MainActivity.java

public class MainActivity extends ActionBarActivity {

    private static final String QUANTITY = "quantity";

    private int quantity = 1;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        if (savedInstanceState != null) {
            quantity = savedInstanceState.getInt(QUANTITY);
        }
    }

    @Override
    protected void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        outState.putInt(QUANTITY, quantity);
    }
}