public class Android extends Fragment {
    @Override

        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                Bundle savedInstanceState) {

            View prime = inflater.inflate(R.layout.android_frag, container, false);

            //I have a spinner with multiple choices here. The Spinner has the id 'convtype'
            Spinner spinner = (Spinner) prime.findViewById(R.id.convtype);
            ArrayAdapter<CharSequence> adapter = ArrayAdapter.createFromResource(getActivity().getBaseContext(),
                    R.array.planets_array, android.R.layout.simple_spinner_item);
            adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
            spinner.setAdapter(adapter);

            return prime;
         }

        private String getGradi(String a) {
            return a+"°";
        }

         //This method is called when I click a button
         public void convertStart(View v) {

             Spinner spinner = (Spinner) v.findViewById(R.id.convtype);
             int indice = spinner.getSelectedItemPosition();
             EditText mEdit = (EditText) v.findViewById(R.id.unita);

             String gradi = getGradi("65");
             mEdit.setText(gradi);

         }

}

<Button
        android:id="@+id/button1"
        style="?android:attr/buttonStyleSmall"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:text="@string/calcola"
        android:onClick="convertStart" />