SAX, the power API

/*
 * XML By Example, chapter 8: SAX
 */

package com.psol.xbe2;

import org.xml.sax.*;
import java.io.IOException;
import org.xml.sax.helpers.*;
import java.text.MessageFormat;

/**
 * SAX event handler to find the cheapest offering in a list of
 * prices.
 */

public class Cheapest
   extends DefaultHandler
{
   /**
    * constants
    */
   protected static final String
      NAMESPACE_URI = "http://www.psol.com/xbe2/listing8.1",
      MESSAGE =
         "The cheapest offer is from {0} ({1,number,currency})",
      PARSER_NAME = "org.apache.xerces.parsers.SAXParser";

   /**
    * properties we are collecting: cheapest price & vendor
    */
   protected double min = Double.MAX_VALUE;
   protected String vendor = null;

   /**
    * startElement event: the price list is stored as price
    * elements with price and vendor attributes
    * @param uri namespace URI
    * @param name local name
    * @param qualifiedName qualified name (with prefix)
    * @param attributes attributes list
    */
   public void startElement(String uri,
                            String name,
                            String qualifiedName,
                            Attributes attributes)
   {
      if(uri.equals(NAMESPACE_URI) && name.equals("price-quote"))
      {
         String attribute =
            attributes.getValue("","price");
         if(null != attribute)
         {
            double price = toDouble(attribute);
            if(min > price)
            {
               min = price;
               vendor = attributes.getValue("","vendor");
            }
         }
      }
   }

   /**
    * helper method: turn a string in a double
    * @param string number as a string
    * @return the number as a double, or 0.0 if it cannot convert
    * the number
    */
   protected static final double toDouble(String string)
   {
      Double stringDouble = Double.valueOf(string);
      if(null != stringDouble)
         return stringDouble.doubleValue();
      else
         return 0.0;
   }

   /**
    * main() method
    * decodes command-line parameters and invoke the parser
    * @param args command-line argument
    * @throw Exception catch-all for underlying exceptions
    */
   public static void main(String[] args)
      throws IOException, SAXException
   {
      // command-line arguments
      if(args.length < 1)
      {
         System.out.println("java com.psol.xbe2.Cheapest file");
         return;
      }

      // creates the event handler
      Cheapest cheapest = new Cheapest();

      // creates the parser
      XMLReader parser =
         XMLReaderFactory.createXMLReader(PARSER_NAME);
      parser.setFeature("http://xml.org/sax/features/namespaces",
                        true);
      parser.setContentHandler(cheapest);

      // invoke the parser against the price list
      parser.parse(args[0]);

      // print the results
      Object[] objects = new Object[]
      {
          cheapest.vendor,
          new Double(cheapest.min)
      };
      System.out.println(MessageFormat.format(MESSAGE,objects));
   }
}