双击退出应用

 @Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {
        if (keyCode == KeyEvent.KEYCODE_BACK) {
            //判断当前界面是否在主界面 ，如果在主界面 弹出土司  ，不是在主界面 返回上一级
            Fragment ft = (Fragment) getSupportFragmentManager().findFragmentById(R.id.container);//realtabcontent为你的容器，装载fragment的
            if (ft instanceof HomePageFragment || ft instanceof CalendarFragment || ft instanceof HandInHomeworkFragment || ft instanceof ArtSceneFragment || ft instanceof MyFragment) {
                if (0 == mExitTime) {
                    Toast.makeText(this, "再按一次退出程序", Toast.LENGTH_SHORT).show();
                    mExitTime = System.currentTimeMillis();
                } else if ((System.currentTimeMillis() - mExitTime) > 2000) {
                    Toast.makeText(this, "再按一次退出程序", Toast.LENGTH_SHORT).show();
                    mExitTime = System.currentTimeMillis();
                } else {
                    finish();
                }
                return true;
            }
        }
        return super.onKeyDown(keyCode, event);
    }