NullPointerException

**JohnST** · 12-31-2009, 08:59 PM

Can someone please explain why I keep getting a null pointer exception in the same place when I'm trying to parse a file (about 1/4 of the way through)? The file is about a 1.5 MB

Java Code:

import java.io.File;
import org.w3c.dom.Document;
import org.w3c.dom.*;

import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.DocumentBuilder;
import org.xml.sax.SAXException;
import org.xml.sax.SAXParseException; 

public class ReadAndPrintGetitems{

   public static void main (String argv []){
    try {

            DocumentBuilderFactory docBuilderFactory = DocumentBuilderFactory.newInstance();
            DocumentBuilder docBuilder = docBuilderFactory.newDocumentBuilder();
             
    

            Document doc = docBuilder.parse (new File("GetItems.xml"));

            // normalize text representation
            doc.getDocumentElement ().normalize ();
            System.out.println ("Root element of the doc is " + 
                 doc.getDocumentElement().getNodeName());


            NodeList listOfItems = doc.getElementsByTagName("item");
            int totalItems = listOfItems.getLength();
            System.out.println("Total no of <item> tags found : " + totalItems);

            for(int s=1; s<listOfItems.getLength() ; s++){


                Node firstItemNode = listOfItems.item(s);
                if(firstItemNode.getNodeType() == Node.ELEMENT_NODE){


                    Element firstItemElement = (Element)firstItemNode;

                    //-------
                    NodeList firstIdList = firstItemElement.getElementsByTagName("id");
                    Element firstIdElement = (Element)firstIdList.item(0);

                    NodeList textIdList = firstIdElement.getChildNodes();
                    System.out.println("id   : " + 
                           ((Node)textIdList.item(0)).getNodeValue().trim());

                    //-------
                    NodeList nameList = firstItemElement.getElementsByTagName("name");
                    Element nameElement = (Element) nameList.item(0);

                    NodeList textLNList = nameElement.getChildNodes();
                    System.out.println("name : " + 
                           ((Node)textLNList.item(0)).getNodeValue().trim());

                    //----
                    NodeList typeList = firstItemElement.getElementsByTagName("type");
                    Element typeElement = (Element) typeList.item(0);

                    NodeList textAgeList = typeElement.getChildNodes();
                    System.out.println("type : " + 
                           ((Node)textAgeList.item(0)).getNodeValue().trim());

                    //------


                }//end of if clause


            }//end of for loop with s var


        }catch (SAXParseException err) {
        System.out.println ("** Parsing error" + ", line " 
             + err.getLineNumber () + ", uri " + err.getSystemId ());
        System.out.println(" " + err.getMessage ());

        }catch (SAXException e) {
        Exception x = e.getException ();
        ((x == null) ? e : x).printStackTrace ();

        }catch (Throwable t) {
        t.printStackTrace ();
        }
        //System.exit (0);

    }//end of main

}

**Eranga** · 01-01-2010, 02:37 AM

Can you please post the complete error you comes with?

**travishein** · 01-01-2010, 04:38 AM

its possible the xml document is missing a tag that the code is expecting to always have there, for one of the nodes way down in the document.

For example, instead of doing :

NodeList nameList = firstItemElement.getElementsByTagName("name");
Element nameElement = (Element) nameList.item(0);

If there was for some reason no "name" tag in the XML, the nameList would be null, and then we get a NPE on namelist.item(0). So instead, do

Java Code:

NodeList nameList = firstItemElement.getElementsByTagName("name");
Element nameElement = null;
if (nameList != null) {
  nameElement = (Element) nameList.item(0);
  // maybe something here that uses nameElement, 
}
else {
  // if this really can not ever be null, generate your own meaningful error message here.
}

where you would want to do this for all the other places of this pattern..

And again, for places like this,

System.out.println("type : " +
((Node)textAgeList.item(0)).getNodeValue().trim()) ;

instead, do something like

Java Code:

String value = null;

Node aNode = null;
if (textAgeList != null) {
  aNode = (Node)textAgeList.item(0);
  if (aNode != null) {
    value = aNode.getNodeValue();
    if (value != null) {
      value = value.trim();
    }
  }
}
System.out.println("type : " + value);

