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Your terminology is confusing.
You don't call classes. You call methods. A method belongs to a class.
You can call methods from within a method.
To call methods in another class, you need a...

Not sure what you mean by "calling" a class.
Do you mean creating an instance of the class? Use the new statement:
AClass aClsRef = new AClass(); // Create an instance of AClass

To call...