How can I add a new Method to the Attribute Class to extend the Functionality?

I want to extend the Attribute Class with a new Method that if you have a specific Value of that Attribute you use this Method you get a permanent Potion Effect. For example if you have a Attribute that has the min Value 0 and the Max Value 1 and want that you get the specified Potion Effect if the Value of that Attribute is 1.

I never made something like extending a certain Class with new Functionality and need some Instructions to get into. Maybe you know a good Tutorial about it.