When you extend a Java class in Clojure and define a method of the same name as one in that class, what is happening?

Question

I've been reading through Programming Clojure, and I've been having some trouble understanding Stuarts primary Java Interop example. He extends DefaultHandler, and creates a startElement method, and then passes that handler to an XML parser. What I don't understand, is what exactly is happening. Does his implementation of startElement override the one defined in DefaultHandler? I'm confused. I have no experience with Java, and little with object orientation.

Thanks!

Answer 1

Glancing over the Java documentation for DefaultHandler answered my own question. http://java.sun.com/j2se/1.4.2/docs/api/org/xml/sax/helpers/DefaultHandler.html#startElement%28java.lang.String,%20java.lang.String,%20java.lang.String,%20org.xml.sax.Attributes%29

By default, do nothing. Application writers may override this method in a subclass to take specific actions at the start of each element (such as allocating a new tree node or writing output to a file).

Answer 2

I don't own the book, but I found the code and it looks like you're right. Here is the function (for others to see):

(def print-element-handler
  (proxy [DefaultHandler] [] 
   (startElement            
     [uri local qname atts] 
     (println (format "Saw element: %s" qname)))))

You're right about what it does. The proxy statement makes a new class, the equivilent of this Java code:

public class SomeNewClass extends DefaultHandler {
    public void startElement(String uri,
                     String localName,
                     String qName,
                     Attributes attributes) {
        System.out.println(*stuff*);
    }
}

So the proxy statement defines that class, and gives you an instance, which is now held in print-element-handler.