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Then you'll need to look at the Skill class code and only use constructors that are available for use.

That's not how you do this, by using statics, and in fact you should strive to avoid use of statics as much as possible except in certain situations where it is useful (and this is not one).

To...

Why do you want to do this? This seems a strange way to organize your code.

Note that in the static world xp has no meaning as it simply doesn't exist. It requires an instance of the class to...