Card card2 = new Holiday( "Bob" ) ; 
Card card3 = new Birthday( "Emily", 18 ) ;

Using Parent Class Reference Variables

A reference variable of a class C can be used for any object that inherits from class C. For example, a Card reference variable card2 can hold a reference to a Holiday object, a Valentine object, or a Birthday object. Of course, if the parent class is abstract, then the variable can only point at class that inherits from the parent.

Important:

When a method is invoked, it is the class of the object (not of the variable) that determines which method is run.

This is what you would expect. The method that is run is part of the object. For example:

Card card = new Valentine( "Joe", 14 ) ;
card.greeting();

Card card2 = new Holiday( "Bob" ) ; 
card2.greeting();

Card card3 = new Birthday( "Emily", 12 ) ; 
card3.greeting();

This will run the greeting() method for a Valentine, then it will run the greeting() method for a Holiday, then it will run the greeting() method for a Birthday. The type of the object in each case determines which version of the method is run.

Card card = new Valentine( "Joe", 14 ) ;
card.greeting();

card = new Holiday( "Bob" ) ; 
card.greeting();

card = new Birthday( "Emily", 12 ) ; 
card.greeting();