Java Basic Syntax
What is Java?
Java is a high-level, object-oriented programming language developed by Sun Microsystems. It’s platform-independent, secure, and widely used for building web, mobile, and desktop applications.
Java Program Structure
A Java program is essentially a collection of classes and objects that interact through methods. Let’s break down these core concepts:
Class — A blueprint for creating objects. It defines properties and behaviors.
Object — An instance of a class. It has a state (variables) and behavior (methods).
Method — A block of code that performs a specific task.
Instance Variable — A variable defined inside a class but outside any method. Each object has its own copy.
First Java Program
MyFirstJavaProgram.java
public class MyFirstJavaProgram {
/* This is my first Java program.
* It will print 'Hello World' as output.
*/
public static void main(String[] args) {
System.out.println("Hello World");
}
}How to Run This Program
Open Notepad and paste the code above.
Save the file as MyFirstJavaProgram.java.
Open Command Prompt and navigate to the file location.
Compile: javac MyFirstJavaProgram.java
Run: java MyFirstJavaProgram
Hello World
Java Syntax Rules
Rule | Description |
Case Sensitivity | Java is case-sensitive. Hello ≠ hello. |
Class Names | Start with uppercase. Example: StudentData. |
Method Names | Start with lowercase. Example: printDetails(). |
File Name | Must match the public class name. Example: MyFirstJavaProgram.java. |
main() Method | Every Java application starts from main() method. |
Java Identifiers
Identifiers are names used for classes, methods, variables, etc.
Must begin with a letter (A–Z or a–z), underscore _, or dollar sign $.
Cannot begin with a digit.
Cannot use Java reserved keywords.
Java Modifiers
Modifiers define access levels and behavior.
Access Modifiers: public, private, protected, default
Non-access Modifiers: final, abstract, static, strictfp
Java Variables
Three types of variables:
Local Variables — Declared inside methods.
Instance Variables — Non-static, unique to each object.
Class Variables — Static, shared across all instances.
Java Arrays
Arrays store multiple values of the same type.
int[] numbers = {1, 2, 3, 4};Java Enums
Enums restrict a variable to predefined constants.
enum Size { SMALL, MEDIUM, LARGE }
Size s = Size.MEDIUM;
System.out.println("Size: " + s);Size: MEDIUM
Java Comments
Single-line:
// This is a comment
Multi-line:
/* This is a multi-line comment */
Reserved Keywords in Java
Java has 50+ reserved keywords. Examples include: class, public, static, void, if, else, switch, try, catch, finally, return, new, this, super, enum, interface, extends, implements.