Java Source File Rules: Naming, Public Classes, and Compilation Explained

When writing a Java program, it's not just your syntax that matters — the structure of your source file plays a critical role in how your code compiles and runs. Let's explore the most important rules Java developers must follow when declaring and organizing a .java file.

📌 1. File Naming and Extensions

Every Java source file must end with the .java extension.
Example:

HelloWorld.java

You can name the file anything as long as there is no public class inside it. However, if a source file contains a public class, the file name must match the class name exactly (including case).

✅ Correct:

public class Main {}
// File name: Main.java

❌ Incorrect:

public class Main {}
// File name: Test.java → Compilation Error

👥 2. Number of Classes per File

You can define multiple classes, interfaces, enums, or annotations in a single .java file. However:

• Only one public type is allowed per file.
• Each non-public type can have any name, and the compiler will still generate separate .class files for each.

class Animal {}
class Dog {}
public class Main {}
// Must be saved as Main.java

🚀 3. The Role of the Main Method

Any class (public or not) can contain a public static void main(String[] args) method.

During execution:

java ClassName

• The JVM looks for the class specified in the command and checks if it contains a valid main() method.
• If found, it executes it. If not, you'll get an error.

🛠️ 4. Compilation Behavior

When compiling your file using:

javac FileName.java

Java performs the following steps:

1. Syntax Check: All code is validated for syntax errors. If any are found, no .class files are generated.
2. Type-by-Type Validation: If syntax is fine, each declared type is checked in order.
3. Class File Generation: If a type is valid, a .class file is created for it.

To see what's happening during compilation, use:

javac -verbose FileName.java

This shows you every class file being loaded and compiled.

📦 5. Package and Import Declarations

• A package statement, if used, must be the first line of the source file.
• import statements come next.
• Class, interface, or enum declarations follow.

✅ Order of declarations:

package com.example;

import java.util.*;

public class Main { ... }

You can have any number of import statements, and both package and import declarations apply to all types declared in the file.

🧠 Final Thoughts

Understanding source file structure isn’t just academic — it’s essential for building real-world Java projects, working with IDEs, or preparing for interviews. These rules ensure your Java code is portable, maintainable, and error-free during compilation.

💡 Pro Tip: Even seasoned Java developers run into simple compilation issues due to naming mismatches or misplaced public classes. Stick to the rules — and your builds will thank you.