PHP: Class not found despite autoloading

Question

I am using the ps-4 autoloader using Composer.

"autoload": {
    "psr-4": {
        "App\\":"app/",
        "Database\\":"database/"
    }
},

So, I have the main index.php file in the root directory like this

require 'vendor/autoload.php';
use App\Server;
$server = new Server();

The root directory has the app folder and a class called Server in it like this

namespace App;
echo "in server<hr>";

class Server{}

I get the echo "in server" so the class file gets included. But I get this error

Fatal error: Uncaught Error: Class "App\Server" not found in /var/www/html/index.php:8 Stack trace: #0 {main} thrown in /var/www/html/index.php on line 8

It looks for a class called "App\Server" instead of "Server". How do I fix this?