Android点击回退键两次退出应用

public class MainActivity extends Activity {
private long exitTime = 0;

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

@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
    if (keyCode == KeyEvent.KEYCODE_BACK) {
        exit();
        return false;
    }
    return super.onKeyDown(keyCode, event);
}

public void exit() {
    if ((System.currentTimeMillis() - exitTime) > 2000) {
        Toast.makeText(getApplicationContext(), "再按一次退出程序",
                Toast.LENGTH_SHORT).show();
        exitTime = System.currentTimeMillis();
    } else {
        finish();
        System.exit(0);
    }
}
}