How to Parse MusicXML file using DOM parser?

Hi everyone, I am trying to parse musicXML file using DOM. However I am getting some error which I cant figure out why. Can someone help me to find this issue I am having and also can someone check if my coding is correct in terms or XML tree structure. The following code I have written sofar. I have attached the xml file that I am trying to work on.

Code Java:

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package de.uos.fmt.musitech.score.musicxml;
 
import java.io.IOException;
import java.net.URL;
 
import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.ParserConfigurationException;
 
import org.w3c.dom.Document;
import org.w3c.dom.Element;
import org.w3c.dom.Node;
import org.w3c.dom.NodeList;
import org.xml.sax.SAXException;
 
import de.uos.fmt.musitech.data.score.NotationStaff;
import de.uos.fmt.musitech.data.score.NotationSystem;
import de.uos.fmt.musitech.data.score.NotationVoice;
import de.uos.fmt.musitech.data.score.ScoreNote;
import de.uos.fmt.musitech.data.structure.Context;
import de.uos.fmt.musitech.data.structure.Note;
import de.uos.fmt.musitech.data.structure.Piece;
import de.uos.fmt.musitech.data.structure.linear.Part;
import de.uos.fmt.musitech.score.mpegsmr.Attributes;
import de.uos.fmt.musitech.utility.math.Rational;
 
public class DomParser<Partwise> {
	private Piece piece;
 
	public DomParser(URL url){
		parseXmlFile(url);	
	}		
 
	private void parseXmlFile(URL url){
		//get the factory	
		DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
		try {
			//Using factory get an instance of document builder
			DocumentBuilder db = dbf.newDocumentBuilder();
			//parse using builder to get DOM representation of the XML file
			Document dom = db.parse(url.openStream());
			//Document dom = db.parse("music.xml");
			piece = parseDocument(dom);
		}catch(ParserConfigurationException pce) {
			pce.printStackTrace();
		}catch(SAXException se) {
			se.printStackTrace();
		}catch(IOException ioe) {
			ioe.printStackTrace();
		}		
	}
	private Piece parseDocument(Document dom){
		//get the root element
		Element docEle =  dom.getDocumentElement();
		// create Musitech piece
		Piece piece = new Piece();
 
		// TODO distinction between partwise and measurewise/timewise (?)	
		//partwise: get a nodelist of elements = parts
		NodeList nl = docEle.getChildNodes();
		if(nl != null && nl.getLength() > 0) {
			for(int i = 0 ; i < nl.getLength();i++) {
				Node node = nl.item(i);
				if(node instanceof Element){
					Element el = (Element)node;					
					// switch  parts-list vs parts
					if( el.getNodeName().equals("part")){
						//create Musitech staff & voice
						NotationStaff staff = new NotationStaff();
						NotationVoice voice = new NotationVoice();
						// go through notes in part
						 NodeList cn = el.getChildNodes();
						 for (int j = 0; j < cn.getLength(); j++) {
							 Node n1 = cn.item(j);
							 n1.getFirstChild();
							 n1.getNextSibling();
							 n1.getLastChild();
							 staff.add(voice);
//-------------------------------------------------------------------------
							 if( el.getNodeName().equals("measure")){						
								 NodeList cn1 = el.getChildNodes();			
								 for (int k = 0; k < cn1.getLength(); k++) {
									 Node n2 = cn1.item(k);	
									 n2.getFirstChild();
									 n2.getNextSibling();
									 n2.getLastChild();
									 //System.out.println(cn1);
 
									 //attributes within measure
									 if(el.getNodeName().equals("attributes")){
										NodeList cn2 = el.getChildNodes();									
										for (int s = 0; s <cn2.getLength(); s++){
											Node n3 = cn2.item(s);
											n3.getFirstChild();
											n3.getNextSibling();
											n3.getLastChild();
											//if(n3.hasChildNodes()){
												//Node n3 = getNote()
											//}
											//division within attributes
											if(el.getNodeName().equals("division")){
												el.getNodeValue();
											}
											//key within attributes
											if(el.getNodeName().equals("key")){
												NodeList cn3 = el.getChildNodes();
												for (int m = 0; m <cn3.getLength(); m++){
													Node n4 = cn3.item(m);
													n4.getFirstChild();
													if(n4.hasChildNodes()){
														n4.getChildNodes();
														n4.getNodeValue();
													}
													//if(el.getNodeName().equals("fifths")){
														//Node n5 = n5.getNodeValue();
													//}	
												}
											}				
										}
										//time within attributes
										if(el.getNodeName().equals("time")){
										 NodeList cn4 = el.getChildNodes();
										 for (int n =0; n<cn4.getLength(); n++){
											 Node n5 = cn4.item(n);
											 if(n5.hasChildNodes()){
												 n5.getChildNodes();
												 n5.getNextSibling().getNodeValue(); 
											 }
 
										 }
										}
										//clef within attributes
										if(el.getNodeName().equals("clef")){
											 NodeList cn5 = el.getChildNodes();
											 for (int o =0; o <cn5.getLength(); o++){
												 Node n6 = cn5.item(o);
												 if(n6.hasChildNodes()){
													 n6.getChildNodes();
													 n6.getNextSibling().getNodeValue();	
												 }
									 }											 
						 		}
										//note within measure 
										if(el.getNodeName().equals("note")){
											NodeList cn6 = el.getChildNodes();
											for(int p = 0; p <cn6.getLength(); p++){
												Node n7 = cn6.item(p);
												n7.getFirstChild().getNextSibling().getNodeValue();
												n7.getLastChild().getNextSibling().getNodeValue();
												//pitch within note
												if(el.getNodeName().equals("pitch")){
													NodeList cn7 = el.getChildNodes();
													for(int q =0; q <cn7.getLength(); q++){
														Node n8 = cn6.item(q);
														if(el.getNodeName().equals("step")){
															el.getNodeValue();
														}
														if(el.getNodeName().equals("octave")){
															el.getNodeValue();
														}
														n8.getFirstChild().getNextSibling().getNodeValue();												
													}
												}
												if(el.getNodeName().equals("duration")){
													el.getNodeValue();
												}
												if(el.getNodeName().equals("type")){
													el.getNodeValue();
												}
											}
										}
									 }
								 }
							 }
 
					}					
				}
			}
		}
	}
	return piece;
}
	public Piece getPiece(){
		return piece;
	}
 
	public Note getNote(Element el) {
		// TODO Auto-generated method stub
		ScoreNote snote = new ScoreNote();
		//Note duration = getValue(el, "Duration");
		//Note = type = getValue(el, "Type");
		//getValue(el,"Duration");
		//getValue(el,"Step");
		snote.setDiatonic('C');
		snote.setAlteration((byte)1);
		snote.setMetricDuration(new Rational(1,4));
		//snote.setOctave(0);
 
		//Note  = (Note) ((DocumentBuilderFactory) el).getAttribute("note");		
		//Create a new song with the value read from the xml nodes
		//music = new Music (pitch,durution,time,key);
		Note note = new Note(snote, null);
		return note;
	}
 
	public Attributes getAttributes(Element el, Attributes tagName) {
		Attributes attributes = null;
		NodeList nl = (NodeList)el.appendChild(el);
		if(nl != null && nl.getLength() > 0) {
			Element e = (Element)nl.item(0);
			//attributes = el.getNodeValue();
		}
 
		return attributes;
	}
	/**
	 * Calls getTextValue and returns a int value
	 */
	public int getIntValue(Element ele, String tagName) {
		//in production application you would catch the exception
		return 0;
	}	
}

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