Java Reference
In-Depth Information
tionship a subclass has with its superclass. Lines 53-54
downcast
currentEmployee
from
type
Employee
to type
BasePlusCommissionEmployee
—this cast is allowed only if the ob-
ject has an
is-a
relationship with
BasePlusCommissionEmployee
. The condition at line 49
ensures that this is the case. This cast is required if we're to invoke subclass
BasePlusCom-
missionEmployee
methods
getBaseSalary
and
setBaseSalary
on the current
Employee
object—as you'll see momentarily,
attempting to invoke a subclass-only method directly on a
superclass reference is a compilation error.
Common Programming Error 10.3
Assigning a superclass variable to a subclass variable is a compilation error.
Common Programming Error 10.4
When downcasting a reference, a
ClassCastException
occurs if the referenced object at ex-
ecution time does not have an
is-a
relationship with the type specified in the cast operator.
If the
instanceof
expression in line 49 is
true
, lines 53-60 perform the special pro-
cessing required for the
BasePlusCommissionEmployee
object. Using
BasePlusCommis-
sionEmployee
variable
employee
, line 56 invokes subclass-only methods
getBaseSalary
and
setBaseSalary
to retrieve and update the employee's base salary with the 10% raise.
Calling
earnings
Polymorphically
Lines 63-64 invoke method
earnings
on
currentEmployee
, which polymorphically calls
the appropriate subclass object's
earnings
method. Obtaining the earnings of the
Sala-
riedEmployee
,
HourlyEmployee
and
CommissionEmployee
polymorphically in lines 63-
64 produces the same results as obtaining these employees' earnings individually in lines
22-27. The earnings amount obtained for the
BasePlusCommissionEmployee
in lines 63-
64 is higher than that obtained in lines 28-30, due to the 10% increase in its base salary.
Getting Each
Employee
's Class Name
Lines 68-70 display each employee's type as a
String
. Every object
knows its own class
and
can access this information through the
getClass
method, which all classes inherit from
class
Object
. Method
getClass
returns an object of type
Class
(from package
ja-
va.lang
), which contains information about the object's type, including its class name.
Line 70 invokes
getClass
on the current object to get its class. The result of the
getClass
call is used to invoke
getName
to get the object's class name.
Avoiding Compilation Errors with Downcasting
In the previous example, we avoided several compilation errors by
downcasting
an
Employ-
ee
variable to a
BasePlusCommissionEmployee
variable in lines 53-54. If you remove the
cast operator
(BasePlusCommissionEmployee)
from line 54 and attempt to assign
Em-
ployee
variable
currentEmployee
directly to
BasePlusCommissionEmployee
variable
em-
ployee
, you'll receive an “
incompatible
types
” compilation error. This error indicates
that the attempt to assign the reference of superclass object
currentEmployee
to subclass
variable
employee
is
not
allowed. The compiler prevents this assignment because a
Com-
missionEmployee
is
not
a
BasePlusCommissionEmployee
—
the
is-a
relationship applies only
between the subclass and its superclasses, not vice versa.
