Android does a great job of providing alternative resources at the right time. However, destroying and re-creating activities on rotation can cause headaches, such as GeoQuiz’s bug of reverting back to the first question when the device is rotated.

To fix this bug, the post-rotation QuizActivity instance needs to know the old value of mCurrentIndex. You need a way to save this data across a runtime configuration change, like rotation. One way to do this is to override the Activity method:

    protected void onSaveInstanceState(Bundle outState)

This method is called before onStop(), except when the user presses the Back button. (Remember, pressing Back tells Android the user is done with the activity, so Android wipes the activity from memory completely and does not make any attempt to save data to re-create it.)

The default implementation of onSaveInstanceState(Bundle) directs all of the activity’s views to save their state as data in the Bundle object. A Bundle is a structure that maps string keys to values of certain limited types.

You have seen this Bundle before. It is passed into onCreate(Bundle):

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    ...
}

When you override onCreate(Bundle), you call onCreate(Bundle) on the activity’s superclass and pass in the bundle you just received. In the superclass implementation, the saved state of the views is retrieved and used to re-create the activity’s view hierarchy.

public class QuizActivity extends AppCompatActivity {

    private static final String TAG = "QuizActivity";
    private static final String KEY_INDEX = "index";

    private Button mTrueButton;

public class QuizActivity extends AppCompatActivity {
    ...
    @Override
    protected void onPause() {
        ...
    }

    @Override
    public void onSaveInstanceState(Bundle savedInstanceState) {
        super.onSaveInstanceState(savedInstanceState);
        Log.i(TAG, "onSaveInstanceState");
        savedInstanceState.putInt(KEY_INDEX, mCurrentIndex);
    }

    @Override
    protected void onStop() {
        ...
    }
    ...
}

public class QuizActivity extends AppCompatActivity {
    ...
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Log.d(TAG, "onCreate(Bundle) called");
        setContentView(R.layout.activity_quiz);

        if (savedInstanceState != null) {
            mCurrentIndex = savedInstanceState.getInt(KEY_INDEX, 0);
        }
        ...
    }
    ...
}