Java First Program

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Every great journey begins with a single step, and in the world of Java, that step is writing and executing your very first program. Before you can build sophisticated applications, it’s essential to understand how a Java program is structured, how it runs, and why the main() method is the gateway for execution. In this chapter, we’ll walk through the process of creating, compiling, and running a Java program, while demystifying the critical role of the main() method.

The Anatomy of Your First Java Program

Let’s start by examining a simple Java program. This example prints a welcome message to the console and demonstrates the basic building blocks of Java code.

package quipohouse;

public class FirstProgram {
    public static void main(String[] args) {
        System.out.println("Welcome to Quipo House");
    }
}

Output:

Welcome to Quipo House

Key Components

1. Package Declaration:
package quipohouse;
Organizes classes into namespaces, making code modular and manageable.

2. Class Declaration:
public class FirstProgram { ... }
Every Java program must have at least one class definition.

3. The main() Method:
The entry point for the program. The JVM looks for this method to start execution.

4. Print Statement:
System.out.println("Welcome to Quipo House");
Outputs text to the console.

Understanding the main () Method

The main() method is more than just a starting point; it’s the bridge between your code and the Java Virtual Machine (JVM). Let’s break down its declaration:

public static void main(String[] args)

Explanation of Each Keyword

public: The method must be accessible to the JVM from outside the class.

static: Allows the JVM to invoke the method without creating an instance of the class.

void: Indicates that the method does not return any value.

main: The name recognized by the JVM as the starting point.

String[ ] args: An array for command-line arguments, enabling user input at runtime.

Why is the main() Method Important?

Predefined Entry Point: The JVM always starts execution from the main() method.

Overloading Possible: You can define multiple main() methods with different parameters, but only the standard signature is used as the entry point.

Command-Line Arguments: Enables dynamic input when running the program.

The Print Statement Explained

The line:

System.out.println("Welcome to Quipo House");

performs a simple yet powerful task-displaying output in the console.

System: A built-in class containing useful members, including out.

out: A static member of System class, representing the standard output stream.

println: A method that prints a message and moves the cursor to a new line.

How to Create and Run a Java Program

Let us walk through the practical steps to write, compile, and execute your first Java program.

Step 1: Write the Program

Open an Editor:
Use Notepad, or an IDE like IntelliJ IDEA, Eclipse, or VS Code.

Type the Code:
Enter your Java code and save the file with a .java extension.

Tip: The filename must match the public class name (e.g., FirstProgram.java).

Save the File:

Choose “All Files” as the file type.
Ensure the extension is .java.

Step 2: Compile and Run the Program

Open Command Prompt:
Press Win + R, type cmd, and hit Enter.

Navigate to the File Location:
Use the cd command to change directories to where your Java file is saved.

Compile the Program:

javac FirstProgram.java

This creates a FirstProgram.class file (bytecode).

Run the Program:

java FirstProgram

You should see the output:

Welcome to Quipo House

What Happens if There is No main () Method ?

If you try to run a Java program without a main() method, the JVM will not know where to begin execution. This results in a runtime error similar to:

Error: Main method not found in class MyClass, please define the main method as: 
   public static void main(String[] args)

Example:

public class MyClass {
    // Not a main method
    public static void myMethod() {
        System.out.println("Welcome to Quipo Tutorials!");
    }
}

Attempting to run this code will produce and error, as there is no valid entry point for the JVM.

Key Point

Every Java application starts with the main() method.

The main() method’s signature must be exact for the JVM to recognize it.

Always save your Java file with the same name as your public class.

Use System.out.println() for console output.

Compilation and execution are two separate steps: first compile, then run.

Two Minute Drill

1. Creating a New File:
Open your IDE, create a new project, and add a new Java class
(e.g., HelloWorld.java).

2. Writing the code:

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

3. Compiling the Program:
Use javac HelloWorld.java to compile.

4. Running the Program:
Use java HelloWorld to execute and see the output.