What are the main principles of OOP? What is the difference between an interface and an abstract class? How does HashMap work internally? What is hash collision? What is the difference between equals() and hashCode()? Why is String immutable in Java? What are checked and unchecked exceptions? What is the difference between ArrayList and LinkedList? What is the difference between HashMap and ConcurrentHashMap? Explain the lifecycle of a thread. What is the difference between Runnable and Callable? What is synchronization? What is the volatile keyword? What are lambda expressions? What is the Stream API in Java 8? 🚀 Spring Boot What is Spring Boot? What is the difference between @Component, @Service, and @Repository? How does dependency injection work in Spring? What is @Autowired? What is the Spring Bean lifecycle? How do you handle exceptions globally in Spring Boot? What is REST and how does it work? How do you secure a Spring Boot application? Explain the flow of a REST API request in Spring Boot. What is Spring Security? 🔐 JWT / Security What is JWT? How does JWT authentication work? What is the difference between authentication and authorization? How do you implement role-based access control? 🧱 Microservices What is the difference between monolithic and microservices architecture? What are the advantages of microservices? How do microservices communicate with each other? What is an API Gateway? What is service discovery? What is a circuit breaker? How do you handle failures in microservices?