**JohnST** · 01-03-2010, 12:01 AM

java.lang.NullPointerException
at parserTest.main(parserTest.java:45)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Nativ e Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknow n Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Un known Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at edu.rice.cs.dynamicjava.symbol.JavaClass$JavaMetho d.evaluate(JavaClass.java:326)
at edu.rice.cs.dynamicjava.interpreter.ExpressionEval uator.handleMethodCall(ExpressionEvaluator.java:92 )
at edu.rice.cs.dynamicjava.interpreter.ExpressionEval uator.visit(ExpressionEvaluator.java:84)
at koala.dynamicjava.tree.StaticMethodCall.acceptVisi tor(StaticMethodCall.java:105)
at edu.rice.cs.dynamicjava.interpreter.ExpressionEval uator.value(ExpressionEvaluator.java:38)
at edu.rice.cs.dynamicjava.interpreter.ExpressionEval uator.value(ExpressionEvaluator.java:37)
at edu.rice.cs.dynamicjava.interpreter.StatementEvalu ator.visit(StatementEvaluator.java:106)
at edu.rice.cs.dynamicjava.interpreter.StatementEvalu ator.visit(StatementEvaluator.java:29)
at koala.dynamicjava.tree.ExpressionStatement.acceptV isitor(ExpressionStatement.java:101)
at edu.rice.cs.dynamicjava.interpreter.Interpreter.ev aluate(Interpreter.java:86)
at edu.rice.cs.dynamicjava.interpreter.Interpreter.in terpret(Interpreter.java:47)
at edu.rice.cs.drjava.model.repl.newjvm.InterpreterJV M.interpret(InterpreterJVM.java:205)
at edu.rice.cs.drjava.model.repl.newjvm.InterpreterJV M.interpret(InterpreterJVM.java:182)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Nativ e Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknow n Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Un known Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at sun.rmi.server.UnicastServerRef.dispatch(Unknown Source)
at sun.rmi.transport.Transport$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at sun.rmi.transport.Transport.serviceCall(Unknown Source)
at sun.rmi.transport.tcp.TCPTransport.handleMessages( Unknown Source)
at sun.rmi.transport.tcp.TCPTransport$ConnectionHandl er.run0(Unknown Source)
at sun.rmi.transport.tcp.TCPTransport$ConnectionHandl er.run(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run Task(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run (Unknown Source)
at java.lang.Thread.run(Unknown Source)

**Eranga** · 01-04-2010, 02:47 AM

What you have on line 45 of the at parserTest.java file?

at parserTest.main(parserTest.java:45)

**JohnST** · 01-04-2010, 03:45 AM

Here is what is on line 45 of parserTest.java

Java Code:

NodeList textIdList = firstIdElement.getChildNodes();

**Eranga** · 01-04-2010, 03:58 AM

Fine, you are retrieving items here. Somehow items are not in. Did you check the content before retrieve.

**Tolls** · 01-04-2010, 10:04 AM

Java Code:

                    NodeList firstIdList = firstItemElement.getElementsByTagName("id");
                    Element firstIdElement = (Element)firstIdList.item(0);

Based on that code, there are no elements inside firstItemElement that have a name of "id", so item(0) is returning null. Are you sure that all <item>s have a sub element that is <id>?

**JohnST** · 01-04-2010, 08:50 PM

Thanks for the reply. Yes you are right not all <item> have a sub element that is <id>. This is what I see printed out:

Java Code:

..........
id   : 302
name : Cisco RNG200
type  : 6
id   : 303
name : Cable
type  : 7
id   : 322
name : WeatherBug Forecasts
type  : 7
java.lang.NullPointerException
	at ReadAndPrintGetitems.main(ReadAndPrintGetitems.java:42)

Here is section where it stops parsing in the XML

Java Code:

