Custom Name for an item

Code

PHP:

class powergem extends Item {

       public function __construct($meta = 0){
                  parent::__construct(467, $meta, "Power Gem");
    }
       public function getCustomName() : String {
               return "Power Gem";
    }

       public function getVanillaName() : String {
               return "Power Gem";
    }

       public function getMaxStackSize() : int {
               return 16;
    }
 }


Everything works when I do /give (name) 467. It shows it gives me Power Gem but when I put item in my hand it still shows Heart Of The Sea.