DRAFT
Observed
A class where all fields are private is immutable
A class where all fields are private can still be mutable
While it is correct that code in other classes cannot touch private fields in our class, the values of private
fields in our class can be changed by methods of that class (e.g., by a setter method).
Thus, there is a way for the fields to change their value, and thus the class is mutable.
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