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

Using Parent Class Reference Variables

A reference variable of class "C" can be used with any object that is related by inheritance to class "C". For example, a Card reference variable card2 can hold a reference to a Holiday object, a Valentine object, or a Birthday object. Usually, references to a parent class are used to hold children objects.

Important Point:

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 (at least conceptually). 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.