As previously explained, we don't want a user of our superhero class to be able to change a superhero's birth year after an instance is initialized because the superhero won't be born again at a different date. In fact, we want to calculate and make the superhero's age available to users. We use an approximated age in order to keep the focus on the properties and don't complicate our lives with the manipulation of complete dates and the NSDate class.
We can define a property called age with a getter method but without a setter method; that is, we will create a read-only computed property. This way, it is possible to retrieve the superhero's age, but we cannot change it because there isn't a setter defined for the property. The getter method returns the result of calculating the superhero's age based on the current year and the value of the birthYear stored property.
The following lines show the new version of the SuperHero class with...
