Java Reference
In-Depth Information
base-salaried commission employee: Bob Lewis
social security number: 444-44-4444
gross sales: $5,000.00; commission rate: 0.04;
base salary: $300.00
new base salary with 10% increase is: $330.00
earned $530.00
Employee 0 is a SalariedEmployee
Employee 1 is a HourlyEmployee
Employee 2 is a CommissionEmployee
Employee 3 is a BasePlusCommissionEmployee
Fig. 10.9
|
Employee
hierarchy test program. (Part 4 of 4.)
Creating the Array of
Employee
s
Line 33 declares
employees
and assigns it an array of four
Employee
variables. Line 36 as-
signs to
employees[0]
a reference to a
SalariedEmployee
object. Line 37 assigns to
employees[1]
a reference to an
HourlyEmployee
object. Line 38 assigns to
employees[2]
a reference to a
CommissionEmployee
object. Line 39 assigns to
employee[3]
a reference
to a
BasePlusCommissionEmployee
object. These assignments are allowed, because a
Sal-
ariedEmployee
is an
Employee
, an
HourlyEmployee
is an
Employee
, a
CommissionEmploy-
ee
is an
Employee
and a
BasePlusCommissionEmployee
is an
Employee
. Therefore, we can
assign the references of
SalariedEmployee
,
HourlyEmployee
,
CommissionEmployee
and
BasePlusCommissionEmployee
objects to
superclass
Employee
variables,
even though
Em-
ployee
is an abstract class
.
Polymorphically Processing
Employee
s
Lines 44-65 iterate through array
employees
and invoke methods
toString
and
earn-
ings
with
Employee
variable
currentEmployee
, which is assigned the reference to a differ-
ent
Employee
in the array on each iteration. The output illustrates that the specific
methods for each class are indeed invoked. All calls to method
toString
and
earnings
are
resolved at
execution
time, based on the
type
of the object to which
currentEmployee
re-
fers. This process is known as
dynamic binding
or
late binding
. For example, line 46
im-
plicitly
invokes method
toString
of the object to which
currentEmployee
refers. As a
result of
dynamic binding
, Java decides which class's
toString
method to call
at execution
time rather than at compile time
. Only the methods of class
Employee
can be called via an
Employee
variable (and
Employee
, of course, includes the methods of class
Object
). A su-
perclass reference can be used to invoke only methods of the
superclass
—the
subclass
meth-
od implementations are invoked
polymorphically
.
Performing Type-Specific Operations on
BasePlusCommissionEmployee
s
We perform special processing on
BasePlusCommissionEmployee
objects—as we encoun-
ter these objects at execution time, we increase their base salary by 10%. When processing
objects
polymorphically
, we typically do not need to worry about the
specifics
, but to adjust
the base salary, we
do
have to determine the
specific
type of
Employee
object at
execution
time
. Line 49 uses the
instanceof
operator to determine whether a particular
Employee
object's type is
BasePlusCommissionEmployee
. The condition in line 49 is
true
if the ob-
ject referenced by
currentEmployee
is a
BasePlusCommissionEmployee
. This would also
be
true
for any object of a
BasePlusCommissionEmployee
subclass because of the
is-a
rela-
