1.9 Union design pattern  (Page 2/5)

The ability for a subclass instance to be used wherever its superclass (or implemented interface) is required. This is because all subclasses are abstractly equivalent to their superclasses. However, the use of different subclass instances in the same situation will produce differing resultant program behaviors because each subclass will behave in their own, differing concrete manners, even though those concrete behaviors are all abstractly equivalent. For more information, see the Wikipedia article on polymorphism and this web page on polymorphism and abstraction .