3. Scanner
有无数 Java 工具能帮助您构建解析器，很多函数语言已成功构建解析器函数库(解析器选择器)。但如果要解析的是逗号分隔值文件，或空格分隔文本文件，又怎么办呢?大多数工具用在此处就过于隆重了，而 String.split() 又不够。(对于正则表达式，请记住一句老话：“ 您有一个问题，用正则表达式解决。那您就有两个问题了。”)
Java 平台的 Scanner 类会是这些类中您最好的选择。以轻量级文本解析器为目标，Scanner 提供了一个相对简单的 API，用于提取结构化文本，并放入强类型的部分。想象一下，如果您愿意，一组类似 DSL 的命令(源自 Terry Pratchett Discworld 小说)排列在文本文件中，如清单 7：
清单 7. Igor 的任务
fetch 1 head
fetch 3 eye
fetch 1 foot
attach foot to head
attach eye to head
admire
fetch 3 eye
fetch 1 foot
attach foot to head
attach eye to head
admire
您，或者是本例中称为 Igor的私仆，能轻松使用 Scanner 解析这组违法命令，如清单 8 所示：
清单 8. Igor 的任务，由 Scanner 解析
import java.io.*;
import java.util.*;
public class Igor
implements IPersonalServant
{
public boolean can(String cmd)
{
if (cmd.equals("fetch body parts"))
return true;
if (cmd.equals("attach body parts"))
return true;
else
return false;
}
public void process(File commandFile)
throws FileNotFoundException
{
Scanner scanner = new Scanner(commandFile);
// Commands come in a verb/number/noun or verb form
while (scanner.hasNext())
{
String verb = scanner.next();
if (verb.equals("fetch"))
{
int num = scanner.nextInt();
String type = scanner.next();
fetch (num, type);
}
else if (verb.equals("attach"))
{
String item = scanner.next();
String to = scanner.next();
String target = scanner.next();
attach(item, target);
}
else if (verb.equals("admire"))
{
admire();
}
else
{
System.out.println("I don't know how to "
+ verb + ", marthter.");
}
}
}
public void fetch(int number, String type)
{
if (parts.get(type) == null)
{
System.out.println("Fetching " + number + " "
+ type + (number > 1 ? "s" : "") + ", marthter!");
parts.put(type, number);
}
else
{
System.out.println("Fetching " + number + " more "
+ type + (number > 1 ? "s" : "") + ", marthter!");
Integer currentTotal = parts.get(type);
parts.put(type, currentTotal + number);
}
System.out.println("We now have " + parts.toString());
}
public void attach(String item, String target)
{
System.out.println("Attaching the " + item + " to the " +
target + ", marthter!");
}
public void admire()
{
System.out.println("It'th quite the creathion, marthter");
}
private Map<String, Integer> parts = new HashMap<String, Integer>();
}
import java.util.*;
public class Igor
implements IPersonalServant
{
public boolean can(String cmd)
{
if (cmd.equals("fetch body parts"))
return true;
if (cmd.equals("attach body parts"))
return true;
else
return false;
}
public void process(File commandFile)
throws FileNotFoundException
{
Scanner scanner = new Scanner(commandFile);
// Commands come in a verb/number/noun or verb form
while (scanner.hasNext())
{
String verb = scanner.next();
if (verb.equals("fetch"))
{
int num = scanner.nextInt();
String type = scanner.next();
fetch (num, type);
}
else if (verb.equals("attach"))
{
String item = scanner.next();
String to = scanner.next();
String target = scanner.next();
attach(item, target);
}
else if (verb.equals("admire"))
{
admire();
}
else
{
System.out.println("I don't know how to "
+ verb + ", marthter.");
}
}
}
public void fetch(int number, String type)
{
if (parts.get(type) == null)
{
System.out.println("Fetching " + number + " "
+ type + (number > 1 ? "s" : "") + ", marthter!");
parts.put(type, number);
}
else
{
System.out.println("Fetching " + number + " more "
+ type + (number > 1 ? "s" : "") + ", marthter!");
Integer currentTotal = parts.get(type);
parts.put(type, currentTotal + number);
}
System.out.println("We now have " + parts.toString());
}
public void attach(String item, String target)
{
System.out.println("Attaching the " + item + " to the " +
target + ", marthter!");
}
public void admire()
{
System.out.println("It'th quite the creathion, marthter");
}
private Map<String, Integer> parts = new HashMap<String, Integer>();
}
