Using Polymorphism

AdultBirthday looks much like YouthBirthday but with a slightly different birthday message. Here is a program fragment:

AdultBirthday ab = new AdultBirthday( "Walter", 47 );
ab.greeting();

It will write out:

Dear Walter,
Happy 47th Birthday

You haven't changed at all!

Now inspect the following code:

Card crd = new YouthBirthday( "Sophia", 7 );
crd.greeting();

crd      = new AdultBirthday( "Walter", 47 );
crd.greeting();

crd      = new Birthday( "Zoe", 30 );
crd.greeting();