<item>
<id>322</id>
<name>WeatherBug Forecasts</name>
<type>7</type>
<itemdata><large_image>devices_sm/contactsingle_humiditysensor_open.gif</large_image><small_image>devices_sm/contactsingle_humiditysensor_open.gif</small_image><config_data_file>weatherbug.c4i</config_data_file><tag>c8969fc5_a55a_4bf5_9d13_1e842be90ea4</tag></itemdata>
<state>&lt;lua_gen_persisting&gt;&lt;properties&gt;&lt;property&gt;&lt;name&gt;Location Code&lt;/name&gt;&lt;value&gt;77040&lt;/value&gt;&lt;/property&gt;&lt;property&gt;&lt;name&gt;Location Type&lt;/name&gt;&lt;value&gt;US (Zip Code)&lt;/value&gt;&lt;/property&gt;&lt;property&gt;&lt;name&gt;Units Type&lt;/name&gt;&lt;value&gt;US Units&lt;/value&gt;&lt;/property&gt;&lt;property&gt;&lt;name&gt;Alert Font Size&lt;/name&gt;&lt;value&gt;Medium&lt;/value&gt;&lt;/property&gt;&lt;property&gt;&lt;name&gt;Current Font Size&lt;/name&gt;&lt;value&gt;Medium&lt;/value&gt;&lt;/property&gt;&lt;property&gt;&lt;name&gt;Forecast Font Size&lt;/name&gt;&lt;value&gt;Medium&lt;/value&gt;&lt;/property&gt;&lt;property&gt;&lt;name&gt;Debug Mode&lt;/name&gt;&lt;value&gt;Off&lt;/value&gt;&lt;/property&gt;&lt;/properties&gt;&lt;LuaPersistentData&gt;PersistData = {}
&lt;/LuaPersistentData&gt;&lt;/lua_gen_persisting&gt;</state><devicedata>
    <copyright>Copyright 2004-2008 Control4 Corporation.  All rights reserved.</copyright>
    <manufacturer>weatherbug</manufacturer>
    <model>WeatherBug Forecasts</model>
    <creator>RyanE</creator>
    <name>WeatherBug Forecasts</name>
    <small>devices_sm/contactsingle_humiditysensor_open.gif</small>
    <large>devices_sm/contactsingle_humiditysensor_open.gif</large>
    <control>lua_gen</control>
    <proxy>weatherbug</proxy>
    <driver>DriverWorks</driver>
    <states/>
    <config>
        <documentation>This is a driver that retrieves weather data from connecting to the WeatherBug API.\n\nYou must first set the Address of this driver (in 'Connections -- Network') to the Hostname.\n\nFor API information, see: NOTE: This driver will not capture more than one active Weather Alert, and should not be used as life-safety alerting system...\n\nTo get a list of Zip or City codes, use the 'Lua' tab, and enter SearchLocations(), and pass in a name or zip code to search for.  For example, to verify a valid zipcode for Draper, UT, enter: SearchLocations("84020").  To Search for a city that sounds like 'Paris', enter: SearchLocations("paris")\n\nThe Font Size Properties do not apply to the Mini-Touch, which only has a single font size.</documentation>
        <properties>
<property>
                <name>Location Code</name>
                <type>STRING</type>
                <default/>
                <readonly>false</readonly>
            </property>
            <property>
                <name>Location Type</name>
                <type>LIST</type>
                <items>
                    <item>US (Zip Code)</item>
                    <item>Foreign (City Code)</item>
                </items>
                <default>US (Zip Code)</default>
                <readonly>false</readonly>
            </property>
            <property>
                <name>Units Type</name>
                <type>LIST</type>
                <items>
                    <item>US Units</item>
                    <item>Metric Units</item>
                </items>
                <default>US Units</default>
                <readonly>false</readonly>
            </property>
            <property>
                <name>Alert Font Size</name>
                <type>LIST</type>
                <items>
                    <item>XX-Small</item>
                    <item>X-Small</item>
                    <item>Small</item>
                    <item>Medium</item>
                    <item>Large</item>
                    <item>X-Large</item>
                    <item>XX-Large</item>
                </items>
                <default>Medium</default>
                <readonly>false</readonly>
            </property>
            <property>
                <name>Current Font Size</name>
                <type>LIST</type>
                <items>
                    <item>XX-Small</item>
                    <item>X-Small</item>
                    <item>Small</item>
                    <item>Medium</item>
                    <item>Large</item>
                    <item>X-Large</item>
                    <item>XX-Large</item>
                </items>
                <default>Medium</default>
                <readonly>false</readonly>
            </property>
            <property>
                <name>Forecast Font Size</name>
                <type>LIST</type>
                <items>
                    <item>XX-Small</item>
                    <item>X-Small</item>
                    <item>Small</item>
                    <item>Medium</item>
                    <item>Large</item>
                    <item>X-Large</item>
                    <item>XX-Large</item>
                </items>
........

How do I go about fixing this? I tried what travishein but I still no luck.

**Fubarable** · 01-04-2010, 08:54 PM

I tried what travishein but I still no luck.

This doesn't tell us much, and in fact tells us nothing about what specifically you've tried and what problems have occurred from it.

**JohnST** · 01-04-2010, 09:56 PM

Here is what I tried.

Java Code:

//-------
                    NodeList IdList = firstItemElement.getElementsByTagName("id");
                    Element IdElement = null;
                    if (IdList != null) {
                    IdElement = (Element) IdList.item(0);
                     
                    }
                    else {
                    // if this really can not ever be null, generate your own meaningful error message here.
                    }
                    NodeList textIdList = IdElement.getChildNodes();
                    String value = null;

                    Node aNode = null;
                    if (textIdList != null) {
                    aNode = (Node)textIdList.item(0);
                    if (aNode != null) {
                    value = aNode.getNodeValue();
                    if (value != null) {
                    value = value.trim();
                     }
                   }
                 }
                    System.out.println("id : " + value);

And I get NPE on this line

Java Code:

NodeList textIdList = IdElement.getChildNodes();

**Fubarable** · 01-04-2010, 10:05 PM

I'd find out what the program is reading at that time and why textIdList is null. Either use a debugger or println statements. With a little sleuthing the answer should come to you. For e.g.,

Java Code:

if (IdElement == null) {
  System.out.println(/* a bunch of stuff here to find out where in the file you are */);
}
NodeList textIdList = IdElement.getChildNodes();

**JohnST** · 01-04-2010, 10:51 PM

Thanks for all of your help :) I got it working!!!

**Eranga** · 01-06-2010, 12:37 AM

Haven't you ever check for null values to sort out such instance John. Or else you should catch the exception and take the proper action, basically move to the next step. Those are the simplest ways to solve this kind of errors.

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