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<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> EmployeeID rhs = obj as EmployeeID;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> if (rhs == null)</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> return false;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> if (prefix == rhs.prefix && number == rhs.number)</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> return true;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> return false;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> }</span></p>
<p class="2" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US"> }</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">类定义的第一部分只存储了</span><span lang="EN-US">ID</span><span style="FONT-FAMILY: 宋体">。</span><span lang="EN-US">ID</span><span style="FONT-FAMILY: 宋体">的格式是</span><span lang="EN-US">B001 </span><span style="FONT-FAMILY: 宋体">或</span><span lang="EN-US"> W234</span><span style="FONT-FAMILY: 宋体">，换言之，它包含一个字母前缀，后面是</span><span lang="EN-US">3</span><span style="FONT-FAMILY: 宋体">个数字字符。把前缀存储为一个字符，用一个</span><span lang="EN-US">int</span><span style="FONT-FAMILY: 宋体">类型的变量存储后面的</span><span lang="EN-US">3</span><span style="FONT-FAMILY: 宋体">个</span><span lang="EN-US"> </span><span style="FONT-FAMILY: 宋体">数字。</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">构造函数仅获取一个字符串，并分解它，组成这些字段。注意，为了使这个示例比较简单，不执行错误检查。假定传递给构造函数的字符串有正确的格式。</span><span lang="EN-US">ToString()</span><span style="FONT-FAMILY: 宋体">方法仅把</span><span lang="EN-US">ID</span><span style="FONT-FAMILY: 宋体">作为一个字符串返回：</span></p>
<p class="a6" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US"> return prefix.ToString() + string.Format("{0,3:000}", number);</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">注意格式说明符</span><span lang="EN-US">(3:000)</span><span style="FONT-FAMILY: 宋体">可以确保包含数字的</span><span lang="EN-US">int</span><span style="FONT-FAMILY: 宋体">用</span><span lang="EN-US">0</span><span style="FONT-FAMILY: 宋体">填满空位，于是就有</span><span lang="EN-US">B001</span><span style="FONT-FAMILY: 宋体">这样的</span><span lang="EN-US">ID</span><span style="FONT-FAMILY: 宋体">，而不是</span><span lang="EN-US">B1</span><span style="FONT-FAMILY: 宋体">。</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">下面是字典需要的两个重写方法，首先重写</span><span lang="EN-US">Equals()</span><span style="FONT-FAMILY: 宋体">，让它比较</span><span lang="EN-US">EmployeeID</span><span style="FONT-FAMILY: 宋体">实例的值：</span></p>
<p class="a6" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US"> public override bool Equals(object obj)</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> {</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> EmployeeID rhs = obj as EmployeeID;</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> if (rhs == null)</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> return false;</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> if (prefix == rhs.prefix && number == rhs.number)</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> return true;</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> return false;</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> }</span></p>
<p class="a6" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US"> }</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">这是我们看到的第一个</span><span lang="EN-US">Equals()</span><span style="FONT-FAMILY: 宋体">重写例子。注意第一个任务是检查作为参数传递的对象是否是一个</span><span lang="EN-US">EmployeeID</span><span style="FONT-FAMILY: 宋体">实例。如果不是，它显然就不等于这个对象，因此返回</span><span lang="EN-US">false</span><span style="FONT-FAMILY: 宋体">，测试类型的方式是使用</span><span lang="EN-US">C#</span><span style="FONT-FAMILY: 宋体">关键字</span><span lang="EN-US">as</span><span style="FONT-FAMILY: 宋体">，把它转换为</span><span lang="EN-US">EmployeeID</span><span style="FONT-FAMILY: 宋体">。有了</span><span lang="EN-US">EmployeeID</span><span style="FONT-FAMILY: 宋体">对象后，就可以比较字段的值，看看它们包含的值是否与当前对象的值相同。</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">下面看看</span><span lang="EN-US">GetHashCode()</span><span style="FONT-FAMILY: 宋体">，它的实现比较短，但乍看上去比较难理解：</span></p>
