Hello World

A "Hello, World!" is a simple program that outputs Hello, World! on the screen. Since it's a very simple program, it's often used to introduce a new programming language to a newbie.

Let's explore how Java "Hello, World!" program works.

Java "Hello, World!" Program

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

Output:

Hello, World!

graph LR
    A[HelloWorld.java] -->|javac| B[HelloWorld.class]
    B -->|java| C[JVM]
    C -->|Output| D[Hello, World!]

    style A fill:#e1f5fe,stroke:#01579b
    style B fill:#fff9c4,stroke:#fbc02d
    style C fill:#fce4ec,stroke:#c2185b
    style D fill:#e0f2f1,stroke:#00695c

How Java "Hello, World!" Program Works?

1. Your First Program

In Java, any line starting with // is a comment. Comments are intended for users reading the code to understand the intent and functionality of the program. It is completely ignored by the Java compiler.

2. class HelloWorld

In Java, every application begins with a class definition. In the program, HelloWorld is the name of the class, and the class definition is:

class HelloWorld {
 ... .. ...
}

Rule: The name of the public class must match the filename. If the class is public class HelloWorld, the file must be named HelloWorld.java.

3. public static void main(String[] args)

This is the main method. Every application in Java must contain the main method. The Java compiler starts executing the code from the main method.

• public: Access modifier. It means the method is visible to everyone.
• static: It means the method belongs to the class, not an instance of the class.
• void: It means the method does not return any value.
• main: The name of the method.
• String[] args: An array of Strings. It stores command-line arguments passed to the program.

4. System.out.println("Hello, World!");

The code above is a print statement. It prints the text Hello, World! to standard output (your screen).

• System: A built-in class that contains useful members.
• out: A static member of the System class (Standard Output Stream).
• println: Method to print a line.

Command Line Arguments

The String[] args parameter in the main method allows you to pass arguments from the command line.

class Main {
  public static void main(String[] args) {
    System.out.println("Argument 1: " + args[0]);
  }
}

If you run this with java Main Hello, it will print Argument 1: Hello.

Challenge

Key Takeaways

• Class Definition: Every Java program must have at least one class.
• Main Method: public static void main(String[] args) is the entry point.
• Printing: System.out.println() prints to the console.
• Case Sensitive: Java is case-sensitive.

Common Pitfalls

[!WARNING] Case Sensitivity: Java is case-sensitive. Main is different from main.

[!WARNING] Missing Semicolons: Every statement in Java must end with a semicolon ;.

What's Next?

You've written your first program! Now, let's learn how to store and manipulate data using Variables. Learn about Variables & Literals →