1. What is Java? Why is it platform-independent? Java runs on JVM → bytecode → machine code → so it works on any OS. 2. Difference between JDK, JRE, and JVM JDK → development kit JRE → runtime environment JVM → executes bytecode 3. OOPS Concepts (Infosys always asks) Encapsulation Inheritance Polymorphism Abstraction 4. Difference between Overloading and Overriding Overloading – compile time, same method name, different params. Overriding – runtime, same signature in subclass. 5. What is this keyword? Refers to current object. 6. What is super keyword? Refers to parent class. 7. What is constructor? Types? Default Parameterized Used to initialize objects. 8. Difference between abstract class and interface Abstract → can have concrete + abstract methods Interface → only abstract methods (Java 8 allows default/static) 9. Can Java support multiple inheritance 10. What is String pool? Area in heap memory that stores unique string literals. 11. Difference between String, StringBuilder, StringBuffer String → immutable StringBuilder → mutable, not thread-safe StringBuffer → mutable, thread-safe 12. What is exception handling? try, catch, finally, throw, throws 13. Checked vs Unchecked Exceptions Checked → compile time (IOException) Unchecked → runtime (ArithmeticException) 14. Collections vs Arrays Collections grow dynamically, array has fixed length. 15. Difference between List, Set, Map List → ordered, duplicates allowed Set → unique, no duplicates Map → key-value pairs 16. HashMap vs Hashtable HashMap → not synchronized, faster Hashtable → synchronized, slower 17. What is a Thread? How to create? Extend Thread OR implement Runnable. 18. What is synchronization