How to parse XML from URL using XMLpullparser in Android

I am a novice Android Developer I am trying to parse Xml from URL.I am able to parse a XML file in assets folder but i am not able to parse url from web page and this is my code given below`public class

public class MainActivity extends ActionBarActivity {
ListView listView;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

listView = (ListView) findViewById(R.id.list);

    List<Device> devices = null;

    try {


         XMLPullParserHandler parser = new XMLPullParserHandler();
         devices = parser.parse(getAssets().open("xml.php"));
         ArrayAdapter<Device> adapter =
                 new ArrayAdapter<Device>(this, R.layout.list_item,devices);
     listView.setAdapter(adapter); 
    } catch (IOException e) {
        e.printStackTrace();
        // TODO: handle exception
    }
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}

//

public class XMLPullParserHandler {
List<Device> devices;

private Device device;
private String text;


public XMLPullParserHandler(){
    devices = new ArrayList<Device>();
}
public List<Device>getDevices(){
    return devices;

}
public List<Device> parse(InputStream is){

    XmlPullParserFactory factory = null;
    XmlPullParser parser = null;

    try {
        factory = XmlPullParserFactory.newInstance();
        factory.setNamespaceAware(true);

        parser = factory.newPullParser();
        parser.setInput(is,null );

        int eventType = parser.getEventType();
        while(eventType != XmlPullParser.END_DOCUMENT){
            String tagname = parser.getName();
            switch (eventType) {

            case XmlPullParser.START_TAG:
                if (tagname.equalsIgnoreCase("device")){
                    device = new Device();
                }

                break;

            case XmlPullParser.TEXT:

                text = parser.getText();
                break;

            case XmlPullParser.END_TAG:
                        if (tagname.equalsIgnoreCase("device")) {
                        devices.add(device);
                        }else if(tagname.equalsIgnoreCase("id")){
                            device.setId(Integer.parseInt(text));
                        }else if(tagname.equalsIgnoreCase("devName")){
                            device.setDevname(text);
                        }else if(tagname.equalsIgnoreCase("devID")){
                            device.setDevid(text);
                        }else if(tagname.equalsIgnoreCase("devDes")){
                            device.setDevDes(text);
                        }else if(tagname.equalsIgnoreCase("img")){
                            device.setDevimg(text);
                        }else if(tagname.equalsIgnoreCase("chart")){
                            device.setDevchart(Integer.parseInt(text));
                        }else if(tagname.equalsIgnoreCase("type")){
                            device.setDevtype(Integer.parseInt(text));
                        }

                        break;
            default:
                break;
            }
            eventType = parser.next();
        }



    } catch (Exception e) {
        // TODO: handle exception
        e.printStackTrace();
    }

    return devices;

}

}