public class Main extends Fragment implements SendStringEvent{
    Button addButton;
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.teacher_category, container, false);
        addButton = (Button) rootView.findViewById(R.id.add_category_button);
        addButton.setOnClickListener(new OnClickListener() {
            @Override
            public void onClick(View arg0) {
                TEACHER_AddCategory teacherAddCategory = new TEACHER_AddCategory();
                teacherAddCategory.show(getFragmentManager(), "TEACHER_AddCategory");
            }
        });
       return rootView;
    }

    //This method from interface SendStringEvent
    @Override
    public void eventAfterMessageClosed(String message) {
        //I get error when executing this
        addButton.setText(message);
    }
}

public class TEACHER_AddCategory extends DialogFragment {
    Button OKButton;
    EditText editText;
    private SendStringEvent eventMessageClosed;
    Fragment fragment = new Main();

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View view = inflater.inflate(seamolec.ujianonline.R.layout.teacher_add_category, container);
        OKButton = (Button) view.findViewById(seamolec.ujianonline.R.id.ok_button);
        try {
            eventMessageClosed = (SendStringEvent) fragment;
        }
        catch(ClassCastException e) {
            e.printStackTrace();
        }
        OKButton.setOnClickListener(new OnClickListener() {
            @Override
            public void onClick(View arg0) {
                    eventMessageClosed.eventAfterMessageClosed("test");
                    dismiss();
                }
            }
        });
        return view;
    }

}

public interface SendStringEvent {
    void eventAfterMessageClosed(String message);
}

try {
        eventMessageClosed = (SendStringEvent) fragment;
    }
    catch(ClassCastException e) {
        e.printStackTrace();
    }

Fragment fr = getFragmentManager().findFragmentById(R.id.where_fragment_is_loaded) // Change as per you layout
if(fr != null && fr instanceOf SendStringEvent)
    eventMessageClosed = (SendStringEvent) fr;

eventMessageClosed.eventAfterMessageClosed("test");

if(eventMessageClosed != null)
    eventMessageClosed.eventAfterMessageClosed("test");