Android 再按一次返回键退出 代码

代码是网上找的，很有用

   private boolean isfinished = false;    @Override    public void onBackPressed() {        //super.onBackPressed();        if (isfinished)        {            ActivityCollector.finishAll();        } else        {            Toast.makeText(this, "再按一次返回键退出", Toast.LENGTH_SHORT).show();            new Thread()            {                public void run()                {                    isfinished = true;                    try                    {                        Thread.sleep(2000);                        isfinished = false;                    } catch (InterruptedException e)                    {                        e.printStackTrace();                    }                };            }.start();        }    }