The soul is dyed the color of its thoughts. Think only on those things that are in line with your principles and can bear the light of day. The content of your character is your choice. Day by day, what you do is who you become. Your integrity is your destiny - it is the light that guides your way. - Heraclitus
I would also consider introducing a factory class for creating different implementations of the Holder class based on the Class you pass to the factory method. Ilja's tip about the Visitor pattern sounds good as well.
The code definitely has a smell. Pretty much any time you have an if condition based on type it should be refactored unless it is in a factory method.
It is a little hard to give a definitive recommendation without knowing a little more about what you are trying to do. In your example the Holder has no value if it just executes the other classes do method.
I would make all 3 classes implement a common interface (I'll call it the DoInterface). Forgive any syntax errors.
Your code would look as follows now:
[ edited to break long line -ds ] [ August 21, 2004: Message edited by: Dirk Schreckmann ]