code: public ProgressBarField<T> addProgressBarField(int viewResId,
                                                     IntegerExtractor<T> progressExtractor,
                                                     IntegerExtractor<T> maxProgressExtractor) {
  
          ProgressBarField<T> field =
                  new ProgressBarField<T>(viewResId, progressExtractor, maxProgressExtractor);
  
          mProgressBarFields.add(field);
  
          return field;
      }
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