Question

我现在有几天遇到这些问题，我不知道我的问题是什么。我现在真的需要一些帮助，跳跃可以帮助我在这里。这些是我的代码。

public class MainActivity extends AppCompatActivity implements LocationListener {

TextView text;
String trip;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    text = (TextView) findViewById(R.id.id_trip);
    scheduleAlarm();

}

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

@Override
public void onProviderEnabled(String provider) {}

@Override
public void onProviderDisabled(String provider) {}


public void onLocationChanged(Location location) {
    new MyAsyncTask ().execute();
}

public class MyAsyncTask extends AsyncTask<String, String, String> {

    @Override
    protected void onPreExecute() {
    }

    @Override
    protected String doInBackground(String... arg0) {
        trip = "Hai";
        return null;
    }

    @Override
    protected void onPostExecute(String strFromDoInBg) {
        text.setText(trip);
    }
}

public void scheduleAlarm() {
    Intent intent = new Intent(getApplicationContext(), Track.class);
    final PendingIntent pIntent = PendingIntent.getBroadcast(this, Track.REQUEST_CODE, intent, PendingIntent.FLAG_UPDATE_CURRENT);
    AlarmManager alarm = (AlarmManager) this.getSystemService(Context.ALARM_SERVICE);
    alarm.setInexactRepeating(AlarmManager.ELAPSED_REALTIME, SystemClock.elapsedRealtime(), 60 * 1000, pIntent);
}
}

服务类

public class Services extends Service {
public LocationManager locationManager;
public MainActivity listener;

@Override
public void onCreate() {

    super.onCreate();
}

@Override
public void onStart(Intent intent, int startId) {
    locationManager = (LocationManager) getSystemService(Context.LOCATION_SERVICE);
    listener = new MainActivity();
    locationManager.requestSingleUpdate(LocationManager.GPS_PROVIDER, listener, null);
}

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

跟踪课程

public class Track extends BroadcastReceiver {
public static final int REQUEST_CODE = 12345;

@Override
public void onReceive(Context context, Intent intent) {
    Intent i = new Intent(context, Services.class);
    context.startService(i);
}}

错误是：

java.lang.NullPointerException：尝试调用虚方法＆＃39; android.view.View android.view.Window.findViewById（int）＆＃39;在null 对象参考

由于

Answer 1

您正在从doInBackground返回null值，这就是您获得NullPointerException的原因。

使用以下代码更改MyAsyncTask代码。

public class MyAsyncTask extends AsyncTask<String, String, String> {

    @Override
    protected void onPreExecute() {
       if(text==null){
       text = MainActivity.this.findViewById(R.id.id_trip);
      }
    }

    @Override
    protected String doInBackground(String... arg0) {
        trip = "Hai";
        return trip;
    }

    @Override
    protected void onPostExecute(String strFromDoInBg) {
        text.setText(strFromDoInBg);
    }
}

Answer 2

这是您的错误：

TextView text = (TextView) main.findViewById(R.id.id_trip);
text.setText(string);

你无法从TextView获得Service这样的对象。将TextView对象创建为静态或使用BroadcastReceiver。

Answer 3

从setText(String str)方法中删除以下2行。

MainActivity main = new MainActivity();
TextView text = (TextView) main.findViewById(R.id.id_trip);

希望这会对你有所帮助。

Android更改TextView文本

3 个答案: