unit testing with rxPreference

[AshokSennanOkCredit]

while writing unit testing, I cannot return the rxPreference.getString() as Preference since RealPreference() class is the only class which implements Preference that too final class.

org.mockito.exceptions.misusing.WrongTypeOfReturnValue:
String cannot be returned by getString()
getString() should return Preference

how to return the Preference in junit