Subclasses - Multimedia Introduction to Programming Using Java - page 150

4.2.2 Explicit widening and narrowing

To cast an expression to some class, use the class name, within parentheses, as a
prefix operator. Such a cast changes the apparent type of an expression. For
example, this expression has apparent class Executive :

Activities
4-2.1, 4-2.2

new Executive("Gries", 1966, 25000)
but this expression has apparent type Employee :

(Employee) ( new Executive("Gries", 1966, 25000))
A cast (C) e is widening if C is already the apparent type of e or if C is a
superclass of the apparent type of e . A class is narrowing if C is a subclass of the
apparent type of e . Below, the cast of x is a widening cast, while the cast of y is
a narrowing cast:

Executive x= new Executive("Gries", 1966, 25000);
Employee y= (Employee) x;
Executive z= (Executive) y;
Widening casts are unnecessary because Java does them automatically.
You have to be careful with narrowing casts. You can cast an expression only
to its real type or any superclass of its real type. To see the errors you might
make, consider this expression:

a9

Employee

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

name Gries

1997

start

50,000

salary

HourlyEmployee

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

1600

hoursWorked

hourlyPay 15.00

PartTimeEmployee

components declared in class PartTimeEmployee

Figure 4.6: An instance of subclass ParttimeEmployee