filter shortcode example - does not work

gistfile1.php

<?php

    /**
    *   plugin name: test shortcode filter
    *   author: travis ballard
    */

    class ShortcodeFilter
    {
        public $shortcode = 'testshortcode';
        
        public function __construct(){
            add_shortcode( apply_filters( 'test_shortcode_filter', $this->shortcode ), array( $this, 'shortcode' ) );
        }

        public function shortcode($a){
            return '<p>The shortcode worked!</p>';
        }
    }

    $sf = new ShortcodeFilter();