Java Hello World Program

Writing Your First Java Program

Create a file named HelloWorld.java. Java is strict about naming — your filename must match the class name if it's public. HelloWorld in HelloWorld.java should be same as HelloWorld in public class HelloWorld.

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

Hello, World!

Hello World

Compiling the Program

Open a terminal or command prompt where your HelloWorld.java file is saved and run the following command:

javac HelloWorld.java

What happens during compilation?

The javac command invokes the Java compiler. Here's what it does behind the scenes:

• It reads your .java source file — which contains human-readable Java code.
• Then it checks for syntax errors or code issues.
• If everything is correct, it converts the source code into bytecode — a platform-independent intermediate form of the code.
• This bytecode is saved in a new file named HelloWorld.class.

This bytecode is what the Java Virtual Machine (JVM) understands and executes — not the original source code.

Running the Program

Now, execute the compiled bytecode using the java command:

java HelloWorld

What happens during execution?

The java command calls the JVM (Java Virtual Machine) to:

• Locate the HelloWorld.class file.
• Load the bytecode into memory.
• Interpret or Just-In-Time (JIT) compile it into native machine code specific to your system.
• Execute the main method of the class.

Hello, World!

You’ve just compiled and executed your first Java program. This process — compile once, run anywhere — is what makes Java so powerful and portable.

Common Issues to Watch For

• Filename mismatch: Ensure your filename is HelloWorld.java if your class is HelloWorld.
• Missing main method: Java won’t run if it doesn’t find the exact public static void main method.
• Case sensitivity: Java is case-sensitive. Main and main are not the same.