In Java, the Simple API for XML (SAX) API provides a streaming-based approach for parsing and processing XML documents. The SAX API is event-driven, meaning that it calls methods on a ContentHandler object as it encounters elements, attributes, and other content in the XML document.

The DefaultHandler class is a convenience implementation of the ContentHandler interface that provides default implementations of all the methods. You can extend the DefaultHandler class and override only the methods that you're interested in handling.

Here's an example of how to use a DefaultHandler to parse an XML document using SAX in Java:

import org.xml.sax.Attributes;
import org.xml.sax.SAXException;
import org.xml.sax.helpers.DefaultHandler;
import javax.xml.parsers.SAXParser;
import javax.xml.parsers.SAXParserFactory;
import java.io.File;

public class MyHandler extends DefaultHandler {
    public void startElement(String uri, String localName, String qName, Attributes attributes)
            throws SAXException {
        System.out.println("Start Element: " + qName);
    }

    public void endElement(String uri, String localName, String qName)
            throws SAXException {
        System.out.println("End Element: " + qName);
    }

    public void characters(char ch[], int start, int length)
            throws SAXException {
        System.out.println("Characters: " + new String(ch, start, length));
    }
}

public class MySaxParser {
    public static void main(String[] args) throws Exception {
        SAXParserFactory factory = SAXParserFactory.newInstance();
        SAXParser saxParser = factory.newSAXParser();
        MyHandler handler = new MyHandler();
        saxParser.parse(new File("example.xml"), handler);
    }
}

In this example, the MyHandler class extends the DefaultHandler class and overrides the startElement(), endElement(), and characters() methods to print information about the elements and content in the XML document.

In the MySaxParser class, a new SAXParser is created using a SAXParserFactory, and a new instance of the MyHandler class is passed to the parse() method of the SAXParser, along with the File object that represents the XML document to parse.

When the parse() method is called, the MyHandler object will receive calls to its overridden methods as SAX encounters elements and content in the XML document.

The DefaultHandler class provides default implementations of several other ContentHandler methods as well, such as startDocument() and endDocument(), which are called at the beginning and end of the document, respectively. By extending DefaultHandler and overriding the methods that you're interested in handling, you can create a simple, efficient, and flexible SAX parser for your XML documents in Java.