Discussions

You should check for 'null' and empty before trying to parse the returned string. Even better, since if the entered string cannot be parsed, you will throw a NumberFormatException error, you could use code similar to the following to catch any and all problems (including 'null' and empty):

So I should put the follow method in the specific greep object class (small,medium,and heavy, the only three classes that extends Greeps) to have the health bar follow them?

No, your revised code still deals with both Gegner1 and Gegner2 instances at the same time. It's supposed to be parameterised so that you can choose which one you want to deal with - either Gegner1 instances, or Gegner2 instances.

So without the 'public void' ...