05_Writing_First_Code

Writing the First Code

Where Do Java Files Go - Java Source File Location :

- All Java source files are typically stored inside a src (source) folder.

- Java source files use the .java extension.

- It's best practice to organize source files using PACKAGES.

src

└── javaprograms

└── MyClass.java

Package

- A PACKAGE in Java is like a FOLDER on our computer.

Just like a folder can contain many files, a package can contain many Java classes (files).

- Helps in:
  - Organizing code
  - Avoiding name conflicts
  - Controlling access

Syntax to declare a package - package javaprograms

package is a RESERVED KEYWORD in Java.

Package names should be written in LOWER CASE by convention.

Java Program Structure

A basic Java class looks like this:

public class Example {

    public static void main(String[] args) {

        // Code block starts

        //  logic here

        // Code block ends
    }
}

Understanding the Block Structure

Class starts here 

public class HelloWorld {


    public static void main(String[] args) {    // Main method starts here 

        System.out.println("Hello, Java!");

    } //  Main method ends here

} //  Class ends here

Key Components Explained

| Part                  | Meaning                          
|-----------------------|--------------------------------------------|
| public class          | Declares a class accessible from anywhere  |
| main(String[] args)   | Entry point of the Java program            |
| { }                   | Code block boundaries                      |
| args                  | Runtime arguments passed during execution  |

Can We Create a private Class?

In Java:

- A top-level class is a class declared directly in a `.java` file (outside of any other class).

- A nested class is a class defined **inside another class**.


| Class Type                  | Allowed? | Explanation                                                               |
|-----------------------------|----------|---------------------------------------------------------------------------|
| Top-level private class     |  No      | Java requires the main class to be accessible — PUBLIC or default only.   |
| Nested private class        | Yes      | A private class inside another class is allowed, used for hiding logic.   |

POINTS

> A NESTED CLASS is like a helper class that lives inside another class.  
> A TOP - LEVEL class is the outer class — and it cannot be PRIVATE because Java needs to access it to run our code.

How many Public Classes allowed per java file?

Only one public class is allowed per .java file, and the file name must match the public class name.

// File name: MyClass.java


public class MyClass {

    public static void main(String[] args) {

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

File name = MyClass.java

One public class = MyClass

Why multiple public classes not allowed in java?

Entry Point Clarity

   - The Java compiler and JVM need a clear, unambiguous entry point for program execution.

   - Allowing multiple public classes would create confusion about which one to load.

What We Cannot Do - Adding multiple public class in a single .java file!

// File name: MyFile.java

public class A {
    
}

public class B {
    //  Compilation error: Only one public class is allowed
}

Summary

Place .java files under src/package_name/.

Always start the file with a package declaration (unless in the default package).

Only one public top-level class is allowed per .java file.

Class name must match the filename when using public.

Q: Why does Java allow only one public class per .java file? What happens if we try to define two public classes in the same file?

Java enforces a one-to-one relationship between the public class and the file name for clarity, maintainability, and proper class loading by the JVM.

- If two public classes are defined in the same .java file, the compiler will throw an error:  The public type X must be defined in its own file.

- Only one class can be public, and its name **must match the file name**.