code: public void setSearchCategory(String searchCategory){
          if (searchCategory != null && getSearchQuery() == null){
              throw new IllegalStateException("SearchQuery must be set before SearchCategory can be set.");
          }
          setParameter(SEARCH_CATEGORY, searchCategory);
      }
docs: Specify a search category with this parameter.  SearchQuery must first be
  set.
  @param searchCategory the search category to set.  A null value will remove this parameter