Navigating in XML Documents

The Node interface available when you use the DOM parser contains all the standard W3C DOM methods for navigating in a document, such as getNextSibling , getPreviousSibling , getFirstChild , getLastChild , and getParent . It's different when you use a SAX parser: This parser does not create a tree of nodes, so those methods don't apply.

Instead, if you want to find a particular element, you have to find it yourself. In the previous chapter, I found the third person's name in this document, which we'll call ch12_08.xml here:

Listing ch12_08.xml

 <?xml version="1.0"?> <MEETINGS>    <MEETING TYPE="informal">        <MEETING_TITLE>XML In The Real World</MEETING_TITLE>        <MEETING_NUMBER>2079</MEETING_NUMBER>        <SUBJECT>XML</SUBJECT>        <DATE>6/1/2002</DATE>        <PEOPLE>            <PERSON ATTENDANCE="present">                <FIRST_NAME>Edward</FIRST_NAME>                <LAST_NAME>Samson</LAST_NAME>            </PERSON>            <PERSON ATTENDANCE="absent">                <FIRST_NAME>Ernestine</FIRST_NAME>                <LAST_NAME>Johnson</LAST_NAME>            </PERSON>            <PERSON ATTENDANCE="present">                <FIRST_NAME>Betty</FIRST_NAME>                <LAST_NAME>Richardson</LAST_NAME>            </PERSON>        </PEOPLE>    </MEETING> </MEETINGS> 

It's not difficult to do the same thing here, but in SAX programming, finding a specific element takes a little code. I start by finding the third <PERSON> element and setting a variable named thirdPersonFlag to true when I find it:

 public void startElement(String uri, String localName, String qualifiedName, Attributes  attributes)  {  if(qualifiedName.equals("PERSON")) {   personCount++;   }   if(personCount == 3) {   thirdPersonFlag = true;   }  .     .     . } 

When the SAX parser is parsing the third person's <FIRST_NAME> element, I'll set a variable named firstNameFlag to true . When it's parsing the third person's <LAST_NAME> element, I'll set a variable named lastNameFlag to true:

 public void startElement(String uri, String localName, String qualifiedName, Attributes  attributes)  {     if(qualifiedName.equals("PERSON")) {         personCount++;     }     if(personCount == 3) {         thirdPersonFlag = true;     }  if(qualifiedName.equals("FIRST_NAME") && thirdPersonFlag) {   firstNameFlag = true;   }   if(qualifiedName.equals("LAST_NAME")  && thirdPersonFlag) {   firstNameFlag = false;   lastNameFlag = true;   }  } 

Watching the variables firstNameFlag and lastNameFlag , I can store the person's first and last names in the character callback:

 public void characters(char characters[], int start, int length)  {     String characterData = (new String(characters, start, length)).trim();     if(characterData.indexOf("\n") < 0 && characterData.length() > 0) {  if(firstNameFlag) {   firstName = characterData;   }   if(lastNameFlag) {   lastName = characterData;   }  } } 

When the SAX parser is done parsing the third <PERSON> element, I'll display that person's name:

 public void endElement(String uri, String localName, String qualifiedName) {  if(thirdPersonFlag && lastNameFlag){   System.out.println("Third name: " + firstName + " " + lastName);   thirdPersonFlag = false;   firstNameFlag = false;   lastNameFlag = false;   }  } 

And that's the technique you use when you're hunting a specific element using a SAX parser. You just wait until the parser hands it to you. Here are the results:

 %java ch12_09 ch12_08.xml  Third name: Betty Richardson 

Here's the full code for this program, ch12_09.java:

Listing ch12_09.java

 import org.xml.sax.*; import org.xml.sax.helpers.DefaultHandler; import javax.xml.parsers.*; import java.io.*; public class ch12_09 extends DefaultHandler {     int personCount;     boolean thirdPersonFlag = false, firstNameFlag = false, lastNameFlag = false;     String firstName, lastName;     public void displayDocument(String uri)     {         DefaultHandler handler = this;         SAXParserFactory factory = SAXParserFactory.newInstance();         try {             SAXParser saxParser = factory.newSAXParser();             saxParser.parse(new File(uri), handler);         } catch (Throwable t) {}     }     public void startElement(String uri, String localName, String qualifiedName,  Attributes attributes)     {         if(qualifiedName.equals("PERSON")) {             personCount++;         }         if(personCount == 3) {             thirdPersonFlag = true;         }         if(qualifiedName.equals("FIRST_NAME") && thirdPersonFlag) {             firstNameFlag = true;         }         if(qualifiedName.equals("LAST_NAME")  && thirdPersonFlag) {             firstNameFlag = false;             lastNameFlag = true;         }     }     public void characters(char characters[], int start, int length)     {         String characterData = (new String(characters, start, length)).trim();         if(characterData.indexOf("\n") < 0 && characterData.length() > 0) {             if(firstNameFlag) {                 firstName = characterData;             }             if(lastNameFlag) {                 lastName = characterData;             }         }     }     public void endElement(String uri, String localName, String qualifiedName)     {         if(thirdPersonFlag && lastNameFlag){             System.out.println("Third name: " + firstName + " " + lastName);             thirdPersonFlag = false;             firstNameFlag = false;             lastNameFlag = false;         }     }     public void warning(SAXParseException exception)     {         System.err.println("WARNING! " +             exception.getMessage());     }     public void error(SAXParseException exception)     {         System.err.println("ERROR! " +             exception.getMessage());     }     public void fatalError(SAXParseException exception)     {         System.err.println("FATAL ERROR! " +             exception.getMessage());     }     public static void main(String args[])     {         ch12_09 obj = new ch12_09();         obj.displayDocument(args[0]);     } }