Here are some basic Java interview questions and answers: 1. What is Java? Answer: Java is a high-level, object-oriented programming language developed by Sun Microsystems (now owned by Oracle). It is platform-independent due to its "Write Once, Run Anywhere" (WORA) capability, thanks to the Java Virtual Machine (JVM). 2. What are the main features of Java? Answer: Platform Independent – Runs on any operating system with JVM. Object-Oriented – Follows OOP concepts like inheritance, encapsulation, and polymorphism. Secure – Provides security features like bytecode verification and no explicit pointers. Multithreaded – Allows concurrent execution of multiple threads. Robust – Handles memory management and exceptions effectively. High Performance – Uses Just-In-Time (JIT) compiler for better performance. 3. What is JVM, JRE, and JDK? Answer: JVM (Java Virtual Machine) – Converts Java bytecode into machine code to run on different platforms. JRE (Java Runtime Environment) – Contains JVM and libraries required to run Java applications. JDK (Java Development Kit) – Includes JRE, compiler (javac), and tools needed for Java development. 4. What is the difference between Primitive and Non-Primitive Data Types? Answer: Primitive Data Types Non-Primitive Data Types Predefined by Java Created by programmers (e.g., Classes, Arrays) Stores actual value Stores reference to an object Memory efficient Requires more memory Example: int, char, float, boolean Example: String, Array, Object 5. What are OOP principles in Java? Answer: Encapsulation – Wrapping data and methods inside a class. Inheritance – Acquiring properties of one class in another (using extends). Polymorphism – Multiple forms of the same method (Method Overloading and Overriding). Abstraction – Hiding implementation details using abstract classes and interfaces. 6. What is the difference between == and .equals()? Answer: == compares memory references (checks if two variables refer to the same object). .equals() checks the actual content of objects (especially for Strings). java Copy Edit String s1 = new String("Java"); String s2 = new String("Java"); System.out.println(s1 == s2); // false (different memory locations) System.out.println(s1.equals(s2)); // true (same content) 7. What is the difference between Array and ArrayList? Answer: Array ArrayList Fixed size Dynamic size (resizable) Can store both primitives and objects Stores only objects Performance is better Uses extra memory for resizing Example of Array: java Copy Edit int[] arr = new int[5]; Example of ArrayList: java Copy Edit ArrayList list = new ArrayList<>(); 8. What are Constructors in Java? Answer: A constructor is a special method used to initialize objects. It has the same name as the class and does not have a return type. Example: java Copy Edit class Example { int x; Example() { // Constructor x = 10; } } 9. What is the difference between "final", "finally", and "finalize"? Answer: Keyword Description final Used to declare constants, prevent method overriding, or class inheritance. finally A block that always executes after try-catch (used in exception handling). finalize() A method called by the garbage collector before destroying an object. 10. What is Exception Handling in Java? Answer: Exception Handling is used to handle runtime errors to maintain normal program flow. Java provides try, catch, finally, throw, and throws keywords for handling exceptions. Example: java Copy Edit try { int a = 10 / 0; // ArithmeticException } catch (ArithmeticException e) { System.out.println("Cannot divide by zero"); } finally { System.out.println("This block always executes"); } 11. What is the difference between Checked and Unchecked Exceptions? Answer: Checked Exception Unchecked Exception Checked at compile-time Checked at runtime Must be handled using try-catch Can be avoided by writing safe code Example: IOException, SQLException Example: NullPointerException, ArithmeticException 12. What is the difference between Interface and Abstract Class? Answer: Abstract Class Interface Can have both abstract and non-abstract methods Only abstract methods (before Java 8) Can have constructors No constructors Uses extends keyword Uses implements keyword Example: abstract class Animal {} Example: interface Animal {} 13. What is Multithreading in Java? Answer: Multithreading allows multiple threads to run concurrently, improving performance. Java provides the Thread class and Runnable interface for implementing multithreading. Example using Thread class: java Copy Edit class MyThread extends Thread { public void run() { System.out.println("Thread is running"); } } MyThread t1 = new MyThread(); t1.start(); Example using Runnable interface: java Copy Edit class MyRunnable implements Runnable { public void run() { System.out.println("Thread is running"); } } Thread t2 = new Thread(new MyRunnable()); t2.start(); 14. What is the difference between HashMap and HashSet? Answer: HashMap HashSet Stores key-value pairs Stores only unique values Allows one null key Allows only one null value Uses put() method Uses add() method