What is OOPs? Object-Oriented Programming organizes code using classes and objects, improving reusability, scalability, security, and maintenance through concepts like inheritance, encapsulation, abstraction, and polymorphism. Difference between == and .equals() in Java == compares memory references, while .equals() compares object content. For strings and objects, .equals() checks actual values. What is Exception Handling? Exception handling manages runtime errors using try, catch, finally, throw, and throws, preventing program crashes and allowing graceful error recovery. What is Multithreading? Multithreading allows multiple threads to execute simultaneously, improving application performance and CPU utilization by executing tasks concurrently. What is SQL JOIN? SQL JOIN combines rows from multiple tables based on related columns, enabling retrieval of meaningful data across tables. What is JVM? JVM executes Java bytecode, manages memory, handles garbage collection, and provides platform independence. Difference between List and Set List allows duplicate elements and maintains insertion order, while Set does not allow duplicates and may not preserve order. What is REST API? REST API enables communication between systems using HTTP methods like GET, POST, PUT, and DELETE with stateless operations.