@Test
public void testDoGetWhenFileNameIsNullShouldWriteErrorToResponse() throws ServletException, IOException {

    //GIVEN
    given(request.getParameter("fileName")).willReturn(null);
    given(response.getWriter()).willReturn(printWriter);
    //WHEN
    underTest.doGet(request, response);
    //THEN
    verify(printWriter).write(underTest.returnHintMessage());
}