According to spec , XPath.evaluate(java.lang.String expression, InputSource source, QName returnType) should throw NPE if expression is null.
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java version "1.5.0-beta2"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0-beta2-b37)
Java HotSpot(TM) Client VM (build 1.5.0-beta2-b37, mixed mode)

javac Test4.java
java Test4
 checkXPath24() - failed , expected NPE - but XPathExpressionException thrown
 checkXPath29() - failed , expected NPE - but XPathExpressionException thrown
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Testcase - Test4.java & widgets.xml attached
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<?xml version="1.0" standalone="yes"?>
<widgets>
<widget name="a" style="red" quantity="6"/>
<widget name="b" style="blue"/>
<widget name="c">
<style>green</style>
</widget>
</widgets>
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//checkXPath24()
//checkXPath29()
import javax.xml.xpath.XPath;
import javax.xml.xpath.XPathFactory;
import javax.xml.xpath.XPathConstants;
import javax.xml.xpath.XPathExpression;
import javax.xml.xpath.XPathFactoryConfigurationException;
import javax.xml.xpath.XPathExpressionException;

import org.xml.sax.InputSource;

import java.io.FileInputStream;


public class Test4 {
   
   String xmlPath = "";
   FileInputStream fis = null;
   InputSource iSource = null;
   Test4 () {
     xmlPath = "./";
   }
    
    // main()
    public static void main(String[] argv) {
        Test4 test4 = new Test4();
        test4.init();
        test4.checkXPath24();
        test4.checkXPath29();
    
    }// end main()
    
    
    // initializing
    private void init(){
        try {
            fis = new FileInputStream(xmlPath+"widgets.xml");
            iSource = new InputSource(fis);
        }catch (Exception e) {
          System.out.println(" Exception thrown in XPath01/init() - Test failed");
        }
    }
    
    // test for XPath.evaluate(java.lang.String expression, InputSource source) - default returnType is String
    // expression is null , should throw NPE
    private void checkXPath24() {
     try{
     XPathFactory xpathFactory = XPathFactory.newInstance();
     if( xpathFactory instanceof XPathFactory ){
     XPath xpath = xpathFactory.newXPath();
if ( xpath instanceof XPath ){
String expression = null ;
String quantity = (String) xpath.evaluate(expression, iSource);
System.out.println("checkXPath24() : failed - expected NPE not thrown ");
}
     else{
     System.out.println("checkXPath24() failed - xpath not instance of XPath");
     }
     }else{
     System.out.println(" checkXPath24() failed - creating instance of XPathFactory ");
     }
     }catch( NullPointerException npe ){
     System.out.println(" checkXPath24() - passed , expected NPE thrown ");
     }catch(XPathFactoryConfigurationException xpfce ){
     System.out.println(" checkXPath24() - failed , Default object model not available ");
     }catch(XPathExpressionException xpee ){
     System.out.println(" checkXPath24() - failed , expected NPE - but XPathExpressionException thrown " );
     }catch(Exception e){
     System.out.println(" Exception thrown - checkXPath24() failed ");
     }
    }
    
    // test for XPath.evaluate(java.lang.String expression, InputSource source, QName returnType)
    // expression is null , should throw NPE
    private void checkXPath29() {
     try{
     XPathFactory xpathFactory = XPathFactory.newInstance();
     if( xpathFactory instanceof XPathFactory ){
     XPath xpath = xpathFactory.newXPath();
if ( xpath instanceof XPath ){
String expression = null ;
String quantity = (String) xpath.evaluate(expression, iSource, XPathConstants.STRING);
System.out.println("checkXPath29() : failed - expected NPE not thrown ");
}
     else{
     System.out.println("checkXPath29() failed - xpath not instance of XPath");
     }
     }else{
     System.out.println(" checkXPath29() failed - creating instance of XPathFactory ");
     }
     }catch( NullPointerException npe ){
     System.out.println(" checkXPath29() - passed , expected NPE thrown ");
     }catch(XPathFactoryConfigurationException xpfce ){
     System.out.println(" checkXPath29() - failed , Default object model not available ");
     }catch(XPathExpressionException xpee ){
     System.out.println(" checkXPath29() - failed , expected NPE - but XPathExpressionException thrown " );
     }catch(Exception e){
     System.out.println(" Exception thrown - checkXPath29() failed ");
     }
    }
    
    
}
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