用dom解析xml文件时遇到的问题，帮帮忙～～～～～～～～～～～～～～

xiaohuoma7620 2006-05-12 11:17:50

Sample.xml

<?xml version="1.0" encoding="gb2312"?>
<dataSet>
<single>
<col>123</col>
</single>
</dataSet>

用dom解析的程序如下：

import java.io.*;
import javax.xml.parsers.*;
import org.w3c.dom.*;
import org.xml.sax.SAXException;

public class ParserSample {
Document document = null;
NodeList sampleNodeListSingle = null;

public ParserSample() {
try {
DocumentBuilderFactory dbFactory = DocumentBuilderFactory.newInstance();
//dbFactory.setIgnoringElementContentWhitespace( true );
DocumentBuilder documentBuilder = dbFactory.newDocumentBuilder();
document = documentBuilder.parse( new File( "E:\\biyesheji", "Sample.xml" ) );
} catch( ParserConfigurationException e ) {
System.out.println( "There's a Parser Config problem." );
} catch( SAXException e ) {
System.out.println( "There's a SAX Exception." );
} catch( IOException e ) {
System.out.println( "There's an IO Exception." );
}

sampleNodeListSingle = document.getElementsByTagName( "single" );
}
public void listSingles() {
String s = sampleNodeListSingle.item( 0 ).getFirstChild().getFirstChild().getNodeValue();
System.out.println( s );
}

public static void main(String[] args) {
ParserSample sampleXML = new ParserSample();

System.out.println( "Singles:" );
sampleXML.listSingles();

}

}
运行结果如下：
Singles:
java.lang.NullPointerException
at ParserSample.listSingles(ParserSample.java:32)
at ParserSample.main(ParserSample.java:45)
Exception in thread "main"

请问是什么原因？

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