code: public ClassFilter build() throws IllegalStateException {
          if(filters.isEmpty()) {
              throw new IllegalStateException("No filters set.");
          }
          if(filters.size() == 1) {
              return filters.get(0);
          }
          ClassFilter[] classFilters = new ClassFilter[filters.size()];
          filters.toArray(classFilters);
          return new ClassFilterList(classFilters);
      }
docs: Builds the filter and returns the result.
  
  @return the ClassFilter built.
  
  @throws IllegalStateException if no filter has been added.