Java Reference
In-Depth Information
♦
When to Use Inheritance
The Employee class has a field named salary, which implies that an employee
has a
salary;
however, hourly employees have an hourly rate, which is quite different from a salary.
A tempting fix for this situation is to add a Boolean field to the Employee class named
isSalary that is true for salary employees and false for hourly employees. The salary field
could be used to represent an annual salary when isSalary is true or an hourly rate when
isSalary is false.
The isSalary field could also be used within the computePay() method to determine
which arithmetic to use because computing pay for salaried employees is different than for
hourly employees. For example,
public float computePay()
{
if(isSalary)
{
//Perform arithmetic for salaried employee
}
else
{
//Perform arithmetic for hourly employee
}
}
Adding a field such as isSalary and trying to use one class to represent two different
types of objects is not a good OOP design. You could make it work, but you are not taking
advantage of the benefits of inheritance.
If you use a field to determine the type of an object, your end result is a class that looks
object-oriented but is really procedural. For example, an Employee object will need to
check this added Boolean field just to know what type of object it is, causing the design of
the program to not be focused on objects.
A bigger problem arises when something needs to be changed. What happens to the
isSalary field when a new type of employee needs to be added? Suppose that the com-
pany starts hiring contractors who are paid by the day. The Boolean no longer works
because it can't be used to distinguish between three types. You could change the field to
an int, name it employeeType, and use an enumeration like 0 for salary, 1 for hourly, and
2 for contractor.
Again, you could make this work, but you have to make serious modifications to the
Employee class. The computePay() method will have to entirely rewritten:
public float computePay()
{
switch(employeeType)
{
case 0:
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