I'm trying to understand how to use PSR-4 with Composer. I have 3 folders in my project: `folder`, `folder2` and `vendor. Outside all these folders there's four files: index.php, composer.phar, composer.json and X.php. Some of these files are descibed below:

index.php

```
require_once 'vendor/autoload.php';

use \folder\in\A;
use \folder\in\B;
use \folder\C;
use \folder2\D;

$a = new A();
$b = new B();
$c = new C();
$d = new D();
$x = new X();

```

X.php

```
<?php

class X {
        public function __construct() {
            echo "Classe X";
        }
}

```

composer.json

```
{
    "autoload":{
        "psr-4": {
            "folder\\": "folder/",
            "folder2\\": "folder2/"
        }
    }
}

```

The index.php file is working well for the files that are in folder and folder2, but the X.php is not being found. What's the problem? How I can found the X.php file in index.php using Composer and PSR-4? I can found the X.php file using require\_once, though, but I want to know how to this with Composer and PSR-4.