Equip\Exception\FormatterException::invalidClass PHP Method

    public static function invalidClass($spec)
    {
        return new static(sprintf('Formatter class `%s` must implement `%s`', $spec, FormatterInterface::class));
    }

 /**
  * @inheritDoc
  *
  * @throws FormatterException
  *  If $classes does not implement the correct interface,
  *  or does not have a quality value.
  */
 protected function assertValid(array $classes)
 {
     parent::assertValid($classes);
     foreach ($classes as $formatter => $quality) {
         if (!is_subclass_of($formatter, FormatterInterface::class)) {
             throw FormatterException::invalidClass($formatter);
         }
         if (!is_float($quality)) {
             throw FormatterException::needsQuality($formatter);
         }
     }
 }