INHERITANCE AND POLYMORPHISM - Java Programming: From Problem Analysis to Program Design - page 670

This statement will throw a ClassCastException because name points to an object of
the class Person . It does not refer to an object of the class PartTimeEmployee .
However, the following statement is legal:
employeeRef = (PartTimeEmployee) nameRef;

Because nameRef refers to the object employee (as set by the statement in Line 5), and
employee is a reference variable of the PartTimeEmployee type, this statement would
make employeeRef point to the object employee . Therefore, the output of the statement:

System.out.println(employeeRef);
is:

Susan Johnson's wages are: $562.50

Operator instanceof
As previously described, an object of a subclass type can be considered an object of the
superclass type. Moreover, by using an appropriate cast operator, you can treat an object of
a superclass type as an object of a subclass type. To determine whether a reference variable
that points to an object is of a particular class type, Java provides the operator instanceof .
Consider the following expression (suppose that p is an object of a class type):

p instanceof BoxShape

This expression evaluates to true if p points to an object of the class BoxShape ;
otherwise, it evaluates to false . The class BoxShape is defined in Example 10-6,
which further illustrates how the operator instanceof works.

EXAMPLE 10-6

Consider the following classes: (The class es RectangleShape and BoxShape are the same
as the class es Rectangle and Box given earlier in this chapter. The only difference is that
the instance variables of the class es Rectangle and Box are private . Because the instance
variables of the class RectangleShape are protected , they can be directly accessed in
the class BoxShape . Therefore, the definitions of the methods area and volume of the
class BoxShape directly access the instance variables length and width of the class
RectangleShape .)
public class RectangleShape
{

protected double length;
protected double width;

public RectangleShape()
{

length = 0;
width = 0;

}