NullPointerException handling

i,
      I have the following code.

package transform;

import oracle.xml.schemavalidator.XSDValidator;
import oracle.xml.parser.schema.XSDException;
import oracle.xml.parser.schema.XMLSchema;
import oracle.xml.parser.schema.XSDBuilder;
import oracle.xml.parser.v2.XMLError;
import org.xml.sax.SAXException;
import org.xml.sax.SAXParseException;
import org.xml.sax.helpers.DefaultHandler;
import java.net.URL;

public class SchemaValidator{

        public void validateSchema(String SchemaUrl, String
XmlDocumentUrl)
        {
                try {
                        XSDValidator xsdValidator=new XSDValidator();
                        XSDBuilder builder = new XSDBuilder();
                        URL url = new URL(SchemaUrl);
                        XMLSchema schemadoc =
(XMLSchema)builder.build(url);
                        xsdValidator.setSchema(schemadoc);

                        Validator handler=new Validator();
                        XMLError xmlError=new XMLError();
                        xmlError.setErrorHandler(handler);
                        xsdValidator.setError(xmlError);
                        xsdValidator.validate(new
URL(XmlDocumentUrl));
                        if(handler.validationError==true)
                                System.out.println("This XML Document
has Error: " + handler.saxParseException.getMessage());
                        else
                                System.out.println("This XML Document
is valid");

                }catch(java.lang.NullPointerException npe)
                 {
                        System.out.println("NullPointerException
"+npe.getMessage());
                 }
                catch(java.io.IOException ioe)
                {
                        System.out.println("IOException
"+ioe.getMessage());
                }catch (SAXException e) {
                        System.out.println("SAXException
"+e.getMessage());
                }
                catch (XSDException e) {
                        System.out.println("SAXException
"+e.getMessage());
                }
        }

        private class Validator extends DefaultHandler{

                public boolean validationError = false;
                public SAXParseException saxParseException=null;
                alidationError = true;
                saxParseException=exception;
                }

                public void fatalError(SAXParseException exception)
throws SAXException{
            validationError = true;
            saxParseException=exception;
                }
                public void warning(SAXParseException exception)
throws SAXException{}

           }

        public static void main(String[] argv){
                try{
                String SchemaUrl=argv[0];
                String XmlDocumentUrl=argv[1];
                SchemaValidator validator=new SchemaValidator();

                validator.validateSchema(SchemaUrl, XmlDocumentUrl);

                        } catch(java.lang.NullPointerException npe)
                        {

System.out.println("NullPointerException");
                        }
        }

Here XmlDocumentUrl is the xml document. Am trying to validating a xml
document(XmlDocumentUrl) with the schema specified by SchemaUrl. But
when I miss some attribute in the xml document(XmlDocumentUrl) its
throwing NullPointerException. After catching the NullPointerException
I want to handle it. I want to print an error message which specifies
in which file(XmlDocumentUrl) it occured and the line in that
file(XmlDocumentUrl). Can anyone tell me how to do this?