按两次返回键退出app

public boolean onKeyDown(int keyCode, KeyEvent event) {        if (keyCode == KeyEvent.KEYCODE_BACK                && event.getAction() == KeyEvent.ACTION_DOWN) {            if ((System.currentTimeMillis() - exitTime) > 2000) {                Toast.makeText(getApplicationContext(), "再按一次退出程序",                        Toast.LENGTH_SHORT).show();                exitTime = System.currentTimeMillis();            } else {                ActivityStack.create().AppExit(ac);            }        }        return true;    }

app退出请参考博客：activity任务栈