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<h3><a href="http://caterpillar.onlyfun.net/GossipCN/index.html">From
Gossip@caterpillar</a></h3>
<h1><a href="CppGossip.html">Design Pattern: Interpreter 模式</a></h1>
對于一個具有層次節點關系的問題來說，如果您要剖析每一個節點，您可以使用Interpreter模式，直譯器模式有些類似演算法中的個別擊破方式，對每一個父節點我們剖析出其子節點組合，然而交給子節點剖析物件繼續剖析，直到剖析至終端節點為止。<br>
<br>
舉個例子來說明好了，先說明的是，這個例子是改寫自 <a href="http://www.drmaster.com.tw/info.asp?NO=PG20214">Design Patterns于Java語言之實習應用</a> 第23章的范例，我將之更簡化了，以讓大家將焦點能集中在如何使用Interpreter模式，以及如何實用。<br>
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假設您要實作一個Interpreter，這個Interpreter可以直譯您文字檔中的程式，并依您自訂的程式文法來執行程式，幾個簡單的程式如下：<br>
<div style="margin-left: 40px;"><span style="font-weight: bold; font-family: Courier New,Courier,monospace;">PROGRAM </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> PRINT dog SPACE </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> PRINT is SPACE </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> PRINT an SPACE </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> PRINT animai </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">END </span><br>
</div>
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您的這式程個會印出"dog is an animal"的文字，再來一個例子是：<br>
<div style="margin-left: 40px;"><span style="font-weight: bold; font-family: Courier New,Courier,monospace;">PROGRAM </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> REPEAT 2 </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> LINEBREAK </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> PRINT dog </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> BREAK </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> END </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">END</span><br>
</div>
<br>
<br>
這個程式要印出：<br>
<table style="text-align: left; background-color: rgb(0, 0, 0); width: 963px; height: 32px; font-family: Times New Roman,Times,serif; color: rgb(255, 255, 255);" border="0" cellpadding="2" cellspacing="2">
<tbody>
<tr>
<td><small>------------------------------ <br style="font-weight: bold;">
dog <br style="font-weight: bold;">
------------------------------ <br style="font-weight: bold;">
dog</small></td>
</tr>
</tbody>
</table>
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"></span><span style="font-weight: bold; font-family: Courier New,Courier,monospace;"></span><br>
您也可以任意的組合程式，例如：<br>
<div style="margin-left: 40px;"><span style="font-weight: bold; font-family: Courier New,Courier,monospace;">PROGRAM </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> PRINT begin </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> BREAK </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> REPEAT 3 </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> REPEAT 2 </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> PRINT dog SPACE </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> PRINT is SPACE </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> PRINT a SPACE </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> PRINT animal </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> BREAK </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> END </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> END </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">END</span><br>
</div>
<br>
<br>
這個程式中的幾個關鍵字是PROGRAM、PRINT、SPACE、BREAK、LINEBREAK、REPEAT、END，
PROGRAM是表示程式開始，以END作結，PRINT可以印出一個無空白的字串，SPACE印出一個空白，BREAK是換行，而LINEBREAK是
畫一個直線并換行，REPEAT是回圈指令，可以指定回圈次數，以END作結。<br>
<br>
觀察程式，可以制定出以下的文法，如下：<br>
<div style="margin-left: 40px;"><span style="font-weight: bold; font-family: Courier New,Courier,monospace;"><program> ::= PROGRAM <command list> </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"><command list> ::= <command>* END </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"><command> ::= <repeat command> | <primitive command> </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"><repeat command> ::= REPEAT <number> <command list> </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"><primitive command> ::= PRINT <string> </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">
| BREAK | SPACE | LINEBREAK</span><br>
</div>
<br>
<br>
程式文法制定需要對程式進行語句分析與定義，在這邊并不討論這個課題，在程式中，command節點由primitive或repeat兩個節點任意組
合，一個command list節點則是零個以上的command節點組合而成，其中repeat還可以組合command
list節點，這是組合模式的應用，可以在程式中組合巢狀回圈。<br>
<br>
在直譯程式時，以讀到PROGRAM作為開始節點，接下來我們剖析程式為command list 節點，并將它們丟給專門剖析command
list的物件繼續剖析，這個物件將之分析，看是不是有repeat command或primitive
command節點，如果有就再往下交由專屬物件進行剖析，如此層層剝開，并由專屬物件負責剖析工作。<br>
<br>
Interpreter模式的基本觀念就如上所示，先來看看如何以程式實現剖析的過程，下面這個程式會剖析您的程式，并將程式加上對應的括號來將同一個區塊組合起來，以表示它完成剖析之后的結果：<br>
<ul>
<li> INode.java
</li>
</ul>
<pre>public interface INode { <br> public void parse(Context context); <br>} <br></pre>
<br>
<ul>
<li> ProgramNode.java
</li>
</ul>
<pre>// <program> ::= PROGRAM <command list> <br>public class ProgramNode implements INode { <br> private INode commandListNode; <br> public void parse(Context context) { <br> context.skipToken("PROGRAM"); <br> commandListNode = new CommandListNode(); <br> commandListNode.parse(context); <br> } <br><br> public String toString() { <br> return "[PROGRAM " + commandListNode + "]"; <br> } <br>} <br></pre>
<br>
<ul>
<li> CommandListNode.java
</li>
</ul>
<pre>import java.util.Vector; <br><br>// <command list> ::= <command>* END <br>public class CommandListNode implements INode { <br> private Vector list = new Vector();<br><br> public void parse(Context context) { <br> while (true) { <br> if (context.currentToken() == null) { <br> System.err.println("Missing 'END'"); <br> break; <br> } else if (<br> context.currentToken().equals("END")) { <br> context.skipToken("END"); <br> break; <br> } else { <br> INode commandNode = new CommandNode(); <br> commandNode.parse(context); <br> list.add(commandNode); <br> } <br> } <br> }<br><br> public String toString() { <br> return "" + list; <br> } <br>} <br></pre>
<br>
<ul>
<li> CommandNode.java
</li>
</ul>
<pre>// <command> ::= <repeat command> | <primitive command> <br>public class CommandNode implements INode { <br> private INode node;<br><br> public void parse(Context context) { <br> if (context.currentToken().equals("REPEAT")) { <br> node = new RepeatCommandNode(); <br> node.parse(context); <br> } else { <br> node = new PrimitiveCommandNode(); <br> node.parse(context); <br> } <br> }<br><br> public String toString() { <br> return node.toString(); <br> } <br>} <br></pre>
<br>
<ul>
<li> RepeatCommandNode.java
</li>
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