Typo3 extbase: inject services

The problem: I create a ViewHelper, which calls a service (calcService) and in this service I want to use another service, named AuthenticationService. I got the following error message:

Fatal error: Call to a member function getUser() on a non-object 

The problem is that I tried to call the functions statically instead of inject or via objectManager. When I tried the inject in my CalcService, I got the errormessage above and the reason is, that I called the service in my viewhelper in the wrong way.

WRONG:

use Ophi\OphiSomething\Service\CalcService;
class CalcSomethingViewHelper extends AbstractViewHelper {

	public function render($str) {
		return CalcService::doSomething($str);
	}
} 

CORRECT:

use Ophi\OphiSomething\Service\CalcService;
class CalcSomethingViewHelper extends AbstractViewHelper {
	/**
	 * @var \Ophi\OphiSomething\Service\CalcService
	 * @inject
	 */
	private $calcService;
	public function render($str) {
		return $this->calcService($str);
	}
}

…good to know.