After writing your first HelloWorld.java, you’ve seen how Java works at a basic level. But real-world Java projects are more than a single file. They involve organizing code, reusing libraries, and automating builds.

In this chapter, you’ll learn how Java programs are structured—from simple packages to large, modular applications—and how to bundle and build them using modern tools like Maven and Gradle.

As your program grows, you’ll have multiple .java files. To avoid naming conflicts and keep things clean, Java uses packages.

Here’s an example of moving from a flat HelloWorld to a structured layout:

Inside src/com/example/app/utils/GreetHelper.java:

package com.example.app.utils;

public class GreetHelper {
    public static void sayHello() {
        System.out.println("Hello from GreetHelper!");
    }
}

And in src/com/example/app/Main.java:

package com.example.app;

import com.example.app.utils.GreetHelper;

public class Main {
    public static void main(String[] args) {
        GreetHelper.sayHello();
    }
}

Modules are a step above packages. Introduced in Java 9, they help structure large applications with clear boundaries and strong encapsulation.

At the root of a module, you define a src/com/example/app/module-info.java:

module com.example.app {
    exports com.example.app.utils;
}

This makes only com.example.app.utils available to other modules—everything else is hidden by default.

To Run this,

java src/com/example/app/Main.java

To distribute or run your app easily, you can package it as a JAR (Java ARchive).

To compile al java classes, you’d use:

javac -d out src/com/example/app/Main.java src/com/example/app/utils/GreetHelper.java

From your compiled classes:

jar --create --file app.jar -C out .

To run the JAR:

java -cp app.jar com.example.app.Main

You can also add a manifest to make it executable:

jar --create --file app.jar --main-class com.example.app.Main -C out .
java -jar app.jar

This is how real-world Java development happenss.