Subclasses - Multimedia Introduction to Programming Using Java - page 151

(HourlyEmployee) y
The real type of y is Executive , so to cast it to HourlyEmployee is a mistake,
and this mistake is sure to lead to a runtime error and abortion of execution.

4.2.3 Operator instanceof

At times, you may want to test what the real class of a variable is, so that you do
not make the mistake of casting an object to something that it is not. Use opera-
tor instanceof to do this. As an example, the following statement determines
whether the real type of x is HourlyEmployee and, if so, executes its then-part.

Activity
4-2.2

if (x instanceof HourlyEmployee) {
...

}
Operator instanceof can be useful. But if you find it necessary to use it fre-
quently, perhaps the structure of your classes is not adequate and some redesign
may be in order. If you find yourself writing code like that below, where you
have to test all possible subclasses, think about redesigning.

a9

Object

equals(Object) toString() wait()

getClass() hashCode() clone()

Employee

Employee(String, int )
getCompensation()
addToSalary( double )
toString() getName()
setName(String) getStart()
setStart( int )

Gries

name

1997

start

salary 50,000

HourlyEmployee

getHourlyPay()
getHoursWorked()
changeSalary( double d)
toString()
HourlyEmployee(String, int ,
double , int )

1600

hoursWorked

hourlyPay 15.00

PartTimeEmployee

components defined in class PartTimeEmployee

An instance of subclass PartTimeEmployee , showing the Object partition Figure 4.7: