Java™ Application Development on Linux - Dator

12.6 Essential and Nonessential285• Does this attribute or method belong here?If you find yourself specifying nearly identical methods in more thanone class, this should make you ask if the classes should have a commonbase class from which they should inherit, or if there should be a newunrelated class that they all share by composition.If the functionality is the same for a set of classes, and the classes arespecific instances of a more general type, the method should be on thegeneral class. For example, a changeName() method should probably beon Person, not on Employee or Customer, because the functionality is thesame for all three classes. By contrast, a changeEmployeeNumber()method should be only on Employee. It should not be on Person, becausenot all Persons are Employees. There may also be methods that are commonto both Employee and Customer types, but are radically different inimplementation. For example, a changePassword() method mightchange a password in a system-wide LDAP server for an Employee, butmight just change a record in a Web site database for a Customer. This iseasily done by writing separate methods in each class.But should you add a changePassword() method on Person? If youwant to be able to call the method when treating either a Customer or anEmployee as a Person, then you should. But you don’t have to implementthe method on Person. You can declare Person.changePassword as abstract,and then, if you call the method on a Person, it will call the correctmethod based on what type of Person (Employee or Customer) the Personis. Note that if a class contains any abstract methods, the class itself mustbe declared abstract and it cannot then be instantiated. Also note thatthis is often best accomplished not through abstract classes, but throughinterfaces (see Eckel, pp. 321–322).These are by no means the only considerations that come to bear on whatclasses to create and how to arrange and implement them, but they do representa good start. They are a foundation on which you can build best practices outof your own experience and environment.Whole books have been written on the topics of object-oriented analysisand object-oriented design. CRC cards are only one part of an array of techniquesthat can be applied to OOA/OOD. The Unified Modeling Language(UML) is popular in many MIS circles. UML consists of a variety of differentdiagrams which are used to model parts of an object-oriented design. They are: