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<td width="90%"><font size="4" face="Verdana, Arial, Helvetica"><b>GOF 模式：Behavioral（续）</b></font></td><td width="200" align="right"><font size="1" face="Verdana, Arial, Helvetica"><nobr>   第 10 页（共18 页）</nobr></font></td>
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<p>
<b>State</b>
</p>
<p>目的：<br>
			让一个对象可以在内部状态改变时改变其行为。这个对象看来像是改变了它所属的类。
			</p>
<p>说明：<br>
			这种模式的主要思想是引入一个抽象类来表示对象的可能状态。这个类声明了一个对所有表示不同的操作状态类都通用的接口。具体类的实现取决于状态的行为。可以基于当前状态，选择并且使用相应的具体类。
			</p>
<p>UML 图：<br>
			
<img src="images/SCEA-StatePattern.gif" alt="State 的 UML 图 ">
			</p>
<p>益处：<br>
			
<ul>
<li>使特定于状态的行为本地化，并对用于不同状态的行为进行区分。<br>
<br>
</li>
<li>使状态转换显式状态。</li>
</ul>
			
</p>
<p>什么时候使用？<br>
			可以在以下情况下使用 State 状态：
			<ul>
<li>对象的行为取决于它的状态，并且它必须在运行时根据这个状态改变其行为。<br>
<br>
</li>
<li>操作中包含大型的、由多个部分组成的条件语句，这取决于对象的状态。</li>
</ul>
			
</p>
<p>
<b>Strategy</b>
</p>
<p>目标：<br>
			定义一组算法，封装每一个算法，并使它们可以互换。Strategy 使算法改变与使用它的客户机无关。
			</p>
<p>说明：<br>
			Strategy 模式定义了一组类，表示一组可能的行为。根据所选的 strategy（算法）而具有不同的功能。
			</p>
<p>UML 图：<br>
			
<img src="images/SCEA-Strategy.gif" alt="Strategy 的 UML 图">
			</p>
<p>益处：<br>
			
<ul>
<li>继承的替代物。</li>
<li>Strategy 消除了条件语句。</li>
<li>可选择的实现。</li>
</ul>
			
</p>
<p>什么时候使用？<br>
			可以在以下情况下使用 Strategy 模式：
			<ul>
<li>许多相关的类只是行为上有差别时。<br>
<br>
</li>
<li>需要一种算法的不同变化。<br>
<br>
</li>
<li>某种算法使用了客户机不该知道的数据。<br>
<br>
</li>
<li>一个类定义了许多行为，并且这些行为在其操作中表现为多条件语句。将相关的条件分支转移到它们自己的 Strategy 类中，而不是加入许多条件。</li>
</ul>
			
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