Chapter 1: Introduction 

1.6 Your First Java Program: Hello World!

The "Hello World" program is a traditional first program for any new programming language. It is a simple yet powerful introduction to the language’s basic syntax, compilation, and execution process. By successfully writing and running this program, you will gain a foundational understanding of how Java works.

1.6.1 Structure of a Simple Java Program

Let’s dissect the components of a typical "Hello World" program in Java. Every executable Java application must have a class that contains a special method called main.

Full HelloWorld.java Program:

Now, let’s break down each line:

public class ClassName { … } |: Understanding the Class Definition

class: This keyword is used to declare a class. In Java, all executable code must reside within a class. A class is a blueprint or a template for creating objects.

HelloWorld: This is the name of our class. By convention, class names in Java start with an uppercase letter and follow PascalCase (e.g., MyAwesomeProgram). It’s crucial that the class name matches the file name (e.g., HelloWorld.java).

public: This is an access modifier. public means that this class is accessible from anywhere. For the main class of a Java application, it must be public.

{ … }: These curly braces define the body of the class. All members (variables and methods) of the class go inside these braces.

public static void main(String[] args) { … } |: The main Method
This is arguably the most important line for a standalone Java application, as it serves as the program’s starting point. When you execute a Java program, the Java Virtual Machine looks for and executes this specific method.

public: This access modifier means the main method can be accessed from outside the HelloWorld class (specifically, by the JVM).

static: This keyword means that the main method belongs to the HelloWorld class itself, rather than to any specific object of the HelloWorld class. This allows the JVM to call this method directly using the class name (HelloWorld.main()) without needing to create an object of HelloWorld.

void: This keyword indicates that the main method does not return any value.

main: This is a special method name recognized by the JVM as the entry point for the program. It must be spelled exactly as main.

(String[] args): This defines a parameter for the main method. It’s an array of String objects, which can be used to receive command-line arguments when the program is run. For "Hello World," we won’t use it, but it’s a standard part of the main method signature.

System.out.println("…"); |: Outputting Text to the Console
This line is responsible for displaying the text "Hello, World!" on the console.

System: This is a built-in final class (meaning it cannot be subclassed) provided by Java’s standard library (java.lang package, which is automatically imported). It provides access to system resources.

out: This is a static member of the System class. It is an object of type PrintStream that represents the standard output stream, typically the console.

println(): This is a method of the PrintStream object (out). It stands for "print line" and is used to print the enclosed text (in this case, "Hello, World!") to the console, followed by a new line character, moving the cursor to the next line. There’s also print() which does not add a new line.

;: The semicolon at the end of the line is a statement terminator in Java, indicating the end of a statement.

1.6.2 Writing the Code

Let’s create the HelloWorld.java file step-by-step:

Open a Text Editor: Use any plain text editor (like Notepad on Windows, TextEdit on macOS, VS Code, or Notepad++). Do not use a word processor (like Microsoft Word), as they add formatting that will interfere with the code.

Type the Code: Carefully type (or copy-paste) the exact code provided above into your text editor:

            Pay close attention to capitalization, punctuation, and curly braces.

Save the File: Save the file with the same name as your public class, followed by the .java extension. In this case, save it as HelloWorld.java. It’s generally a good practice to save it in a simple, easy-to-access directory (e.g., C:\java_programs on Windows or ~/java_programs on Linux/macOS).

1.6.3 Compiling and Running from the Command Line

Once you have saved your HelloWorld.java file, you can compile and run it using the command-line tools provided by the JDK. This is a crucial skill for understanding the underlying Java environment.

Open Command Prompt / Terminal:

Windows: Search for "cmd" or "Command Prompt" and open it.

macOS / Linux: Open the "Terminal" application.

Navigate to Your File’s Directory: Use the cd (change directory) command to go to the folder where you saved HelloWorld.java.

Example:

○      Example (macOS/Linux):

Compile the Java Source Code:
Use the javac command to compile your .java file into bytecode.

What happens: If there are no syntax errors in your code, this command will generate a new file named HelloWorld.class in the same directory. This .class file contains the Java bytecode that the JVM can understand. If javac reports errors, review your HelloWorld.java file for typos, incorrect capitalization, or missing semicolons.

Run the Compiled Java Program:
Use the java (Java interpreter/launcher) command to execute the compiled bytecode.

What happens: This command instructs the JVM to load the HelloWorld.class file and start execution from its main method.

Expected Output: You should see the following text displayed in your command prompt/terminal:

1.6.4 Running in an IDE

Integrated Development Environments like Eclipse and NetBeans automate the compilation and execution process, making development much faster and more convenient. While the underlying steps (compilation, JVM execution) are the same, the IDE handles them behind the scenes.

If you have followed the setup instructions in Section 1.5.2, you can easily run your "Hello World" program within your chosen IDE:

In Eclipse or NetBeans:

Create a New Java Project: As detailed in Section 1.5.2, create a new Java project (e.g., MyHelloWorldProject).

Create a New Class: Within that project, create a new Java class named HelloWorld and ensure the public static void main(String[] args) method is automatically generated.

Write the Code: Inside the main method, add the line:

Save the File: The IDE usually saves automatically or prompts you to save.

Run the Program:

■      Eclipse: Right-click anywhere in the HelloWorld.java editor window, then select Run As > Java Application.

■      NetBeans: Click the green "Run Project" button in the toolbar, or right-click on the project in the "Projects" window and select Run.

View Output: The output will appear in the IDE’s built-in console window (often at the bottom of the screen):

By successfully completing this "Hello World" exercise, you have taken your first significant step into Java programming! You now understand the basic structure of a Java program and how it moves from source code to executable output.