Android: Fragment cannot get activity

I have an activity which does a fragment transaction

DetailFragment newFragment = new DetailFragment();
transaction.replace(R.id.mylist, newFragment);
transaction.addToBackStack(null);
transaction.commit();

that works fine. Now I know within my activity a dynamic string I need to replace in the layout I have in newFragment. I thought that I can call after transaction.commit() just something like

newFragment.setMyString("my dynamic value");

And in the newFragment.java I have

public void setMyString(String s)
{
 TextView tv = (TextView) getActivity().findViewById(R.id.myobject);
 tv.setText(s);
}

The point is that getActivity() returns null. How can I get the context I need to find my layout elements?

Edit:

I try to follow the route using a bundle as this seems to be the cleanest way. So I have changed my code:

Bundle b = new Bundle();
b.putString("text", "my dynamic Text");
DetailFragment newFragment = new DetailFragment();
transaction.replace(R.id.mylist, newFragment);
transaction.addToBackStack(null);
transaction.commit();

My fragment onCreateView looks like this:

public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) 
{
   View v = inflater.inflate(R.layout.mylayout, container, false);
   TextView t = (TextView) v.findViewById(R.id.texttobereplaced);
   t.setText(savedInstanceState.getString("text");
}

It seems that savedInstranceState is empty. Where should I find my bundle?

Edit2:

missed the getArguments() in the reply. is working now.