Answer:

Card      c;
Birthday  b;
Valentine v;
Holiday   h

c = new Valentine("Debby", 8);    //OK
b = new Valentine("Elroy", 3);    //WRONG
v = new Valentine("Fiona", 3);    //OK
h = new Birthday ("Greg",  35);   //WRONG

Another Hierarchy

Here is a picture of another hierarchy. There are no abstract classes in this hierarchy, so each class can be instantiated. As seen previously, the following is OK:

// OK
parentReferenceVariable = referenceToChild ;

As is this:

// OK
parentReferenceVariable = referenceToChildofChild ;

A reference variable of a parent type can hold a reference to an object of one of its child types (or a reference to one of its further descandants.) However, the opposite direction does not work:

// don't do this
childReferenceVariable = referenceToParent ;

Also, siblings are not descendants, so this does not work

// don't do this
referenceVariable = referenceToSibling ;

QUESTION 18:

Here are some variables:

Rodent rod;
Rat    rat;
Mouse  mou;

Look at the table and decide if each section of code is correct or not.

code section	OK or Not?	code section	OK or Not?
rod = new Rat();		rod = new FieldMouse();
mou = new Rat();		mou = new Rodent();
rat = new Rodent();		rat = new LabRat();
rat = new FieldMouse();		rat = new Mouse();

There is still much more to learn about inheritance and polymorphism.