Java assigning values to variables in an Interface class through xml

I have an interface class the contains variables that needs to be implemented by other classes.

Interfaces can't contain variables as such - they can only contain constants, so it makes no sense to try to change the value of them.

From JLS 9.3:

Every field declaration in the body of an interface is implicitly public, static, and final.

Your interface should contain appropriate getters/setters instead - or have an abstract superclass which contains the appropriate fields.