Android Tutorial => Saving and Restoring Activity State

Example

As your activity begins to stop, the system calls onSaveInstanceState() so your activity can save state information with a collection of key-value pairs. The default implementation of this method automatically saves information about the state of the activity's view hierarchy, such as the text in an EditText widget or the scroll position of a ListView.

To save additional state information for your activity, you must implement onSaveInstanceState() and add key-value pairs to the Bundle object. For example:

public class MainActivity extends Activity {
    static final String SOME_VALUE = "int_value";
    static final String SOME_OTHER_VALUE = "string_value";

    @Override
    protected void onSaveInstanceState(Bundle savedInstanceState) {
        // Save custom values into the bundle
        savedInstanceState.putInt(SOME_VALUE, someIntValue);
        savedInstanceState.putString(SOME_OTHER_VALUE, someStringValue);
        // Always call the superclass so it can save the view hierarchy state
        super.onSaveInstanceState(savedInstanceState);
    }
}

The system will call that method before an Activity is destroyed. Then later the system will call onRestoreInstanceState where we can restore state from the bundle:

@Override
protected void onRestoreInstanceState(Bundle savedInstanceState) {
    // Always call the superclass so it can restore the view hierarchy
    super.onRestoreInstanceState(savedInstanceState);
    // Restore state members from saved instance
    someIntValue = savedInstanceState.getInt(SOME_VALUE);
    someStringValue = savedInstanceState.getString(SOME_OTHER_VALUE);
}

Instance state can also be restored in the standard Activity#onCreate method but it is convenient to do it in onRestoreInstanceState which ensures all of the initialization has been done and allows subclasses to decide whether to use the default implementation. Read this stackoverflow post for details.

Note that onSaveInstanceState and onRestoreInstanceState are not guaranteed to be called together. Android invokes onSaveInstanceState() when there's a chance the activity might be destroyed. However, there are cases where onSaveInstanceState is called but the activity is not destroyed and as a result onRestoreInstanceState is not invoked.

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