Validation is a process of checking the source XML document to correspond to the DOCTYPE it referes. If the document corresponds to its doc-type it is calles valid, otherwise the document is invalid. In order to tell the SAX to check the document for validity a correspodning property of SAX-parser factory should be set to true, like follows:

 // create new factory is ordinary way:
  SAXParserFactory factory = SAXParserFactory.newInstance();

  // specifies that the parser produced by this factory will
  // validate documents as they are parsed.
  factory.setValidating(true);
  
  // this parser will validate XML-input:
  SAXParser parser = factory.newSAXParser();