尝试运行多个线程时导致强制关闭的空指针异常

Question

我正在尝试在后台运行一项服务，该服务会发送有关手机位置的短信。在服务中，我使用计时器。发送消息的代码运行正常，但是当我尝试将代码放入以查找用户位置时，我总是遇到错误。

我尝试调用一个新线程并使用了一个循环器，但现在我得到了一个指向这两行代码的空指针异常：

sendSmsMessage();

在这个方法中：

if (coordinates.equals(null) || coordinates.equals("") || coordinates == null || coordinates == ""){
        coordinates = "Could Not Receive Location";
    }

所以，我要问的问题是为什么会发生此错误？如果您能帮助我，将不胜感激。如果我只是忽略了一些非常简单的事情，请指出。谢谢！

代码如下：

    public class MessageService extends Service{
int counter = 0;
private Timer timer = new Timer();
public String textTime, phoneNumber;
public int updateInterval;
int lat, lng;
String coordinates, latitude, longitude;
LocationManager locationManager;

@Override
public IBinder onBind(Intent arg0) {
    return null;
}

@Override
public int onStartCommand(Intent intent, int flags, int startId){
    //receives the intent extras from the calling intent
    textTime = intent.getStringExtra("textTime");
    phoneNumber = intent.getStringExtra("phone");

    phoneNumber = "5556";

    //the following if statement has to do with transferring the string textTime into a number that can be used
    if (textTime.equals("15 Minutes")) {
        updateInterval = (15 * (60000));
    }else if (textTime.equals("30 Minutes")) {
        updateInterval = (30 * (60000));
    }else if (textTime.equals("1 Hour")){
        updateInterval = (60 * (60000));
    }else {
        updateInterval = (15 * (60000));
    }

    locationManager = (LocationManager) this.getSystemService(Context.LOCATION_SERVICE);

    new Thread(){
        public void run(){
            Looper.prepare();
            // Define a listener that responds to location updates
            LocationListener locationListener = new LocationListener() {
                public void onLocationChanged(Location location) {
                  // Called when a new location is found by the network location provider.
                  //makeUseOfNewLocation(location);
                    lat = (int) (location.getLatitude() * 1E6);
                    lng = (int) (location.getLongitude() * 1E6);

                    latitude = Integer.toString(lat);
                    longitude = Integer.toString(lng);

                    coordinates = "Coordinates: " + latitude + ", " + longitude + ". Latitude: " + latitude + " Longitude: " + longitude + ". Respond 'END' to stop texts."; 
                }

                public void onStatusChanged(String provider, int status, Bundle extras) {}

                public void onProviderEnabled(String provider) {}

                public void onProviderDisabled(String provider) {}
              };

              Looper.loop();

              if (locationManager.isProviderEnabled(LocationManager.GPS_PROVIDER)){
                    locationManager.requestLocationUpdates(LocationManager.GPS_PROVIDER, 0, 0, locationListener);
              }else{
                    locationManager.requestLocationUpdates(LocationManager.NETWORK_PROVIDER, 0, 0, locationListener);
              }

        }
    }.start();


    //the following method should use a timer to send a sms message in a timed interval. It also should implement using a different thread
    doSomethingRepeatedly();

    return START_STICKY;

}

public void doSomethingRepeatedly(){
    timer.scheduleAtFixedRate(new TimerTask() {
        public void run() {
            //the following code should be what is done repeatedly
            //Log.d("MessageService", String.valueOf(++counter));

            sendSmsMessage();
        }
    }, 0, updateInterval);
}

//the following code handles sending the text message
public void sendSmsMessage(){
    SmsManager sms = SmsManager.getDefault();
    if (coordinates.equals(null) || coordinates.equals("") || coordinates == null || coordinates == ""){
        coordinates = "Could Not Receive Location";
    }

    sms.sendTextMessage(phoneNumber, null, "test" , null, null);

}

public void onDestroy() {
    super.onDestroy();

    if (timer != null) {
        timer.cancel();
    }
}




}//end of service

这是 logcat 的输出：

    10-19 20:23:28.096: E/AndroidRuntime(944): FATAL EXCEPTION: Timer-0
10-19 20:23:28.096: E/AndroidRuntime(944): java.lang.NullPointerException
10-19 20:23:28.096: E/AndroidRuntime(944):  at com.app.MessageService.sendSmsMessage(MessageService.java:113)
10-19 20:23:28.096: E/AndroidRuntime(944):  at com.app.MessageService$2.run(MessageService.java:105)
10-19 20:23:28.096: E/AndroidRuntime(944):  at      java.util.Timer$TimerImpl.run(Timer.java:284)

Answer 1

你需要更换

coordinates.equals(null)  

与

coordinates == null

因为第一个永远不会评估为true，所以每次coordinates 是null 时都会抛出一个NullPointerException()。

Answer 2

如果您的coordinates 为空，那么第一个条件oordinates.equals(null) 将导致NullPointerException。

另外，我还有一个观察。比较coordinates == "" 没有用，因为您已经以正确的方式使用它，即坐标.equals("")。

从

更改您的 if

if (coordinates.equals(null) || coordinates.equals("") || 
                                    coordinates == null || coordinates == ""){
    coordinates = "Could Not Receive Location";
}

到

if (coordinates == null || coordinates.equals("")){
    coordinates = "Could Not Receive Location";
}