Got NullPointerException when applying DOM Technique in XML Parser

I ran this program and it reported NullPointerException at this line

NodeList nodeList = root.getChildNodes();

Here is the whole thing:

package com.example.dom_technique;

import java.io.IOException;
import java.io.InputStream;

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 android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

public class DOM extends Activity {

private Button button;
private TextView textView;

protected void onCreate(Bundle x) {
    super.onCreate(x);
    setContentView(R.layout.activity_dom);

    button = (Button) findViewById(R.id.button1);
    textView = (TextView) findViewById(R.id.textView1);

    button.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View v) {
            try {
                readXML();
            } catch (ParserConfigurationException e) {
                e.printStackTrace();
            } catch (SAXException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }

    });
}

private void readXML() throws ParserConfigurationException, SAXException, IOException {
    String output = "";

    DocumentBuilder builder = DocumentBuilderFactory.newInstance().newDocumentBuilder();

    InputStream inputStream = getResources().openRawResource(R.drawable.student);
    Document doc = builder.parse(inputStream);

    Element root = doc.getDocumentElement();
    NodeList nodeList = root.getChildNodes();

    for (int i = 0; i < nodeList.getLength(); i++) {
        Node node = nodeList.item(i);

        if (node instanceof Element) {
            Element student = (Element) node;
            String name = student.getAttribute("name");
            String student_id = student.getAttribute("student_id");

            NodeList subNodeList = student.getElementsByTagName("school");  
            String school = subNodeList.item(0).getTextContent();

            subNodeList = student.getElementsByTagName("major");
            String major = subNodeList.item(0).getTextContent();

            output += name + "  " + student_id + "\n" + school + "\n" + major +"\n" + "     ***********" + "\n";
        }
    }
    textView.setText(output);
}

}

And XML file I wrote by myself:

<?xml version="1.0" encoding="utf-8"?><students>

<student name="Henry Thompson" student_id="63827614">

    <school>University of California, Berkeley</school>
    <major>Computer Science</major>

</student>

<student name="Sheila Lopez" student_id="02817281">

    <school>University of California, Los Angeles</school>
    <major>Electrical Engineering</major>

</student>

<student name="Gabriel Carter" student_id="92717290">

    <school>University of California, Davis</school>
    <major>Mathematics</major>

</student>

<student name="Stephanie Nguyen" student_id="83729337">

    <school>University of California, San Diego</school>
    <major>Business Administraion, Concentration in Marketing</major>

</student>

</students>

Barely can I understand why, I thought I went through every requirements. I suspect the problem may come from the XML file. At first, I tried to create a XML file in drawable folder and it reported bug. Therefore I wrote in notepad, saved with xml extension and imported into drawable, then it was fine. Please help me!!! Thank you so much!!!!!!