code: public double getFitness(String candidate,
                               List<? extends String> population)
      {
          int errors = 0;
          for (int i = 0; i < candidate.length(); i++)
          {
              if (candidate.charAt(i) != targetString.charAt(i))
              {
                  ++errors;
              }
          }
          return errors;
      }
docs: Assigns one "penalty point" for every character in the candidate
  string that differs from the corresponding position in the target
  string.
  @param candidate The evolved string to evaluate.
  @param population {@inheritDoc}
  @return The fitness score (how many characters are wrong) of the
  specified string.