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I am using SharedPreferences to get some saved values. I only get a null pointer exception on some devices. When I test the code in my own device I don't get any problem. But some of my user get the error.

Here is my class to get the value of SharedPreference:

public class SavingData {
     public static final String PREFS_NAME = "MyPrefsFile";
     public static MainActivity mainActivity;
public static int getRestTime() {
    // Restore preferences
    SharedPreferences settings = mainActivity.getSharedPreferences(PREFS_NAME, 0);
    int restTime = settings.getInt("resttime", defaultRestTime); // 0 is the default
                                                        // value
    return restTime;
In my main class I declare the mainActivity variable.
  SavingData.mainActivity = this;
Here is the error I get:
  
java.lang.RuntimeException: Unable to start activity

  ComponentInfo{com.playsimple.fitnessapp/com.playsimple.fitnessapp.ExerciseStartActivity}:
  java.lang.NullPointerException at
  android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2198)
  android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2257)
  at android.app.ActivityThread.access$800(ActivityThread.java:139) at
  android.app.ActivityThread$H.handleMessage(ActivityThread.java:1210)
  at android.os.Handler.dispatchMessage(Handler.java:102) at
  android.os.Looper.loop(Looper.java:136) at
  android.app.ActivityThread.main(ActivityThread.java:5086) at
  java.lang.reflect.Method.invokeNative(Native Method) at
  java.lang.reflect.Method.invoke(Method.java:515) at
  com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785)
  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601) at
  dalvik.system.NativeStart.main(Native Method) Caused by:
  java.lang.NullPointerException at
  com.playsimple.fitnessapp.data.SavingData.getDifficulty(SavingData.java:91)
  com.playsimple.fitnessapp.ExerciseStartActivity.initExercise(ExerciseStartActivity.java:69)
  com.playsimple.fitnessapp.ExerciseStartActivity.onCreate(ExerciseStartActivity.java:64)
  at android.app.Activity.performCreate(Activity.java:5248) at
  android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1110)
  android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2162)
  ComponentInfo{com.playsimple.fitnessapp/com.playsimple.fitnessapp.ExerciseStartActivity}:
  java.lang.NullPointerException at
  android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1647)
  android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1663)
  at android.app.ActivityThread.access$1500(ActivityThread.java:117) at
  android.app.ActivityThread$H.handleMessage(ActivityThread.java:931) at
  android.os.Handler.dispatchMessage(Handler.java:99) at
  android.os.Looper.loop(Looper.java:130) at
  android.app.ActivityThread.main(ActivityThread.java:3683) at
  java.lang.reflect.Method.invokeNative(Native Method) at
  java.lang.reflect.Method.invoke(Method.java:507) at
  com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:889)
  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:647) at
  dalvik.system.NativeStart.main(Native Method) Caused by:
  java.lang.NullPointerException at
  com.playsimple.fitnessapp.data.SavingData.getDifficulty(SavingData.java:91)
  com.playsimple.fitnessapp.ExerciseStartActivity.initExercise(ExerciseStartActivity.java:69)
  com.playsimple.fitnessapp.ExerciseStartActivity.onCreate(ExerciseStartActivity.java:64)
  android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
  android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1611)
You are creating an instance of your main activity here:
public static MainActivity mainActivity;
and then calling shared Prefs.. using that. Get the context object instead from your main activity and then call it.
Also use this including MODE_PRIVATE argument:
SharedPreferences userDetails = context.getSharedPreferences("userdetails", MODE_PRIVATE);
See this detailed answer.
Also see the docs for [Context getSharedPreferences](http://developer.android.com/reference/android/content/Context.html#getSharedPreferences(java.lang.String, int) )
                from whereever you are calling the function, also get the context object, getRestTime(Context context), then context.getSharedPreferences(...). About the static thing i do not know if it should be used or not. But variable mainActivity seems to be the Problem.
– Mohammed Ali
                Nov 12, 2014 at 19:02
                When I do mainActivity.MODE_PRIVATE I get this warning: "The static field Context.MODE_PRIVATE should be accessed in a static way". Is it safe to do this Context.MODE_PRIVATE?
– user3730218
                Nov 12, 2014 at 19:13
                What do you mean by calling from another Activity? Do you mean I should use that instead of SavingData.getRestTime();
– user3730218
                Nov 12, 2014 at 18:50
                savingData is not an activity in the code that you posted. Since it is a class, using getSharedPreferences with the preference name and  mode should work.
– vembutech
                Nov 12, 2014 at 18:51
        
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