XML validation using DOM

Hi... i am new to XML validation using XML...
   I am trying to Validate the XML document using XMLSchema.. The code
i am using is ...but its giving Error like
  could any one tell me where i went wrong.....
Error:
ValidationExample.java:27: cannot resolve symbol
symbol : variable XMLConstants
location: class ValidationExample
             SchemaFactory factory =
SchemaFactory.newInstance(XMLConstants.W3C_XML_SCHEMA_NS_URI);
                                                               ^
ValidationExample.java:33: cannot resolve symbol
symbol : method Validate (org.w3c.dom.Document)
location: class javax.xml.validation.Validator
         valid.Validate(doc);
              ^
Programme:

import java.io.*;
import javax.xml.validation.*;
import org.xml.sax.*;
import javax.xml.transform.stream.*;
import javax.xml.transform.*;
import javax.xml.parsers.*;
import java.lang.*;
import org.w3c.dom.*;

public class ValidationExample{
           public static void main (String args[]){
//parse the XML document in to XML
      try {

            // DOMParser parser = new DOMParser();
            // parser.parse("DEMMODTD.xml"));
             // Document doc = parser.getDocument();

             File docfile = new File("DEMODTD.XML");
             Document doc = null;
             DocumentBuilderFactory dbf =
DocumentBuilderFactory.newInstance();
             DocumentBuilder db = dbf.newDocumentBuilder();
             doc = db.parse(docfile);
             dbf.setValidating(true);

               // set the Schema Factory
             SchemaFactory factory =
SchemaFactory.newInstance(XMLConstants.W3C_XML_SCHEMA_NS_URI);
             Source schemaFile = new StreamSource(new
File("DEMODTD.xsd"));
             Schema schema = factory.newSchema(schemaFile);
             Validator valid = schema.newValidator();
   //validate the DOm tree

         valid.Validate(doc);
}catch (SAXException e) {
  System.out.println("error"+e.getMessage());
 }catch (IOException e){
   System.out.println("error"+e.getMessage());
 }catch (ParserConfigurationException e) {
      System.out.println("error"+e.getMessage());
 }catch (FactoryConfigurationError e) {
    System.out.println("error"+e.getMessage());
 }

}
}