Please define namespace with first line and use like we do it below.

    
<?php 
namespace Appsense\\popop;
class shortcode{
public function gernerateShortcode(){
add_shortcode(\'appsense_popop\',array(\'Appsense\\popop\\shortcode\',\'popop_shortcode\'));
}
public function popop_shortcode(){
$output = \\\"Hello world!\\\";
return $output;
}
}