Nak.  This is now public, and mutable, and a singleton.

Change the enum to be:

  public enum SupportedAlgorithm {
    MYERS, HISTOGRAM
  }

  public static DiffAlgorithm getAlgorithm(SupportedAlgorithm alg) {
    switch(alg) {
    case MYERS:
      return MyersDiff.INSTANCE;
    case HISTOGRAM:
      return new HistogramDiff();
    }
  }