NullPointerException on calling a public method from another activity

I have an Activity SaveData.class with a public method addEvent() use to add some information in a DataBase table as follows:

public class SaveData extends Activity implements OnClickListener {

public SoftCopyDatabase dB ;
public static String FILE_NAME;
String _subject, _topic,_lecturenumber,_date;


protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.save);
    View add = findViewById(R.id.saveSave);
    add.setOnClickListener(this);
    View home = findViewById(R.id.saveBack);
    home.setOnClickListener(this);

}public void onStart() {
    su开发者_运维百科per.onStart();
    dB = new SoftCopyDatabase(this);
}

public void onStop() {
    super.onStop();
    if (dB.getReadableDatabase().isOpen()) {
        //dB.close();
    }
}
public void onDestroy() {
    super.onDestroy();
    if (dB.getReadableDatabase().isOpen()) {
        dB.close();
    }
}

public void onClick(View v) {
    switch (v.getId()) {
    case R.id.saveBack:
        Intent i = new Intent(this, OpenScreen.class);
        startActivity(i);
        break;
    case R.id.saveSave:

        EditText subject = (EditText) findViewById(R.id.subjectid);
        EditText topic = (EditText) findViewById(R.id.topicid);
        EditText lecturenumber = (EditText) findViewById(R.id.lecturenumberid);
        EditText date = (EditText) findViewById(R.id.dateid);
         _subject = ((TextView) subject).getText().toString();
         _topic = ((TextView) topic).getText().toString();
         _lecturenumber = ((TextView) lecturenumber).getText()
                .toString();
         _date = ((TextView) date).getText().toString();
        FILE_NAME = _subject + _topic + _lecturenumber;
        //addEvent();

        Intent j = new Intent(this, LectureNoting.class);
        startActivity(j);

        break;

    }
}

public void addEvent() {


    ContentValues values = new ContentValues();
    values.put(SUBJECT, _subject);
    values.put(TOPIC, _topic);
    values.put(LECTURENUMBER, _lecturenumber);
    values.put(DATE, _date);
    values.put(_DATA, FILE_NAME + ".png");
    dB.getWritableDatabase().insertOrThrow(TABLE_NAME, null, values);
}

}

Another activity LectureNoting.class is used to save Drawings on the disk and updates the entry in Database Table as follows:

public class LectureNoting extends Activity implements View.OnTouchListener{

private SaveData sD=new SaveData();
private File directory = new File("/sdcard/SoftCopy"); 
//...remaining code
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.drawing_activity);
    }
//...remaining code
public void onClick(View view){
    switch (view.getId()){
case R.id.saveBtn:
addEvent();
            final Activity currentActivity  = this;
    Handler saveHandler = new Handler(){
        @Override
        public void handleMessage(Message msg) {

            Toast.makeText(currentActivity, "Lecture Saved",   Toast.LENGTH_SHORT).show();

        }
    } ;
   new ExportBitmapToFile(this,saveHandler, softCopyInterface.getBitmap()).execute();
        break;
//...remaining code
}
private class ExportBitmapToFile extends AsyncTask<Intent,Void,Boolean> {
    private Context mContext;
    private Handler mHandler;
    private Bitmap nBitmap;

    public ExportBitmapToFile(Context context,Handler handler,Bitmap bitmap) {
        mContext = context;
        nBitmap = bitmap;
        mHandler = handler;
    }

    @Override
    protected Boolean doInBackground(Intent... arg0) {

        try {
            if (!directory.exists()) {
                directory.mkdirs();
            }

            final FileOutputStream out = new FileOutputStream(new File(directory + "/"+SaveData.FILE_NAME+".png"));

            nBitmap.compress(Bitmap.CompressFormat.PNG, 90, out);
            out.flush();
            out.close();

            return true;
        }

        catch (FileNotFoundException e)
        {
            e.printStackTrace();
        }
        catch (IOException e)
        {
            e.printStackTrace();
        }
        return false;
    }


    @Override
    protected void onPostExecute(Boolean bool) {
        super.onPostExecute(bool);
        if ( bool ){
            mHandler.sendEmptyMessage(1);
        }
    }
 }
}

And I am receiving following error:

       Unable to start activity componentInfo(com.ned.LectureNoting):NullPointerException

At the addEvent(), used in the onClick method of LectureNoting.

Kindly tell me where I am going wrong. One point I would like to mention is if addEvent() is called from the same activity in which it was defined, this error does not appear.

Couple of things:

1. Logcat should be giving more information about the error. You may have to scroll down a bit to see the source of the problem in your code, but there should be more info.
2. you shouldnt be defining public methods inside of classes that extend Activity to be used by other classes. If you want to expose some database method to multiple activities, then create a separate class for that and then call that method inside of your activity. You said LectureNoting extends Activity. You sure about this? You must have it extending SaveData if you are just calling addEvent() like that.

Either way, DON'T CALL METHODS FROM ONE ACTIVITY INSIDE OF ANOTHER. If you want to expose a method to multiple activities, create it in it's own class with a sensible name related to the group of functions that you expose.