Can't Instantiate an Abstract Class

You can't do the following:

   . . . .

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

    Card card = new Card() ;        // can't instantiate abstract class
    card.greeting() ;

     . . . .
  }

Because Card is an abstract class, the compiler flags this as a syntax error. Card does have a constructor that is (implicitly) invoked by its children, but it cannot be invoked directly. However, the following is OK:

   . . . .

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

    Card card = new Valentine( "Joe", 14 ) ;      // a Valentine is-a Card
    card.greeting() ;

     . . . .
  }

It is OK to save a reference to a Valentine object in a reference variable of type Card because Valentine is-a Card. You can think of the reference variable Card as being a card rack designed to hold any type of a Card.

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