Strange exception on a button

So, i just want to save to a file when taping on a button, however, i have a FC, here is the code:

final Button button = (Button) findViewById(R.id.button);

 button.setOnClickListener(new View.OnClickListener() {
     public void onClick(View v) {
 String value1 = vol.getText().toString();
 String value2 = kil.getText().toString();


                     if (value1 != null && value1.trim().length() > 0 && value2 != null && value2.trim().length() > 0)
                     {
                             float q1=Float.parseFloat(vol.getText().toString());
                             float q2=Float.parseFloat(kil.getText().toString());
                             float x=((q1 / q2)* 100);
                             String y= Float.toString(x);
                             cons.setText(y);

                             SimpleDateFormat format = new SimpleDateFormat("dd/MM/yyyy");
                             data = format.format(new Date());
                             data = data + " : " + y + " L/100km\n" + value1 + " litres "+ value2 + " km\n";
      SharedPreferences preferences = PreferenceManager.getDefaultSharedPreferences(context);
      if (data != "" ) {
      String fileName = getResources().getString(R.string.fileName);
      String fileDir = ""+ preferences.getString("login", "") + "."+ preferences.getString("marque", "") + ".";
      myIO.WriteSettings(context, fileDir + fileName, data);
      data = "";
                                }

                     }
                     else 
                     {
                     Toast.makeText(carburant.this, "Veuillez vérifier les deux champs", Toast.LENGTH_LONG).show();
                     }
     }

});

Logcat: 03-16 20:52:59.080: ERROR/AndroidRuntime(1019): FATAL EXCEPTION: main 03-16 20:52:59.080: ERROR/AndroidRuntime(1019): java.lang.NullPointerException 03-16 20:52:59.080: ERROR/AndroidRuntime(1019): at android.preference.PreferenceManager.getDefaultSharedPreferencesName(PreferenceManager.java:353) 03-16 20:52:59.080: ERROR/AndroidRuntime(1019): at android.preference.PreferenceManager.getDefaultSharedPreferences(PreferenceManager.java:348) 03-16 20:52:59.080: ERROR/AndroidRuntime(1019): at carburant.android.com.carburant$1.onClick(carburant.java:118) 03-16 20:52:59.080: ERROR/AndroidRuntime(1019): at android.view.View.performClick(View.java:2485) 03-16 20:52:59.080: ERROR/AndroidRuntime(1019): at android.view.View$PerformClick.run(View.开发者_运维百科java:9080) 03-16 20:52:59.080: ERROR/AndroidRuntime(1019): at android.os.Handler.handleCallback(Handler.java:587) 03-16 20:52:59.080: ERROR/AndroidRuntime(1019): at android.os.Handler.dispatchMessage(Handler.java:92) 03-16 20:52:59.080: ERROR/AndroidRuntime(1019): at android.os.Looper.loop(Looper.java:123) 03-16 20:52:59.080: ERROR/AndroidRuntime(1019): at android.app.ActivityThread.main(ActivityThread.java:3683) 03-16 20:52:59.080: ERROR/AndroidRuntime(1019): at java.lang.reflect.Method.invokeNative(Native Method) 03-16 20:52:59.080: ERROR/AndroidRuntime(1019): at java.lang.reflect.Method.invoke(Method.java:507) 03-16 20:52:59.080: ERROR/AndroidRuntime(1019): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839) 03-16 20:52:59.080: ERROR/AndroidRuntime(1019): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597) 03-16 20:52:59.080: ERROR/AndroidRuntime(1019): at dalvik.system.NativeStart.main(Native Method) 03-16 20:52:59.080: WARN/ActivityManager(68): Force finishing activity carburant.android.com/.carburant 03-16 20:52:59.854: WARN/ActivityManager(68): Activity pause timeout for HistoryRecord{406e89b8 carburant.android.com/.carburant}

Any help please ? Thanks :) .

According to your stack trace and extra information, the error is a null pointer at the line

SharedPreferences preferences = PreferenceManager.getDefaultSharedPreferences(context); 

I suspect that the context parameter is null. Is context a variable name which you've declared and assigned to a context already?

EDIT:

you need to assign a Context to your context variable. Just declaring it as a global variable without assigning it makes it null.

Please try this:

private Context context = getApplicationContext();

Well, I forgot to add this :

context = getApplicationContext();