How to use a button in a fragment

To notify Activity so that it has no more coupling, use the Observer / Listener pattern.

1. Create an interface that has all the events that can occur in Fragment and that depend on other class (es).

public interface FragmentListener {
    public void metodo(); // Metodo que sera chamado
}

2. Implement the interface in your Activity or any other class you want to use:

public SuaActivity extends Activity implements FragmentListener {
    @Override
    public void metodo() {
        // Implementação
    }
}

3. Create a property to save an object that implements this interface in its Fragment :

public class SeuFragment extends Fragment {
    private FragmentListener mListener;

    // Getters e Setters omitidos
}

4. Use the interface when the button is clicked:

btAnterior.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {
        if(mListener != null) mListener.metodo();
    }
});

5. Pass the implementation of this interface to Fragment :

// Se a Activity implementar, voce pode usar o metodo onAttach do Fragment para setar
public void onAttach(Activity a) {
    super.onAttach(a);

    if(a instanceof FragmentListener) {
        mListener = (FragmentListener) a;
    }
}

// Ou com um setter
public void setFragmentListener(FragmentListener listener) {
    mListener = listener;
}

6. Release the interface instance when Fragment is destroyed, thus avoiding Memory Leak:

public void onDestroy() {
    super.onDestroy();
    mListener = null;
}

// ou
public void onDestroyView() {
    super.onDestroyView();
    mListener = null;
}

// ou quando for dettached
public void onDetach() {
    super.onDetach();
    mListener = null;
}

you can expose btPrevious as follows. In its fragment you can write something of the genre:

private View view;
private TextView btAnterior;

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    ...
    this.view = inflater.inflate(...);
    this.btAnterior = (Button)findViewById(R.id.anterior);
    ...
}

public TextView getBtAnterior() {
    return this.btAnterior;
}

In your MainActivity, you can access btPrevious as follows:

private SparseArray<Fragment> fragments = new SparseArray<Fragment>();
private ListItensFragment fragmentListItens;

@Override
public void onAttachFragment(Fragment fragment) {
    super.onAttachFragment(fragment);
    this.fragments.put(fragment.getId(), fragment);
}

@Override
protected void onCreate(Bundle savedInstanceState) {
    ...
    this.fragmentListItens = (ListItensFragment)this.fragments.get(R.id.fragmentListItens);
    this.fragmentListItens.getBtAnterior().setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Log.e("TAG","ta aki");
        }
    });
    ...
}

Just to update if anyone else needs it. I solved this problem by doing so:

Within the Fragment class (before):

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the layout for this fragment
    return inflater.inflate(R.layout.fragment_dom, container, false);
}

Within the Fragment class (After):

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    View view = inflater.inflate(R.layout.fragment_dom, container, false);
    listView = (ListView) view.findViewById(R.id.ID_ListView_test);
    // Inflate the layout for this fragment
    return view;
}

I get what is returned by default from the fragment class and set it to a "view" variable. And I use this variable to access the methods I would have in an activity class, such as findViewById. In the case of the Button, you do the same thing I did with the listview and then use the normal bt.setOnClickListner as if you were in an activity. I do not know if it's the right one, but that's what worked for me.