Java Reference
In-Depth Information
PITFALL: Use of Private Instance Variables from the Base Class
An object of the class
HourlyEmployee
( Display 7.3 ) inherits, among other things, an
instance variable called
name
from the class
Employee
( Display 7.2 ). For example, the
following would set the value of the instance variable
name
of the
HourlyEmployee
object
joe
to
"Josephine"
:
joe.setName("Josephine");
But you must be a bit careful about how you manipulate inherited instance variables
such as
name
. The instance variable
name
of the class
HourlyEmployee
was inherited
from the class
Employee
, but the instance variable
name
is a private instance variable
in the definition of the class
Employee
. That means that
name
can only be accessed by
name within the definition of a method in the class
Employee
. An instance variable
(or method) that is private in a base class is not accessible
by name
in the definition of
a method in
any other class, not even in a method definition of a derived class
.
For example, notice the following method defi nition taken from the defi nition of
the class
HourlyEmployee
in Display 7.3 :
public
String toString()
{
return
(getName() + " " + getHireDate().toString()
+ "\n$" + wageRate + " per hour for " + hours + " hours");
}
You might wonder why we needed to use the methods
getName
and
getHireDate
.
You might be tempted to rewrite the method definition as follows:
public
String toString() //
Illegal version
{
return
(name + " " + hireDate.toString()
+ "\n$" + wageRate + " per hour for " + hours + " hours");
}
As the comment indicates, this will not work. The instance variables
name
and
hireDate
are private instance variables in the class
Employee
, and although a derived
class such as
HourlyEmployee
inherits these instance variables, it cannot access them
directly. You must instead use some public methods to access the instance variable
name
or
hireDate
, as we did in Display 7.3 .
In the defi nition of a derived class, you cannot mention a private inherited instance
variable by name. You must instead use public accessor and mutator methods (such as
getName
and
setName
) that were defi ned in the base class.
The fact that a private instance variable of a base class cannot be accessed in the def-
inition of a method of a derived class often seems wrong to people. After all, if you are
an hourly employee and you want to change your name, nobody says, “Sorry,
name
is
a private instance variable of the class
Employee
.” If you are an hourly employee, you
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