Hello World - Your First Program

Write and understand your first Java program: the anatomy of a class, the main method, and printing output.

Overview

Every Java program starts with a class. Inside that class you need a special method called main - this is the entry point where Java begins executing your code. The simplest possible program has one class with one main method that prints a message. Understanding the Hello World program line by line is important. Each keyword serves a specific purpose and Java is strict about the syntax. Missing a brace, misspelling "main", or leaving out "static" will prevent the program from compiling or running. Java has three print methods on System.out: println (prints and moves to a new line), print (prints without a newline), and printf (prints formatted text). You will use println most often.

Key Points

•Every Java program must have at least one class; the class name must match the filename
•Execution starts at public static void main(String[] args) - this exact signature is required
•System.out.println() prints a line and moves the cursor to the next line
•System.out.print() prints without adding a newline at the end
•System.out.printf() prints formatted output using format specifiers like %s and %d
•Java is case-sensitive: Main, main, and MAIN are all different identifiers

Examples

The classic Hello World program

public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    }
}

public class HelloWorld - the class must be public and match the filename (HelloWorld.java).

Difference between println, print, and printf

public class PrintDemo {
    public static void main(String[] args) {
        System.out.println("Line 1");   // prints + newline
        System.out.println("Line 2");
        System.out.print("No ");        // no newline
        System.out.print("newline\n"); // manual newline
        System.out.printf("Hello, %s! You are %d years old.%n", "Alice", 25);
    }
}

\n is an escape sequence for a newline character. %n in printf also adds a platform-correct newline.

Anatomy of the main method - each keyword explained

public class Anatomy {
    //  ↓ accessible from outside the class
    // public
    //        ↓ belongs to the class, not an object
    //       static
    //              ↓ returns nothing
    //             void
    //                  ↓ the method name Java looks for
    //                 main
    //                      ↓ command-line arguments
    //                      (String[] args)
    public static void main(String[] args) {
        System.out.println("I understand main!");
    }
}

All four parts - public, static, void, main - must be present and in the correct order.

Common Mistakes

Writing `System.out.Println` with a capital P - Java is case-sensitive; it must be `println`
Forgetting the semicolon at the end of a statement - every statement in Java ends with ;
Class name mismatch: if the class is `HelloWorld` but the file is `helloworld.java`, compilation will fail

Note

In IntelliJ IDEA, type `sout` and press Tab - it auto-expands to `System.out.println()`. This shortcut saves a lot of typing.

Hello World - Your First Program

Overview

Key Points

Examples

Common Mistakes

Note

Exercises

Knowledge Check

Which method signature is the correct Java main method?