Throws NullPointerException while retrieving data from SharedPreferences

Question

First of all, sorry for my bad English !

I have an Activity ( say MainActivity.java ) and a class ( say AsynkTaskClass.java ).

AsynkTaskClass.java contains an AsynkTask . I am calling this AsynkTask from MainActivity.java .

Before calling this AsynkTask , I am saving some data using SharedPreferences .

From the onPostExecute function of AsynkTask , I am calling a normal function ( say displaySharedData() )which defined in MainActivity.java . There I am trying to retrieve the data I stored using SharedPreferences . Then It shows a NullPointerException . But I can access the Shared data from anywhere other than this displaySharedData() function.

This is the Log Output:

W/dalvikvm(3896): threadid=1: thread exiting with uncaught exception
E/AndroidRuntime(3896): FATAL EXCEPTION: main
E/AndroidRuntime(3896): java.lang.NullPointerExceptionE/AndroidRuntime(3896):   at com.samApp.project.samApp.ui.MainActivity.displaySharedData(MainActivity.java:52)
E/AndroidRuntime(3896):     at com.samApp.project.samApp.s3.AsynkTaskClass$UploadImage.onPostExecute(AsynkTaskClass.java:264)
E/AndroidRuntime(3896):     at com.samApp.project.samApp.s3.AsynkTaskClass$UploadImage.onPostExecute(AsynkTaskClass.java:1)
E/AndroidRuntime(3896):     at android.os.AsyncTask.finish(AsyncTask.java:417)
E/AndroidRuntime(3896):     at android.os.AsyncTask.access$300(AsyncTask.java:127)
E/AndroidRuntime(3896):     at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:429)
E/AndroidRuntime(3896):     at android.os.Handler.dispatchMessage(Handler.java:99)
E/AndroidRuntime(3896):     at android.os.Looper.loop(Looper.java:130)
E/AndroidRuntime(3896):     at android.app.ActivityThread.main(ActivityThread.java:3683)
E/AndroidRuntime(3896):     at java.lang.reflect.Method.invokeNative(Native Method)
E/AndroidRuntime(3896):     at java.lang.reflect.Method.invoke(Method.java:507)
E/AndroidRuntime(3896):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
E/AndroidRuntime(3896):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
E/AndroidRuntime(3896):     at dalvik.system.NativeStart.main(Native Method)

MainActivity.java

public class MainActivity extends Activity implements TextView.OnEditorActionListener {
    public SharedPreferences myData;
    public String mySharedData="com.samApp.project.samApp.ui";
    public SharedPreferences.Editor editor;

protected void onCreate(Bundle savedBundle) {
super.onCreate(savedBundle);
    myData = PreferenceManager.getDefaultSharedPreferences(this);
editor = myData.edit();
editor.commit();


attachfileButton.setOnClickListener(new View.OnClickListener() {
       @Override
        public void onClick(View v) {
            Intent intent = new Intent(Intent.ACTION_GET_CONTENT);
            intent.setType("image/*");
            startActivityForResult(intent, PHOTO_SELECTED);

        }
    });

    }

    protected void onActivityResult(int requestCode, int resultCode,
            Intent imageReturnedIntent) {
        super.onActivityResult(requestCode, resultCode, imageReturnedIntent);

        switch (requestCode) {
        case PHOTO_SELECTED:
            if (resultCode == RESULT_OK) {

                Uri selectedImage = imageReturnedIntent.getData();
                Log.d("URIsssss", String.valueOf(selectedImage));
                AsynkTaskClass obj = new AsynkTaskClass ();
                AsynkTaskClass.s3context=this;
                obj.new UploadImage().execute(selectedImage);

            }
        }
    }
}
 protected void onResume() {
    super.onResume();
    editor.putString("name", "MeSSi");
    editor.commit();

    }


public void displaySharedData()
    {  

        Log.d("SharedData",myData.getString("name","")); // line No : 52
    }
}

AsynkTaskClass.java

public class AsynkTaskClass {

MainActivity mainActvtyObj = new MainActivity();
public static Context s3context;


protected void onPostExecute(S3TaskResult result) {
            if(dialog != null)
                dialog.dismiss();

            if (result.getErrorMessage() != null) {

                displayErrorAlert(
                        AsynkTaskClass.s3context.getString(R.string.upload_failure_title),
                    result.getErrorMessage());
            }

            mainActvtyObj.displaySharedData();

        }
 }

So, Any Help ??

Answer 1

MainActivity mainActvtyObj = new MainActivity();

Never instantiate activity classes with new .

For example, their lifecycle callbacks such as onCreate() are not run. You init your myData in onCreate() and it remains null in this activity instance.

Instead, pass a reference to your activity object to the async task as a parameter.

Answer 2

Finally I got a working solution.

I don't know whether It is perfect! But it works well..

I replaced the displaySharedData() calling statement ie mainActvtyObj.displaySharedData(); to the following :

((MainActivity) s3context).displaySharedData();

Now It Works Great !!