/**	 * 菜单、返回键响应	 */	@Override	public boolean onKeyDown(int keyCode, KeyEvent event) {		if (keyCode == KeyEvent.KEYCODE_BACK) {			exitBy2Click();// 调用双击退出函数		}		return false;	}	private long exitTime = 0;	/**	 * 双击退出:利用计算时间差实现	 */	public void exitBy2Click() {		if ((System.currentTimeMillis() - exitTime) > 2000) {			Toast.makeText(this, "再按一次退出程序", Toast.LENGTH_SHORT).show();			exitTime = System.currentTimeMillis();		} else {			finish();		}	}