Writing modular code using a command-line interface

As a tradition, let's create a simple module called helloApp, which will have a simple message and will be required by another module called helloClient. Here, we will use a command line interface to create and run the module.

Create a helloApp module folder named com.packt.helloapp and a package folder named com\packt\helloapp:

mkdir com.packt.helloapp
mkdir com.packt.helloapp\com\packt\helloapp

Now, create a HelloApp.java component class under the package name com.packt.helloapp\com\packt\helloapp and a modue-info.java file at root folder com.packt.helloapp:

HelloApp.java

package com.packt.helloapp;

public class HelloApp {
  public String sayHelloJava() {
      return "Hello Java 9 Module System";
  }
}

module-info.java

module com.packt.helloapp {
 // define exports or requires.
}

Now, we will create another module called helloClient. Create a helloClient module with the folder name com.packt.hello.client and a package with the folder name com\packt\hello\client:

mkdir com.packt.hello.client
mkdir com.packt.hello.client\com\packt\hello\client

Let's create another component class called HelloClient.java under the com.packt.hello.client\com\packt\hello\client package and create a module-info.java file at root folder com.packt.hello.client:

helloClient.java

package com.packt.hello.client;

public class HelloClient {
  public static void main (String arg[]) {
     //code 
  }
}

module-info.java
module com.packt.hello.client {
 //define exports or requires
}

Both modules are independent modules so are not dependent on each other. But if we want to use a method called sayHelloJava() in the HelloClient class, then we have to import the module, otherwise it will give a compile time error package com.packt.helloapp is not visible.