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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"> <html> <head> <title>page_932</title> <link rel="stylesheet" href="reset.css" type="text/css" media="all"> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> </head> <body> <table summary="top nav" border="0" width="100%"> <tr> <td align="left" width="30%" style="background: #EEF3E2"><a style="color: blue; font-size: 120%; font-weight: bold; text-decoration: none; font-family: verdana;" href="page_931.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_932</strong></td> <td align="right" width="30%" style="background: #EEF3E2"><a style="color: blue; font-size: 120%; font-weight: bold; text-decoration: none; font-family: verdana;" href="page_933.html">next page ></a></td> </tr> <tr> <td align="left" colspan="3" style="background: #ffffff; padding: 20px;"> <table border="0" width="100%" cellpadding="0"><tr><td align="center"> <table border="0" cellpadding="2" cellspacing="0" width="100%"><tr><td align="left"></td> <td align="right"></td> </tr></table></td></tr><tr><td align="left"><p></p><table border="0" cellspacing="0" cellpadding="0" width="100%"><tr><td align="right"><font face="Times New Roman, Times, Serif" size="2" color="#FF0000">Page 932</font></td></tr></table><table border="0" cellspacing="0" cellpadding="0"><tr><td rowspan="5"></td> <td colspan="3" height="12"></td> <td rowspan="5"></td></tr><tr><td colspan="3"></td></tr><tr><td></td> <td><font face="Times New Roman, Times, Serif" size="3">and </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">customer</font><font face="Times New Roman, Times, Serif" size="3">. The operations on a </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">savingsAccount</font><font face="Times New Roman, Times, Serif" size="3"> object are suggested by the list of verb phrasesnamely, </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">Deposit</font><font face="Times New Roman, Times, Serif" size="3">, </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">Withdraw</font><font face="Times New Roman, Times, Serif" size="3">, and </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">PayInterest</font><font face="Times New Roman, Times, Serif" size="3">. What are the operations on a </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">customer</font><font face="Times New Roman, Times, Serif" size="3"> object? We would need more information from the rest of the problem definition in order to answer this question. In fact, </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">customer</font><font face="Times New Roman, Times, Serif" size="3"> may not turn out to be a useful object at all. The nouns-and-verbs technique is only a starting pointit points us to <i>potential</i> objects and operations.</font></td><td></td></tr><tr><td colspan="3"></td></tr><tr><td colspan="3" height="1"></td></tr></table><table border="0" cellspacing="0" cellpadding="0"><tr><td rowspan="5"></td> <td colspan="3" height="12"></td> <td rowspan="5"></td></tr><tr><td colspan="3"></td></tr><tr><td></td> <td><font face="Times New Roman, Times, Serif" size="3">Determining which nouns and verbs are significant is one of the most difficult aspects of OOD. There are no cookbook formulas for doing so, and there probably never will be. Not all nouns become objects, and not all verbs become operations. The nouns-and-verbs technique is imperfect, but it does give us a first approximation to a solution.</font></td><td></td></tr><tr><td colspan="3"></td></tr><tr><td colspan="3" height="1"></td></tr></table><table border="0" cellspacing="0" cellpadding="0"><tr><td rowspan="5"></td> <td colspan="3" height="12"></td> <td rowspan="5"></td></tr><tr><td colspan="3"></td></tr><tr><td></td> <td><font face="Times New Roman, Times, Serif" size="3">The solution domain includes not only objects drawn from the problem domain but also <i>implementation-level</i> objects. These are objects that do not model the problem domain but are used in building the program itself. In systems with graphical user interfacesMicrosoft Windows or the Macintosh operating system, for examplea program may need several kinds of implementation-level objects: window objects, menu objects, objects that respond to mouse clicks, and so on. Objects such as these are often available in class libraries so that we don't need to design and implement them from scratch each time we need them in different programs.</font></td><td></td></tr><tr><td colspan="3"></td></tr><tr><td colspan="3" height="1"></td></tr></table><table border="0" cellspacing="0" cellpadding="0"><tr><td rowspan="5"></td> <td colspan="3" height="12"></td> <td rowspan="5"></td></tr><tr><td colspan="3"></td></tr><tr><td></td> <td><font face="Times New Roman, Times, Serif" size="3"><i>Step 2: Determine the Relationships Among Objects</i></font></td><td></td></tr><tr><td colspan="3"></td></tr><tr><td colspan="3" height="1"></td></tr></table><table border="0" cellspacing="0" cellpadding="0"><tr><td rowspan="5"></td> <td colspan="3" height="12"></td> <td rowspan="5"></td></tr><tr><td colspan="3"></td></tr><tr><td></td> <td><font face="Times New Roman, Times, Serif" size="3">After selecting potential objects and operations, the next step is to examine the relationships among the objects. In particular, we want to see whether certain objects might be related either by inheritance or by composition. Inheritance and composition relationships not only pave the way for code reuseas we emphasized in our discussion of OOPthey also simplify the design and allow us to model the problem domain more accurately. For example, the banking problem may require several kinds of savings accountsone for general customers, another for preferred customers, and another for children under the age of 12. If these are all variations on a basic savings account, the is-a relationship (and, therefore, inheritance) is probably appropriate. Starting with a </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">SavingsAccount</font><font face="Times New Roman, Times, Serif" size="3"> class that provides operations common to any savings account, we could design each of the other accounts as a child class of </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">SavingsAccount</font><font face="Times New Roman, Times, Serif" size="3">, concentrating our efforts only on the properties that make each one different from the parent class.</font></td><td></td></tr><tr><td colspan="3"></td></tr><tr><td colspan="3" height="1"></td></tr></table><table border="0" cellspacing="0" cellpadding="0"><tr><td rowspan="5"></td> <td colspan="3" height="12"></td> <td rowspan="5"></td></tr><tr><td colspan="3"></td></tr><tr><td></td> <td><font face="Times New Roman, Times, Serif" size="3">Sometimes the choice between inheritance and composition is not immediately clear. Earlier we wrote a </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">TimeCard</font><font face="Times New Roman, Times, Serif" size="3"> class to represent an employee's time card. Given an existing </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">Time</font><font face="Times New Roman, Times, Serif" size="3"> class, we used composition to relate </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">TimeCard</font><font face="Times New Roman, Times, Serif" size="3"> and </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">Time</font><font face="Times New Roman, Times, Serif" size="3">the private part of the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">TimeCard</font><font face="Times New Roman, Times, Serif" size="3"> class is composed of a </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">Time</font><font face="Times New Roman, Times, Serif" size="3"> object (and an ID number). We could also have used inheritance. We could have derived class </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">TimeCard</font><font face="Times New Roman, Times, Serif" size="3"> from </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">Time</font><font face="Times New Roman, Times, Serif" size="3"> (inheriting the hours, minutes, and seconds members) and then specialized it by adding an extra data mem-</font></td><td></td></tr><tr><td colspan="3"></td></tr><tr><td colspan="3" height="1"></td></tr></table></td></tr></table><p><font size="0"></font></p>聽 </td> </tr> <tr> <td align="left" width="30%" style="background: #EEF3E2"><a style="color: blue; font-size: 120%; font-weight: bold; text-decoration: none; font-family: verdana;" href="page_931.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_932</strong></td> <td align="right" width="30%" style="background: #EEF3E2"><a style="color: blue; font-size: 120%; font-weight: bold; text-decoration: none; font-family: verdana;" href="page_933.html">next page ></a></td> </tr> </table> </body> </html>⌨️ 快捷键说明
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