Polymorphism

Polymorphism means "having many forms." In Java, it means that a single variable might be used with several objects of related classes at different times in a program. When the variable is used with "dot notation" variable.method() to invoke a method, exactly which method is run depends on the object that the variable currently refers to. Here is an example:

 . . . .                           // class definitions as before

public class CardTester
{
  public static void main ( String[] args ) throws IOException
  {

    Card card = new Holiday( "Amy" );
    card.greeting();                      //Invoke a Holiday greeting()

    card = new Valentine( "Bob", 3 );
    card.greeting();                      //Invoke a Valentine greeting()

    card = new Birthday( "Cindy", 17 );
    card.greeting();                      //Invoke a Birthday greeting()

  }
}