Implementing Polymorphism
The ItemInfo class must have display declared in it.
But the OxfordShirtInfo and ChinoInfo classes must supply
specific versions of display.
This can be done by:
- Declaring display to be virtual in ItemInfo, then
- Overriding display in both OxfordShirtInfo
and ChinoInfo.
Declaring a method to be virtual causes the behavior of the method to
depend on the type of its object at run time.