Thanks Dennis,
This almost it 
The only thing that is missing that ControlA is inherit/based/extends from other control of UI5 like auto complete control .(like multiple inheritance...) .
from abstract it inherit some basic things like you already did (like text and sayHello) and from auto complete the already implemented logic which is provided by the default ui5 Control .
Is it possible ? I know that this is not trivial ...
Cheers Expert!