There something ambiguous about this idea and I need some clarifications.
My problem is when using this code:
public class B {
private void don(
is this because the main function is in the same class as the method "don"
No, it's because A's don() is unrelated to B's don() method, in spite of having the same name and argument list. private methods are hidden inside their class. They cannot be invoked directly by outside callers, such as main method in your case, because they are encapsulated inside the class. They do not participate in method overrides.