code: public ScoreDifferencePercentage add(ScoreDifferencePercentage addend) {
          if (percentageLevels.length != addend.getPercentageLevels().length) {
              throw new IllegalStateException("The addend (" + addend + ")'s levelsLength (" +
                      addend.getPercentageLevels().length + ") is different from the base (" +
                      this + ")'s levelsLength (" + percentageLevels.length + ").");
          }
          double[] newPercentageLevels = new double[percentageLevels.length];
          for (int i = 0; i < percentageLevels.length; i++) {
              newPercentageLevels[i] = percentageLevels[i] + addend.percentageLevels[i];
          }
          return new ScoreDifferencePercentage(newPercentageLevels);
      }
docs: ************************************************************************