<products>
   <product pid="01"/>
   <product pid="05"/>
   <product pid="09"/>
</products>

import java.io.*;
import javax.xml.namespace.QName;
import javax.xml.stream.*;
import javax.xml.stream.events.XMLEvent;

public class Merger {

   private static final QName prodName = new QName("product");
   private static final QName pidName = new QName("pid");

   public static void main(String[] args)
      throws FileNotFoundException, XMLStreamException {
         
      // Use  the reference implementation for the  XML input factory
      System.setProperty(
         "javax.xml.stream.XMLInputFactory",
         "com.bea.xml.stream.MXParserFactory");
      // Create the XML input factory
      XMLInputFactory factory = XMLInputFactory.newInstance();
      // Create XML event reader 1
      XMLEventReader r1 = 
         factory.createXMLEventReader(new FileReader("prodList1.xml"));
      // Create XML event reader 2
      XMLEventReader r2 = 
         factory.createXMLEventReader(new FileReader("prodList2.xml"));

      // Create the output factory
      XMLOutputFactory xmlof = XMLOutputFactory.newInstance();
      // Create XML event writer
      XMLEventWriter xmlw = xmlof.createXMLEventWriter(System.out);

      // Read to first <product> element in document 1
      // and output to result document
      String pid1 = readToNextElement(r1, xmlw, false);
      // Read to first <product> element in document 1
      // without writing to result document
      String pid2 = readToNextElement(r2, null, false);
      // Loop over both XML input streams
      while (pid1 != null || pid2 != null) {
         // Compare merge criteria
         if (pid2 == null || (pid1 != null && pid1.compareTo(pid2) <= 0))
            // Continue in document 1
            pid1 = readToNextElement(r1, xmlw, pid2 == null);
         else
            // Continue in document 2
            pid2 = readToNextElement(r2, xmlw, pid1 == null);
      }
      xmlw.close();
   }

   /**
    * @param reader - the document reader
    * @param writer - the document writer
    * @param processEnd - forces the document end to be written
    * @return - the next merge criterion value
    * @throws XMLStreamException
    */
   private static String readToNextElement(XMLEventReader reader,
         XMLEventWriter writer, boolean processEnd) throws XMLStreamException {
      // Nesting level
      int level = 0;
      while (true) {
         // Read event to be written to result document
         XMLEvent event = reader.next();
         // Avoid double processing of document end
         if (!processEnd)
            switch (event.getEventType()) {
               case XMLEvent.START_ELEMENT :
                  ++level;
                  break;
               case XMLEvent.END_ELEMENT :
                  if (--level < 0)
                     return null;
                  break;
            }
         // Output event
         if (writer != null)
            writer.add(event);
         // Look at next event
         event = reader.peek();
         switch (event.getEventType()) {
            case XMLEvent.START_ELEMENT :
               // Start element - stop at <product> element
               QName name = event.asStartElement().getName();
               if (name.equals(prodName)) {
                  return event
                     .asStartElement()
                     .getAttributeByName(pidName)
                     .getValue();
               }
               break;
            case XMLEvent.END_DOCUMENT :
               // Stop at end of document
               return null;
         }
      }
   }
}