📘LESSON 1

3.1 Lesson Outcomes

After completing this lesson, learners will be able to:

• Explain the purpose of the Java main method.
• Describe the role of the Java Virtual Machine (JVM).
• Create a simple Java class declaration.
• Run Java applications using the main() method.
• Pass arguments into the main() method.
• Explain the relationship between Java classes and the JVM.

3.2 Overview

Java applications rely on a structured execution process controlled by the Java Virtual Machine (JVM). Every Java application begins with a class declaration and starts execution from the main() method.

This lesson introduces learners to the structure of Java programs and explains how the JVM processes and executes Java applications. Learners will explore the purpose of Java classes, how the main() method works, and how command-line arguments are passed into Java applications.

These concepts form the foundation of all Java programming and are essential for developing desktop applications, enterprise systems, web applications, Android applications, and cloud-based Java solutions.

KT0101 — A Simple Java Class Declaration

A Java class is a blueprint used to define the structure and behaviour of objects in a Java application.

Every Java program is built using classes. A class contains:

• variables,
• methods,
• constructors,
• and program logic.

The class keyword is used to declare a Java class.

Structure of a Simple Java Class

public class Main {

}

Explanation of the Structure

Rules for Java Class Names

Java class names:

• must begin with a letter,
• cannot contain spaces,
• should use PascalCase naming convention,
• and should clearly describe the purpose of the class.

Examples of valid class names:

• Student
• Employee
• BankAccount
• ProductManager

Examples of invalid class names:

• 1Student
• student name
• @Class

Importance of Java Classes

Classes help developers:

• organize code,
• reuse functionality,
• simplify maintenance,
• and build scalable applications.

Without classes, Java applications cannot function properly because Java is a class-based programming language.

Example of a Simple Java Class

public class Student {

}

KT0102 — The main() Method

The main() method is the starting point of a Java application.

When a Java application runs, the JVM searches for the main() method and starts executing code from that method.

Without the main() method, a Java application cannot start execution normally.

Syntax of the main() Method

public static void main(String[] args) {

}

Components of the main() Method

Why the main() Method is Important

The main() method:

• acts as the entry point of the application,
• controls application startup,
• and allows developers to execute program instructions.

Every standalone Java application requires a main() method.

Example of the main() Method

public class Main {

    public static void main(String[] args) {

        System.out.println("Java Application Started");
    }
}

KT0103 — Running the main() Method

Java applications go through two important stages:

1. Compilation
2. Execution

The Java compiler converts Java source code into bytecode, and the JVM executes the bytecode.

Steps to Run a Java Program

Step 1 — Write the Java Program

public class Main {

    public static void main(String[] args) {

        System.out.println("Hello World");
    }
}

Step 2 — Save the File

The file name must match the class name.

Example:

Main.java

Step 3 — Compile the Program

javac Main.java

The compiler generates:

Main.class

Step 4 — Run the Program

java Main

Output

Hello World

Role of the JVM During Execution

The JVM:

• loads Java bytecode,
• verifies code security,
• manages memory,
• and executes Java instructions.

The JVM allows Java applications to run on multiple operating systems.

KT0104 — Passing Arguments to the main() Method

The main() method can receive input values called command-line arguments.

These arguments are passed into the String[] args parameter during execution.

Why Command-Line Arguments are Useful

Command-line arguments allow applications to:

• receive user input,
• process external data,
• and customize execution behaviour.

Example of Passing Arguments

public class Main {

    public static void main(String[] args) {

        System.out.println("Welcome " + args[0]);
    }
}

Running the Program

java Main Bukhosi

Output

Welcome Bukhosi

Multiple Arguments Example

public class Main {

    public static void main(String[] args) {

        System.out.println(args[0]);
        System.out.println(args[1]);
    }
}

Command

java Main Java Programming

Output

Java
Programming

KT0105 — The Java Main Class

The main class is the class containing the main() method.

The JVM starts application execution from this class.

Large Java applications may contain many classes, but only one class normally acts as the application entry point.

Characteristics of the Java Main Class

The main class:

• contains the main() method,
• controls application startup,
• coordinates program execution,
• and may call other classes and methods.

Example of a Java Main Class

public class Main {

    public static void main(String[] args) {

        Student learner = new Student();

        learner.study();
    }
}

Supporting Class Example

public class Student {

    void study() {

        System.out.println("Student is studying");
    }
}

Real-World Example

In a banking system:

• Main.java starts the application,
• Customer.java manages customer data,
• Account.java manages accounts,
• Transaction.java processes transactions.

The main class controls the overall application flow.

3.5 Key Notes / Summary

• Java applications are built using classes.
• The main() method is the entry point of a Java application.
• The JVM executes compiled Java bytecode.
• Java programs must be compiled before execution.
• Command-line arguments are stored in String[] args.
• The main class controls application startup and execution.
• Java supports platform independence through the JVM.