Top of the Hierarchy

The "top" of the hierarchy of cards is the class Card. Sometimes this is called the root of the hierarchy. A variable of the root type, Card, can be used with any of its descendants:

Card crd = new YouthBirthday( "Valerie", 7 );
crd.greeting();             // print a youth birthday greeting

crd = new AdultBirthday( "Walter", 47 );
crd.greeting();             // print an adult birthday greeting

crd = new Birthday( "Zoe", 30 );
crd.greeting();             // print a generic birthday greeting

Holiday  hol = new Holiday( "Kelly" );
Valentine val = new Valentine( "Jill", 42 );

crd = hol;
crd.greeting();             // print a holiday greeting

crd  = val;
crd.greeting();             // print a valentine greeting

When a program uses several related objects, it is often useful to have a reference variable than can be used with any of them.