SAX (Simple API for XML) is a Java API for parsing and processing XML documents. It provides a low-level, event-driven approach to processing XML documents, in which the parser generates a stream of events as it reads the document, and the application responds to these events by executing code that processes the document's contents.

SAX provides a simple, lightweight, and efficient API for parsing XML documents, and is suitable for processing large XML files. It also provides a mechanism for validating XML documents against a DTD (Document Type Definition) or an XML Schema.

To use SAX, you need to create a handler class that extends the DefaultHandler class provided by the SAX API. This handler class should override the methods that correspond to the events generated by the SAX parser as it reads the XML document. For example, you might override the startElement() method to handle the start of an XML element, or the characters() method to handle the text content of an element.

Here is an example of using SAX to parse an XML file:

import org.xml.sax.*;
import org.xml.sax.helpers.*;

import java.io.*;

public class SAXExample extends DefaultHandler {

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

    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 {
        String text = new String(ch, start, length);
        System.out.println("Text content: " + text);
    }
}

In this example, we create a new SAX parser, and use it to parse an XML file called "example.xml". We also create a handler class that overrides the startElement(), endElement(), and characters() methods to handle the events generated by the SAX parser. When the parser encounters the start of an XML element, it calls the startElement() method, and we print out the name of the element. When it encounters the end of an element, it calls the endElement() method, and we print out the name of the element again. When it encounters text content, it calls the characters() method, and we print out the text content.

This is just a simple example, but it demonstrates the basic principles of using SAX to parse an XML document.