<p class="2" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US"> public override int GetHashCode()</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> {</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> string str = this.ToString();</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> return str.GetHashCode();</span></p>
<p class="2" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US"> }</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">前面列出了计算散列代码必须满足的一些相当苛刻的要求。当然，可以用许多方式设计出简单而高效的散列算法。一般情况下，比较好的方式是获取字段，把它们与较大的素数相乘，再把结果加起来。但</span><span lang="EN-US">Microsoft</span><span style="FONT-FAMILY: 宋体">已经为</span><span lang="EN-US">String</span><span style="FONT-FAMILY: 宋体">类提供了一种虽然复杂、但很有效的散列算法，所以也可以利用这个算法。</span><span lang="EN-US">String.GetHashCode()</span><span style="FONT-FAMILY: 宋体">会根据字符串的内容生成分布均匀的数字，满足散列代码的所有要求。</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">采用这个方法的惟一缺点是在把</span><span lang="EN-US">EmployeeID</span><span style="FONT-FAMILY: 宋体">类转换为</span><span lang="EN-US">string</span><span style="FONT-FAMILY: 宋体">时会有一些性能损失。如果很在意这一点，需要在散列算法中尽可能减少性能损失，就应设计自己的散列算法。设计散列算法是一个很复杂的议题，本书没有深入探讨。但这里有一些解决该问题的简单方法，即把基于类中成员字段的数字乘以不同的素数</span><span lang="EN-US">(</span><span style="FONT-FAMILY: 宋体">在数学上看，乘以不同的素数有助于防止字段值的不同组合给出相同的散列代码</span><span lang="EN-US">)</span><span style="FONT-FAMILY: 宋体">。</span><span lang="EN-US">GetHashCode()</span><span style="FONT-FAMILY: 宋体">的这种实现如下所示：</span></p>
<p class="2" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US"> public override int GetHashCode() // alternative implementation</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> {</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> return (int)prefix*13 + (int)number*53;</span></p>
<p class="2" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US"> }</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体; LETTER-SPACING: -0.1pt">这个示例比示例所使用的基于</span><span lang="EN-US" style="LETTER-SPACING: -0.1pt">ToString()</span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: -0.1pt">的算法速度更快，但其缺点是由不同</span><span lang="EN-US" style="LETTER-SPACING: -0.1pt">EmployeeID</span><span style="FONT-FAMILY: 宋体">生成的散列代码分布在</span><span lang="EN-US">int</span><span style="FONT-FAMILY: 宋体">取值范围内的均匀度比较低。基本数字类型也定义了</span><span lang="EN-US">GetHashCode()</span><span style="FONT-FAMILY: 宋体">方法，但这些方法只返回变量的值，因此不是很有效。基本类型不应用作键。</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">注意这里的</span><span lang="EN-US">GetHashCode() </span><span style="FONT-FAMILY: 宋体">和</span><span lang="EN-US"> Equals()</span><span style="FONT-FAMILY: 宋体">实现满足前面提出的相等要求：使用</span><span lang="EN-US">Equals()</span><span style="FONT-FAMILY: 宋体">重写方法，当且仅当两个</span><span lang="EN-US">EmployeeID</span><span style="FONT-FAMILY: 宋体">对象有相同的前缀和数字时，它们才是相等的，但如果是这样，</span><span lang="EN-US">ToString()</span><span style="FONT-FAMILY: 宋体">会给这两个对象提供相同的值，所以它们有相同的散列代码。我们必须进行这样的测试，才能确保它们满足条件。</span></p>
<p class="MsoNormal"><span lang="EN-US"> </span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">下面看看包含员工数据的类。该类的定义相当基本和直观：</span></p>
<p class="2" style="MARGIN-TOP: 4.9pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US"> class EmployeeData</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 13.5pt; FTEL: 18.45pt"><span lang="EN-US"> {</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 13.5pt; FTEL: 18.45pt"><span lang="EN-US"> private string name;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 13.5pt; FTEL: 18.45pt"><span lang="EN-US"> private decimal salary;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 13.5pt; FTEL: 18.45pt"><span lang="EN-US"> private EmployeeID id;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 13.5pt; FTEL: 18.45pt"><span lang="EN-US"> </span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 13.5pt; FTEL: 18.45pt"><span lang="EN-US"> public EmployeeData(EmployeeID id, string name, decimal salary)</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 13.5pt; FTEL: 18.45pt"><span lang="EN-US"> {</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 13.5pt; FTEL: 18.45pt"><span lang="EN-US"> this.id = id;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 