Tutorials : Java and XML: putting SAX to work :

Sorting the data

It'd be nice to have the DVD's listed in alphabetical order--or in movie length order. In my article "Working with files and directories in Java" I showed how to sort a collection of objects by letting them implement the "Comparable" interface. Since we now have to sort according to two different criteria we'll need another technique. Comparison of DVD-elements can be done in a separate class that implements the Comparator interface and has a "compare" method:

	package hansen.playground;
	
	import java.util.*;
	
	class CompareTitle implements Comparator {
	  public int compare(Object o1, Object o2) {
	    DVD d1 = (DVD)o1;
	    DVD d2 = (DVD)o2;
	    return (d1.getTitle()).compareToIgnoreCase(d2.getTitle());
	  }  
	} 

The MySAXParser class can now sort the titles with a statement like this:

Collections.sort(dvdList, new CompareTitle()); 

To also sort according to movie length we define yet another class:

	package hansen.playground;
	
	import java.util.*;
	
	class CompareLength implements Comparator {
	  public int compare(Object o1, Object o2) {
	    DVD d1 = (DVD)o1;
	    DVD d2 = (DVD)o2;
	    return d1.getLength() - d2.getLength();
	  }  
	}

To control which sorting class to use it's convenient to code a sort-method:

	public void sort(String type) {
	  if (type.equals("length")) 
	      Collections.sort(dvdList, new CompareLength()); 
	  if (type.equals("title")) 
	      Collections.sort(dvdList, new CompareTitle()); 
	} 

Presenting the results in a browser

It's now a simple matter to use the MySAXParser class in a servlet application so we can show the list of DVD's in a browser. Let me first show what it should look like:

By clicking on the "Title" or "Length" heading the list will be sorted. To make it simple we'll make a jsp-page that uses the MySAXParser class. To display the DVD's we'll have to code something like this:

	sax.processFile("d:\\ex1.xml");
	c = sax.getDVDs();
	for (Iterator i = c.iterator(); i.hasNext();) {
	  DVD dvd = (DVD)i.next();
	  . . . get title, length and actors and show this in the HTML    
	}

The complete source can be found in the Resources section. The hyperlinks on "Title" and "Length" calls a JavaScript function that calls another jsp-page (a "controller") that'll call the sort routine. The links for "Title" and "Length" are these:

	<a href="javascript:sort('title')">Title</a>
	<a href="javascript:sort('length')">Length</a>

The JavaScript function is simple:

	<script>
	function sort(type) {
	  location.href="ctlpage1.jsp?sort="+type;
	}
	</script>

The "ctlpage1.jsp" file contains the call to the sort-method:

	<%
	String type = request.getParameter("sort");
	if (type != null && !type.equals("")) sax.sort(type);
	pageContext.forward("page1.jsp");
	%>

Conclusion

The SAX API will read an XML document and return the information bit by bit to your program. This gives you full control of what you want to keep from the document and what you want to skip. You may build a complete tree structure in memory (like the DOM API does) or you may pick just the elements you're interested in. Note however, that when the complexity of the XML-file grows, coding also become more complex, and you should consider using the DOM API instead.

Resources

• The code for all the examples in the article
• The official web site for SAX: http://www.saxproject.org/
  It contains links to several parsers: http://www.saxproject.org/?selected=links
• The Xerces Java parser: http://xml.apache.org/xerces-j/
• SUN's JAXP API (includes SAX and DOM): http://java.sun.com/xml/jaxp/index.html
• Top 10 SAX tips
• STOP PRESS: SAX is now part of the newly announced JDK1.4

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