Free web hosting services - 144 Chapter 6 A better design for the
144 Chapter 6 A better design for the Salary and Hourly classes is to take their common elements and put them in a parent class, leaving the unique elements in the child classes. For example, the mailCheck() method could appear in the Employee parent class, and the computePay() method could appear in each of the child classes. The assumption is that the process of mailing a check is the same for all employees, but computing their pay is directly affected by how they are paid. Employees do not share a common computePay() method. By placing computePay() in each child class, the method will be written twice. This is not repeating code, though, because computePay() in Salary is quite different than computePay() in Hourly. The is a Relationship The is a relationship is a simple but powerful rule for testing if your inheritance is a good design. Whenever you use inheritance, you should be able to say that a child is a parent. If this statement is true, your inheritance is likely a good design. For example, it is true that a salaried employee is an employee. Similarly, an hourly employee is an employee; therefore, it is reasonable that the Salary and Hourly classes extend the Employee class. Let s look at an example where inheritance is not a good idea. Suppose that you have a Date class that represents a calendar date, and you want to use that class to keep track of the date when an employee was hired. Because inheritance has so many benefits, you decide to have the Employee class extend the Date class. When you instantiate an Employee object, you will also get a Date object for storing the employee s hire date; however, is it true that an employee is a date? The is a relationship clearly fails here. Although the result might work for us, an Employee class inheriting from a Date class is not a good design and should not be used. The solution to the improper use of inheritance with the Employee and Date classes is to realize that an employee has a hire date, not that an employee is a hire date. If an object has an attribute, the attribute should be a field in the class. The Employee class should add a field of type Date to represent the hire date of an employee, as opposed to extending the Date class.
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