Android Null Pointer Exception for shared Preferences

Question

I am trying to save a integer value in the Shared preferences and load it again when the application is restarted.

I have created a shared preferences in my main activity and trying to save integer value in a another Menu Listener class on menu item click. The Menu listener class is in a different package and the Menu Listener class extends the Main Activity.

But I am getting a Null pointer exception when I try to do that.

I have also gone through this link!. But it doesn't seem to be helping me.

Here is my code:

Main Activity:

    protected SharedPreferences mPrefs;
protected SharedPreferences.Editor mEditor;
protected Context  main_activityclass_context; //EDIT
public static int count=0;

@Override
protected void onCreate(final Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);


     mPrefs = this.getSharedPreferences("THEME", 0);
     mEditor = mPrefs.edit();
    }

Menu Listener class:

    case  mainActivity.MENU_ITEM_THEME_DARK:
    {

        mainActivity.count=1;

        mPrefs =main_activityclass_context.getSharedPreferences("THEME", 0); //EDIT
        mEditor = mPrefs.edit();
        mEditor.putInt("Theme_count", mainActivity.count).commit();


    break;
    }

I am getting a Null pointer exception on the lines in the Menu Listener :

       mPrefs = getSharedPreferences("THEME", 0);
        mEditor = mPrefs.edit();    

My Logcat:

      08-03 10:54:03.650: E/AndroidRuntime(31258): FATAL EXCEPTION: main
      08-03 10:54:03.650: E/AndroidRuntime(31258): java.lang.NullPointerException
      08-03 10:54:03.650: E/AndroidRuntime(31258):  at               android.content.ContextWrapper.getSharedPreferences(ContextWrapper.java:153)
      08-03 10:54:03.650: E/AndroidRuntime(31258):  at de.exb.ptpt.tablet.menu.TabletMenuListener.onMenuItemClick(TabletMenuListener.java:215)
      08-03 10:54:03.650: E/AndroidRuntime(31258):  at com.android.internal.view.menu.MenuItemImpl.invoke(MenuItemImpl.java:144)
       08-03 10:54:03.650: E/AndroidRuntime(31258):     at com.android.internal.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:874)
       08-03 10:54:03.650: E/AndroidRuntime(31258):     at com.android.internal.view.menu.MenuPopupHelper.onItemClick(MenuPopupHelper.java:156)
       08-03 10:54:03.650: E/AndroidRuntime(31258):     at android.widget.AdapterView.performItemClick(AdapterView.java:292)
       08-03 10:54:03.650: E/AndroidRuntime(31258):     at android.widget.AbsListView.performItemClick(AbsListView.java:1058)
      08-03 10:54:03.650: E/AndroidRuntime(31258):  at android.widget.AbsListView$PerformClick.run(AbsListView.java:2514)
      08-03 10:54:03.650: E/AndroidRuntime(31258):  at android.widget.AbsListView$1.run(AbsListView.java:3168)
      08-03 10:54:03.650: E/AndroidRuntime(31258):  at android.os.Handler.handleCallback(Handler.java:605)
      08-03 10:54:03.650: E/AndroidRuntime(31258):  at android.os.Handler.dispatchMessage(Handler.java:92)
      08-03 10:54:03.650: E/AndroidRuntime(31258):  at android.os.Looper.loop(Looper.java:137)
      08-03 10:54:03.650: E/AndroidRuntime(31258):  at android.app.ActivityThread.main(ActivityThread.java:4424)
      08-03 10:54:03.650: E/AndroidRuntime(31258):  at java.lang.reflect.Method.invokeNative(Native Method)
      08-03 10:54:03.650: E/AndroidRuntime(31258):  at java.lang.reflect.Method.invoke(Method.java:511)
      08-03 10:54:03.650: E/AndroidRuntime(31258):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
      08-03 10:54:03.650: E/AndroidRuntime(31258):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
      08-03 10:54:03.650: E/AndroidRuntime(31258):  at dalvik.system.NativeStart.main(Native Method)

Can anyone please help me?

Thank You.

Solution 1

One of the good ways is, you create a helper class to declare, save and retrive preferences. You can refer following code sample:

public class PreferencesHelper {

private SharedPreferences sharedPreferences;
private Editor editor;

public PreferencesHelper(Context context) {
this.sharedPreferences = getPreferences(MODE_PRIVATE);    
this.editor = sharedPreferences.edit(); }

public String GetPreferences(String key) {
    return sharedPreferences.getString(key, "");
}

public void SavePreferences(String key, String value) {
editor.putString(key, value);    
editor.commit();  
}
}  

In onCreate event of an activity, you can create of object of PreferencesHelper and get all saved preferences like:

Prefs = new PreferencesHelper(getApplicationContext());  

Prefs.GetPreferences("YOURARGUMENT");  

and you can save the preferences like:

Prefs.SavePreferences(key, value);  

Hope this will help you.

Solution 2

Write This code outside the onCreate method.

mPrefs = this.getSharedPreferences("THEME", 0);
     mEditor = mPrefs.edit();

mostly because of this it generating nullpointer exception.