INHERITANCE AND POLYMORPHISM - Java Programming: From Problem Analysis to Program Design - page 669

After the statement in Line 12 executes, shapeRef points to the object box . The
statement in Line 13 executes the method toString . Because shapeRef points to an
object of the class BoxFigure , the method toString of the class BoxFigure
executes. When the method toString of the class BoxFigure executes, it also executes
the method area . In this case, the method area of the class BoxFigure executes, which
outputs the surface area of the box.

If a method of a class is declared final, it cannot be overridden with a new definition in
a derived class. You declare a method of a class final by using the keyword final . For
example, the following method is final:

public final void doSomeThing()
{

//...

}

Similarly, you can also declare a class final using the keyword final . If a class is
declared final, then no other class can be derived from this class ; that is, it cannot be
the superclass of any other classes.

Java does not use late binding for methods that are marked private , final ,or static .

As illustrated above, a reference variable of a superclass type can point to an object of its
subclass. However, you cannot automatically consider a superclass object to be an object
of a subclass. In other words, you cannot automatically make a reference variable of a
subclass type point to an object of its superclass.
Suppose that supRef is a reference variable of a superclass type. Moreover, suppose that
supRef points to an object of its subclass. You can use an appropriate cast operator on
supRef and make a reference variable of the subclass point to the object. On the other
hand, if supRef does not point to a subclass object and you use a cast operator on supRef
to make a reference variable of the subclass point to the object, then Java will throw a
ClassCastException —indicating that the class cast is not allowed.
Suppose name , nameRef , employee , and employeeRef are as declared in the begining of
this section, that is:

1
0

Person name, nameRef; //Line 1

PartTimeEmployee employee, employeeRef; //Line 2

name = new Person("John", "Blair"); //Line 3

employee = new PartTimeEmployee("Susan", "Johnson",
12.50, 45); //Line 4

nameRef = employee; //Line 5

Now consider the following statement:

employeeRef = (PartTimeEmployee) name; //Illegal