@Override    public boolean onKeyDown(int keyCode, KeyEvent event) {        if (keyCode == KeyEvent.KEYCODE_BACK) {            exit();            return false;        }        return super.onKeyDown(keyCode, event);    }    private long exitTime = 0;    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);        }    }

按二次返回键退出