In the Rockette example world, all the items in the list were instances of the Rockette class. In Exercise 1 above, however, the items in a list are...

In the Rockette example world, all the items in the list were instances of the Rockette class. In Exercise 1 above, however, the items in a list are instances of different people.  Having items in a list that are not instances of the same class could cause a problem when the program is executed. What do each of the items in a list need to have in common to avoid a problem at runtime?