How to use onSaveInstanceState() and onRestoreInstanceState()?

When your activity is recreated after it was previously destroyed, you
can recover your saved state from the Bundle that the system passes
your activity. Both the onCreate() and onRestoreInstanceState()
callback methods receive the same Bundle that contains the instance
state information.

Because the onCreate() method is called whether the system is creating
a new instance of your activity or recreating a previous one, you must
check whether the state Bundle is null before you attempt to read it.
If it is null, then the system is creating a new instance of the
activity, instead of restoring a previous one that was destroyed.

static final String STATE_USER = "user";
private String mUser;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    // Check whether we're recreating a previously destroyed instance
    if (savedInstanceState != null) {
        // Restore value of members from saved state
        mUser = savedInstanceState.getString(STATE_USER);
    } else {
        // Probably initialize members with default values for a new instance
        mUser = "NewUser";
    }
}

@Override
public void onSaveInstanceState(Bundle savedInstanceState) {
    savedInstanceState.putString(STATE_USER, mUser);
    // Always call the superclass so it can save the view hierarchy state
    super.onSaveInstanceState(savedInstanceState);
}

http://developer.android.com/training/basics/activity-lifecycle/recreating.html