Write and Run Your First Java Program

What you need before writing a Java program

• A configured Java Development Kit (JDK) containing javac and java.
• A text editor such as Notepad, Notepad++, VS Code, or gedit.
• Alternatively, a Java IDE such as BlueJ, NetBeans, Eclipse, or IntelliJ IDEA.
• A folder where you can save the source file and open a terminal.

Steps to write and run a Java program

Writing and executing a Java program is typically a three-step process: writing the source code, compiling it into bytecode, and running it on the JVM.

Step 1: Create the Java source file

Open a text editor and save the following program as HelloWorld.java. The spelling and letter case must match the public class name exactly.

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

Expected Output

Hello World

Step 2: Compile the Java program

Open Command Prompt or a terminal, then use the cd command to navigate to the exact folder containing HelloWorld.java. For example, if you saved the file on your Desktop, use:

cd Desktop

Compile the source file with:

javac HelloWorld.java

If compilation succeeds, javac normally prints no success message. It creates HelloWorld.class in the same folder. This file contains Java bytecode.

Step 3: Run the Java program

Run the compiled class by passing its class name to the Java launcher:

java HelloWorld

Output

Hello World

Explanation of the HelloWorld program

Java source code facts

• Java source code is written and maintained by the programmer.
• Java source files use the .java extension.
• A source file can contain more than one top-level class, subject to Java's declaration rules.
• If a source file contains a public top-level class, the filename must match that class name.

package declaration;

import statements;

class ClassName {
    // class members
}

Java bytecode facts

• The Java compiler generates bytecode after successful compilation.
• Bytecode is stored in .class files.
• The class-file format enables the same compiled code to run on supported systems with compatible JVMs.
• The JVM loads, verifies, and executes the bytecode.
• Each compiled class normally has its own corresponding class file, including nested-class outputs where applicable.

Common first-program errors

java HelloWorld.class

java HelloWorld