Core Java & OOP What are the four pillars of Object-Oriented Programming? Can you explain each with examples? Difference between an abstract class and an interface? When do you use which one? What is polymorphism? Explain compile-time vs run-time polymorphism with examples. What is encapsulation and how do you achieve it in Java? What is the difference between HashMap, LinkedHashMap and TreeMap? Explain the difference between Collection and Collections in Java. What is multithreading? How do you create and manage threads in Java? What is synchronization and why is it needed? DSA & Coding Logic Write a program to find the second largest element in an array without sorting. Write a Java program to reverse a string without using built-in methods. How do you detect a cycle in a linked list? Write a program to check if a string is a palindrome. Implement binary search and explain its time complexity. Write a program to find duplicate elements in an array. Spring Boot / Backend Explain the Spring Boot architecture and its key components. What are REST APIs and the common HTTP methods? Where are they used? What is the difference between @Component, @Service, @Repository, and @Controller? How does dependency injection (DI) work in Spring? What is JPA/Hibernate? Explain lazy vs eager loading. What is a ResponseEntity and when do you use it? How do you handle exceptions in Spring Boot? Explain @ControllerAdvice and @ExceptionHandler. Explain how Spring Boot handles security & authentication (basic understanding of Spring Security). Database & Deployment Difference between SQL and NoSQL databases? What are database indexes? How do they improve performance? What are ACID properties in DB transactions? How does CI/CD work? Explain tools like Jenkins, GitHub Actions, Docker. Testing / Debugging What is Unit Testing? Explain JUnit & Mockito usage. How do you debug a production issue that occurs randomly? Project & Real-Time Scenarios Describe your recent project architecture — technologies used, API flow, database design, challenges faced. What performance optimization or improvement have you implemented in your project?