Question

我正在开发一个Android应用程序并找到一些关于NullPointerException的奇怪之处。相同的方法retrieveCampusFromXml（）可以在onLocationChanged（）中运行，但它会在onCreate（）中抛出NPE。有人知道为什么吗？

如果我把它放在onCreate（）：

public class XXXX extends Activity {
    XmlResourceParser xrpCampus;
    ArrayList<Destination> campusDestinations = new ArrayList<Destination> ();
    firstRunForXML = true;
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    appState = ((MyApp)this.getApplication());
    mContext = this;
    xrpCampus = getResources().getXml(R.xml.campus);
    campusDestinations = retrieveCampusFromXml(xrpCampus);campusDestinations = retrieveCampusFromXml(xrpCampus);
    mLocationManager = (LocationManager) getSystemService(Context.LOCATION_SERVICE);

    Criteria criteria = new Criteria();
    criteria.setAccuracy(Criteria.NO_REQUIREMENT);
    criteria.setPowerRequirement(Criteria.NO_REQUIREMENT);

    best = mLocationManager.getBestProvider(criteria, true);

    mLocationManager.requestLocationUpdates(best, 1000, 0, mLocationListener);

retrieveCampusFromXml（）（其中包含大量自定义对象，例如Destination）：

private ArrayList<Destination> retrieveCampusFromXml(XmlResourceParser xrp) {
    ArrayList<Destination> listOfCampus = new ArrayList<Destination> ();

    String tagName = "";
    try {
        Destination temp=null;
        int latXml=0;
        int lngXml=0;
        int eventType = xrp.getEventType();
        while (eventType != XmlPullParser.END_DOCUMENT) {
            if (eventType == XmlPullParser.START_TAG) {
                tagName = xrp.getName();
                if(tagName.equals("destination")) {
                    temp = new Destination();
                }
            }
            else if (eventType == XmlPullParser.END_TAG) {
                tagName="";

                if(xrp.getName().equals("destination")) {
                    temp.setGeoPoint(latXml, lngXml);
                    listOfCampus.add(temp);
                    latXml=0;
                    lngXml=0;
                }
            }
            else if (eventType == XmlPullParser.TEXT) {
                if (tagName.equals("name")) {
                    temp.setName(xrp.getText());
                }
                else if (tagName.equals("latitude")) {
                    latXml = (int) (Double.parseDouble(xrp.getText())*1E6);
                }
                else if (tagName.equals("longitude")) {
                    lngXml = (int) (Double.parseDouble(xrp.getText())*1E6);
                }
                else if (tagName.equals("altitude") && !xrp.getText().equals("undefined")) {
                    temp.setAltitude(Double.parseDouble(xrp.getText()) );
                }
                else if (tagName.equals("drawableID") ) {
                    temp.setDrawable( mContext, Integer.decode(xrp.getText()) );
                }
            }
            eventType = xrp.next();
        }

    } catch (XmlPullParserException e) {
        Log.i("LUN", "XML");
        e.printStackTrace();
    } catch (IOException e) {
        Log.i("LUN", "IO");
        e.printStackTrace();
    } catch (NullPointerException e) {
        Log.i("LUN", "NULL");
        e.printStackTrace();
    }
    return listOfCampus;
}

如果我把它放在onLocationChanged（）中，那一切都没问题：

final LocationListener mLocationListener = new LocationListener() {
    public void onLocationChanged(Location arg0) {
        myLocation = arg0;
        if (myLocation != null) {
            if (firstRunForXML) {
                firstRunForXML = false;
                campusDestinations = retrieveCampusFromXml(xrpCampus);
            }
       }
}

我是Android新手，是我应该提供的堆栈跟踪吗？：

Thread [<3> main] (Suspended (exception NullPointerException))  
ViewRoot.draw(boolean) line: 1373   
ViewRoot.performTraversals() line: 1114 
ViewRoot.handleMessage(Message) line: 1633  
ViewRoot(Handler).dispatchMessage(Message) line: 99 
Looper.loop() line: 123 
ActivityThread.main(String[]) line: 4363    
Method.invokeNative(Object, Object[], Class, Class[], Class, int, boolean) line: not available [native method]  
Method.invoke(Object, Object...) line: 521  
ZygoteInit$MethodAndArgsCaller.run() line: 860  
ZygoteInit.main(String[]) line: 618 
NativeStart.main(String[]) line: not available [native method]

Answer 1

那是完整的onCreate吗？在onCreate中调用retrieveCampusXml时，mContext可能为null。在调用retrieveCampusXml（）之后，您必须在onCreate中进一步设置mContext。

Answer 2

由我自己修复。改变

ArrayList<Destination> campusDestinations = new ArrayList<Destination> ();

到

ArrayList<Destination> campusDestinations;

然后可以将方法retrieveCampusFromXml放入onCreate（）中。

感谢谁非常回答我的问题！：）

onCreate（）中的NullPointerException，但不在onLocationChanged（）中

2 个答案: