fragment嵌套问题

private void doClick(View v) {
FragmentManager fm = getFragmentManager();
FragmentTransaction transaction = fm.beginTransaction();
switch (v.getId())
{
case R.id.tv_mess:
if (mess == null) {
mess = new FragmentPage_chat_mess();
transaction.replace(R.id.id_content, mess);
}
else if(list == null) {
list = new FragmentPage_chat_list();
}
transaction.show(mess).hide(list);
break;
case R.id.tv_list:
transaction.hide(mess).show(list);
break;
}
transaction.commit();
}
mess和list是两个fragment，我想在主页的第二个fragment里面嵌套两个fragment，这么写哪里有问题？