13.5pt; FTEL: 18.45pt"><span lang="EN-US"> this.name = name;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 13.5pt; FTEL: 18.45pt"><span lang="EN-US"> this.salary = salary;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 13.5pt; FTEL: 18.45pt"><span lang="EN-US"> }</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 13.5pt; FTEL: 18.45pt"><span lang="EN-US"> </span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 13.5pt; FTEL: 18.45pt"><span lang="EN-US"> public override string ToString()</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 13.5pt; FTEL: 18.45pt"><span lang="EN-US"> {</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 13.5pt; FTEL: 18.45pt"><span lang="EN-US"> StringBuilder sb = new StringBuilder(id.ToString(), 100);</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 13.5pt; FTEL: 18.45pt"><span lang="EN-US"> sb.Append(": ");</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 13.5pt; FTEL: 18.45pt"><span lang="EN-US"> sb.Append(string.Format("{0,-20}", name));</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 13.5pt; FTEL: 18.45pt"><span lang="EN-US"> sb.Append(" ");</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 13.5pt; FTEL: 18.45pt"><span lang="EN-US"> sb.Append(string.Format("{0:C}", salary));</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 13.5pt; FTEL: 18.45pt"><span lang="EN-US"> return sb.ToString();</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 13.5pt; FTEL: 18.45pt"><span lang="EN-US"> }</span></p>
<p class="2" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US"> }</span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 15pt"><span style="FONT-FAMILY: 宋体">注意由于性能的原因，我们使用</span><span lang="EN-US">StringBuilder</span><span style="FONT-FAMILY: 宋体">对象来生成</span><span lang="EN-US">EmployeeData</span><span style="FONT-FAMILY: 宋体">对象的字符串表示。最后，创建测试工具，定义类</span><span lang="EN-US">TestHarness</span><span style="FONT-FAMILY: 宋体">：</span></p>
<p class="2" style="MARGIN-TOP: 4.9pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">class TestHarness</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 13.5pt; FTEL: 18.45pt"><span lang="EN-US">{</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 13.5pt; FTEL: 18.45pt"><span lang="EN-US"> </span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 13.5pt; FTEL: 18.45pt"><span lang="EN-US"> Hashtable employees = new Hashtable(31);</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 13.5pt; FTEL: 18.45pt"><span lang="EN-US"> </span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 13.5pt; FTEL: 18.45pt"><span lang="EN-US"> public void Run()</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 13.5pt; FTEL: 18.45pt"><span lang="EN-US"> {</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 13.5pt; FTEL: 18.45pt"><span lang="EN-US"> EmployeeID idMortimer = new EmployeeID("B001");</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 13.5pt; FTEL: 18.45pt"><span lang="EN-US"> EmployeeData mortimer = new EmployeeData(idMortimer, "Mortimer",</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 13.5pt; FTEL: 18.45pt"><span lang="EN-US"> 100000.00M);</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 13.5pt; FTEL: 18.45pt"><span lang="EN-US"> EmployeeID idArabel = new EmployeeID("W234");</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 13.5pt; FTEL: 18.45pt"><span lang="EN-US"> EmployeeData arabel= new EmployeeData(idArabel, "Arabel Jones",</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 13.5pt; FTEL: 18.45pt"><span lang="EN-US"> 10000.00M);</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 13.5pt; FTEL: 18.45pt"><span lang="EN-US"> </span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 13.5pt; FTEL: 18.45pt"><span lang="EN-US"> employees.Add(idMortimer, mortimer);</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 13.5pt; FTEL: 18.45pt"><span lang="EN-US"> employees.Add(idArabel, arabel);</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 13.5pt; FTEL: 18.45pt"><span lang="EN-US"> </span></p>
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