private function concatenateObjects()
    {
        $output = "";

        if (empty($this->objectList)) return $output;

        foreach ($this->objectList as $object) {
            if ($object->isValid()) {
                $output .= $object->getData();

                $modifiedTime = $object->getLastModifiedTimestamp();

                if (!empty($modifiedTime) && $modifiedTime > $this->lastModifiedTime) {
                    $this->lastModifiedTime = $modifiedTime;
                }
            } else {
                $output .= $this->handleInvalidFile($object->getValue());
            }
        }

        if (!empty($this->lastModifiedTime)) {
            $output = $this->formatLastModifiedTime() . $output;
        }

        return $output;
    }