NullPointerException in Activity

Question

I am somewhat new to Android and am trying to develop my first production app. It is a user defined data storage app that consists of a list activity and an add dialog activity as well as a db adapter. All my "CRUD" (create, read, update, delete) functions seem to work perfectly. But I am running into the following error when I change the orientation of the add dialog when the data has not been saved to the db. In other words, my onSavedInstanceState() function is failing when I change the orientation from portrait to landscape or vice versa.

02-16 11:42:05.321: E/AndroidRuntime(347): java.lang.RuntimeException: Unable to pause activity {net.learn2develop.Homework/net.learn2develop.Homework.HomeworkEditActivity}: java.lang.NullPointerException

here is my code:

public class HomeworkEditActivity extends Activity{
          private Long mRowId;

       // SQLite Db Helper
            private HomeworkDbAdapter mDbHelper ;




// declarations for SQLitedatabase actions
private EditText mSubject;
private EditText mDueText;
private EditText mDescText;



private Button btnSave = null;

/** 
 * Called when the activity 
 *  is first created
 */ 
@Override
protected void onCreate(Bundle savedInstanceState){

    // called to display the reminder edit class
    super.onCreate(savedInstanceState);

    // open the database
    mDbHelper = new HomeworkDbAdapter(this);

    // set the content view by the xml layout
    setContentView(R.layout.homeworkedit);


    // initialize the save button
    btnSave     = (Button) findViewById(R.id.btnSave);

    // initiate the edit text fields
    mSubject = (EditText) findViewById(R.id.txtSubject);
    mDueText  = (EditText) findViewById(R.id.txtDueDate);
    mDescText = (EditText)  findViewById(R.id.txtDesc);


    mRowId = savedInstanceState != null
        ? savedInstanceState.getLong(HomeworkDbAdapter.KEY_ROWID)
        : null;

    // register listeners and set the text of the buttons to their default
    registerButtonListenersAndSetDefaultText();

}
private void setRowIdFromIntent(){
    if(mRowId == null){
        Bundle extras = getIntent().getExtras();
        mRowId = extras != null
        ? extras.getLong(HomeworkDbAdapter.KEY_ROWID)
        : null;
    }
}
@Override
protected void onPause(){

    mDbHelper.close();
    super.onPause();
}

@Override
protected void onResume(){
    super.onResume();
    mDbHelper.open();
    setRowIdFromIntent();

        populateFields();

}

private void registerButtonListenersAndSetDefaultText() {
    btnSave.setOnClickListener(new View.OnClickListener(){
        @Override
        public void onClick (View v){
            mSubject.getText();
            mDescText.getText();
            mDueText.getText();

            saveState();
            setResult(RESULT_OK);
            Toast.makeText(HomeworkEditActivity.this, 
                    getString(R.string.task_saved_message), 
                    Toast.LENGTH_SHORT).show();
            finish();                                               // return to list activity
        }   



            });

}

private void populateFields()  {
    if(mRowId != null){
        Cursor reminder = mDbHelper.fetchHomework(mRowId);
            startManagingCursor(reminder);
            mSubject.setText(reminder.getString(
                         reminder.getColumnIndexOrThrow(HomeworkDbAdapter.KEY_SUBJECT)));
            mDueText.setText(reminder.getString(
                    reminder.getColumnIndexOrThrow(HomeworkDbAdapter.KEY_DUE)));
            mDescText.setText(reminder.getString(
                    reminder.getColumnIndexOrThrow(HomeworkDbAdapter.KEY_DESC)));
    }
    if(mRowId == null){
        onCreate(null);
    }
}

@Override
protected void onSaveInstanceState(Bundle outState){

    try{
    outState.putLong(HomeworkDbAdapter.KEY_ROWID, mRowId);
    }catch(NullPointerException npe){
        npe.printStackTrace();
    }
    super.onSaveInstanceState(outState);
}



private void saveState() {
    // add the save of the text reference here
    String subject = mSubject.getText().toString();
    String due_date = mDueText.getText().toString();
    String desc  = mDescText.getText().toString();



    if(mRowId == null){ 
        long id = mDbHelper.createHomework(subject, due_date, desc);
        if(id > 0){
            mRowId = id;
        }
    }else{
        mDbHelper.updateHomework(mRowId, subject, due_date, desc);
    }
}

}

Answer 1

Try switching mDbHelper.close() and super.onPause()

Like so:

@Override
protected void onPause(){
    super.onPause();
    mDbHelper.close();
}

99% of the time you should call the super.onPause() method first. Calling the super.onPause() does a lot of things behind the scenes. I have had rare occasions where I needed to actually remove the super method call altogether. It doesn't look like this is one of them though.

See here for more details:

http://developer.android.com/training/basics/activity-lifecycle/pausing.html#Pause

Answer 2

The fact that the NullPointerException message contains "Unable to pause activity" means the issue is with your onPause(..) override. I assume this happens when mDbHelper is null, although looking at your code, I can't see why it would be.