Every programming language follows a particular structure, and Java is no exception. Even the smallest Java application has a well-defined format that helps the compiler understand the program.

In this lesson, we'll learn how a Java program is organized and understand each part using simple explanations and real-world examples.

A Simple Java Program

Let's begin with the same program we created in the previous lesson.

public class Main {

    public static void main(String[] args) {

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

    }

}

Although this program is very small, it contains all the basic building blocks of a Java application.

Class

public class Main

Every Java program is written inside a Class.

A class acts like a container that groups related data and functions together.

Without a class, Java programs cannot exist.

Main Method

public static void main(String[] args)

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

Whenever you run a Java program, the JVM first looks for the main() method.

Execution always begins from here.

Think of it as the main entrance to a shopping mall.

No matter how many stores are inside, everyone enters through the main gate.

Statements

Inside the main method are Java statements.

Example:

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

This statement instructs Java to display text on the console.

Every statement ends with a semicolon (;).

Curly Braces

Notice the { } symbols.

These braces define blocks of code.

Everything inside the class braces belongs to the class.

Everything inside the main method braces belongs to the main method.

They help Java understand where a block starts and ends.

Real-World Example

Imagine your smartphone.

The phone contains several applications.

Examples:

Camera
Gallery
Calculator
Music Player

Think of these as Classes.

Now open the Music application.

Inside it, you can perform actions like:

Play Music
Pause Music
Search Songs
Create Playlist

These actions are called Methods.

Similarly, in Java:

Package → Mobile Phone
Class → Individual App
Method → Actions inside the App

The main() method is simply the first action that Java performs.

Program Structure Diagram

Package
   │
   ▼
Class
   │
   ▼
Main Method
   │
   ▼
Statements
   │
   ▼
Output

Every Java application follows this basic hierarchy.

Rules to Remember

The filename must match the public class name.
Every Java program is written inside a class.
Execution always begins with the main() method.
Statements end with semicolons.
Java is case-sensitive.

Common Mistakes

Beginners often make these mistakes:

Class name and filename are different.
Missing semicolons.
Incorrect capitalization.
Writing code outside the class.
Forgetting curly braces.

Avoiding these mistakes will save you a lot of debugging time.

Key Takeaways

Every Java program is built using classes.
Classes contain methods.
The main() method is the entry point of every Java application.
Statements perform actions inside methods.
Curly braces organize code into logical blocks.
Following Java's structure makes programs easier to read and maintain.

Conclusion

Understanding the structure of a Java program is just as important as writing one. Once you know how classes, methods, and statements fit together, reading and writing Java code becomes much easier. These concepts form the foundation for object-oriented programming, which you'll explore throughout the rest of this course.