository → database layer

💻 Round 6: Coding Round

Q1: Reverse a String

👉 Input: “Java” → Output: “avaJ”

public class Main {
    public static void main(String[] args) {
        String s = "Java";
        String rev = "";        for(int i = s.length()-1; i >= 0; i--) {
            rev += s.charAt(i);
        }        System.out.println(rev);
    }
}

Q2: Find Second Largest Element

👉 Logic:

  • Track largest and second largest

🧠 Round 7: Scenario-Based

Interviewer: What will you do if production issue occurs?

👉 Answer:

  • Check logs
  • Identify root cause
  • Fix issue quickly (hotfix if needed)
  • Deploy patch
  • Write RCA (Root Cause Analysis)

Interviewer: How do you optimize performance?

👉 Answer:

  • Proper indexing in DB
  • Use caching
  • Optimize queries
  • Avoid unnecessary object creation

🎯 Final HR Question

Interviewer: Do you have any questions for us?

👉 Good questions:

  • What is the project architecture?
  • What technologies will I work on?
  • Is there any training for new tools?

⚡ Interview Tips (Important)

  • Speak confidently, even if unsure
  • Always connect answers to your project
  • Don’t memorize—understand logic
  • Keep answers short and structured
  • Coding = logic first, syntax second

op 50 Java Coding Questions with Solutions

🧠 1–10: Basic Logic

1. Reverse a String

String rev = new StringBuilder(s).reverse().toString();

2. Palindrome String

return s.equals(new StringBuilder(s).reverse().toString());

3. Factorial

int fact = 1;
for(int i=1;i<=n;i++) fact*=i;

4. Fibonacci Series

int a=0,b=1;
for(int i=0;i<n;i++){
    System.out.print(a+" ");
    int c=a+b;
    a=b;b=c;
}

5. Sum of digits

while(n>0){ sum+=n%10; n/=10; }

6. Count digits

while(n>0){ count++; n/=10; }

7. Swap two numbers

a=a+b; b=a-b; a=a-b;

8. Prime number check

boolean isPrime=true;
for(int i=2;i<=Math.sqrt(n);i++)
    if(n%i==0) isPrime=false;

9. Largest in array

int max=arr[0];
for(int i:arr) if(i>max) max=i;

10. Smallest in array

int min=arr[0];
for(int i:arr) if(i<min) min=i;

🔄 11–20: Arrays

11. Second largest

int max=Integer.MIN_VALUE, second=max;

12. Remove duplicates (array)

Set<Integer> set=new HashSet<>();

13. Sort array

Arrays.sort(arr);

14. Reverse array

int i=0,j=n-1;
while(i<j){ swap; }

15. Frequency of elements

Map<Integer,Integer> map=new HashMap<>();

16. Missing number in array

sumExpected - sumActual

17. Move zeros to end

Use two pointers

18. Find duplicate elements

Use HashSet

19. Union of arrays

Set.addAll()

20. Intersection of arrays

retainAll()

🧩 21–30: Strings Advanced

21. Count vowels

if("aeiou".contains(c)) count++;

22. Remove spaces

s.replace(" ","");

23. Character frequency

HashMap<Character,Integer>

24. Anagram check

sort both strings and compare

25. First non-repeating char

use LinkedHashMap

26. Reverse words in sentence

split + reverse

27. Uppercase to lowercase

s.toLowerCase();

28. Remove duplicates in string

LinkedHashSet

29. Longest word

split and compare length

30. Palindrome sentence

remove spaces + check reverse

⚡ 31–40: Collections

31. ArrayList example

List<Integer> list=new ArrayList<>();

32. HashMap iteration

for(Map.Entry e:map.entrySet())

33. LinkedList usage

addFirst(), addLast()

34. Sort list

Collections.sort(list);

35. Reverse list

Collections.reverse(list);

36. PriorityQueue

Queue<Integer> pq=new PriorityQueue<>();

37. Stack operations

push(), pop()

38. HashSet usage

Set<Integer> set=new HashSet<>();

39. Remove duplicates (list)

new HashSet<>(list)

40. Frequency count map

map.put(key, map.getOrDefault(key,0)+1);

🔥 41–50: Advanced Logic

41. Find missing number (1 to N)

n*(n+1)/2 - sum

42. Check Armstrong number

sum of digits^power

43. Binary search

while(low<=high)

44. Linear search

simple loop

45. Merge two arrays

System.arraycopy()

46. Matrix addition

nested loops

47. Transpose matrix

swap rows and columns

48. Count even/odd

if(n%2==0)

49. GCD of two numbers

Euclid algorithm

50. LCM of two numbers

(a*b)/gcd

⚡ Interview Tip

For 3 years experience, interviewers focus on:

  • Arrays + Strings (VERY IMPORTANT)
  • HashMap + Collections
  • Basic DSA logic
  • Real project coding problems
  • Clean, optimized code (not just working code)