Do you think it would be OK to do the following?

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

A good answer might be:

Yes, both are correct, since Holiday is-a Card and Birthday is-a Card.

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.
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.

QUESTION 12:

Is it necessary to use three different variables for this program fragment?

Click Here after you have answered the question