null pointer exception in battery percent code

Question

here is my code i get null pointer exception in battery percentage code here: what im triyng to do is to put the battery percentage in a textview thahts all

public class MainActivity extends Activity implements SurfaceHolder.Callback {

public static CFlashLight flash = new CFlashLight();
public static SurfaceView surfaceView;
public static SurfaceHolder surfaceHolder;
private final String sCRLF = System.getProperty("line.separator");
boolean bDarkBackground;

//teststrob
StrobeRunner runner;
Thread bw;
Handler mHandler = new Handler();
Runnable mShowToastRunnable = new Runnable() {
    public void run() {
        showMessage();
    }
};
//test battery
private TextView batteryPercent= (TextView) this.findViewById(R.id.TextView01);

private void getBatteryPercentage() {
      BroadcastReceiver batteryLevelReceiver = new BroadcastReceiver() {
             public void onReceive(Context context, Intent intent) {
                 context.unregisterReceiver(this);
                 int currentLevel = intent.getIntExtra(BatteryManager.EXTRA_LEVEL, -1);
                 int scale = intent.getIntExtra(BatteryManager.EXTRA_SCALE, -1);
                 int level = -1;
                 if (currentLevel >= 0 && scale > 0) {
                     level = (currentLevel * 100) / scale;
                 }

                batteryPercent.setText("Battery: " + level + "%");
             }
         }; 
         IntentFilter batteryLevelFilter = new IntentFilter(Intent.ACTION_BATTERY_CHANGED);
         registerReceiver(batteryLevelReceiver, batteryLevelFilter);
      }
//test

@SuppressWarnings("deprecation")
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    bDarkBackground = true;

    getBatteryPercentage();

and here is the log cat out put permissoins are ok .

09-22 14:12:40.417: E/AndroidRuntime(17552): FATAL EXCEPTION: main
09-22 14:12:40.417: E/AndroidRuntime(17552): Process: com.soheil.prolight, PID: 17552
09-22 14:12:40.417: E/AndroidRuntime(17552): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.soheil.prolight/com.soheil.prolight.MainActivity}: java.lang.NullPointerException
09-22 14:12:40.417: E/AndroidRuntime(17552):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2121)
09-22 14:12:40.417: E/AndroidRuntime(17552):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
09-22 14:12:40.417: E/AndroidRuntime(17552):    at android.app.ActivityThread.access$800(ActivityThread.java:135)
09-22 14:12:40.417: E/AndroidRuntime(17552):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
09-22 14:12:40.417: E/AndroidRuntime(17552):    at android.os.Handler.dispatchMessage(Handler.java:102)
09-22 14:12:40.417: E/AndroidRuntime(17552):    at android.os.Looper.loop(Looper.java:136)
09-22 14:12:40.417: E/AndroidRuntime(17552):    at android.app.ActivityThread.main(ActivityThread.java:5017)
09-22 14:12:40.417: E/AndroidRuntime(17552):    at java.lang.reflect.Method.invokeNative(Native Method)
09-22 14:12:40.417: E/AndroidRuntime(17552):    at java.lang.reflect.Method.invoke(Method.java:515)
09-22 14:12:40.417: E/AndroidRuntime(17552):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
09-22 14:12:40.417: E/AndroidRuntime(17552):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
09-22 14:12:40.417: E/AndroidRuntime(17552):    at de.robv.android.xposed.XposedBridge.main(XposedBridge.java:132)
09-22 14:12:40.417: E/AndroidRuntime(17552):    at dalvik.system.NativeStart.main(Native Method)
09-22 14:12:40.417: E/AndroidRuntime(17552): Caused by: java.lang.NullPointerException
09-22 14:12:40.417: E/AndroidRuntime(17552):    at android.app.Activity.findViewById(Activity.java:1884)
09-22 14:12:40.417: E/AndroidRuntime(17552):    at com.soheil.prolight.MainActivity.<init>(MainActivity.java:56)
09-22 14:12:40.417: E/AndroidRuntime(17552):    at java.lang.Class.newInstanceImpl(Native Method)
09-22 14:12:40.417: E/AndroidRuntime(17552):    at java.lang.Class.newInstance(Class.java:1208)
09-22 14:12:40.417: E/AndroidRuntime(17552):    at android.app.Instrumentation.newActivity(Instrumentation.java:1061)
09-22 14:12:40.417: E/AndroidRuntime(17552):    at    android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2112)
09-22 14:12:40.417: E/AndroidRuntime(17552):    ... 12 more

Answer 1

initialize your variable in OnCreate method after setting onContentView So Change on create method like this

 private void getBatteryPercentage() {
    final TextView batteryPercent = (TextView) this.findViewById(R.id.TextView02);
      BroadcastReceiver batteryLevelReceiver = new BroadcastReceiver() {

             public void onReceive(Context context, Intent intent) {

                 context.unregisterReceiver(this);
                 int currentLevel = intent.getIntExtra(BatteryManager.EXTRA_LEVEL, -1);
                 int scale = intent.getIntExtra(BatteryManager.EXTRA_SCALE, -1);
                 int level = -1;
                 if (currentLevel >= 0 && scale > 0) {
                     level = (currentLevel * 100) / scale;
                 }

                batteryPercent.setText("Battery: " + level + "%");
             }
         }; 
         IntentFilter batteryLevelFilter = new IntentFilter(Intent.ACTION_BATTERY_CHANGED);
         registerReceiver(batteryLevelReceiver, batteryLevelFilter);
      }