The contract for Comparator#compare and Comparable#compareTo states that the result is an integer which is < 0 for less than, == 0 for equality and > 0 for greater than. While most implementations return -1, 0 and +1 for those cases respectively, this is not guaranteed. Always comparing to 0 is the safest use of the return value.

  boolean <T> isLessThan(Comparator<T> comparator, T a, T b) {
    // Fragile: it's not guaranteed that `comparator` returns -1 to mean
    // "less than".
    return comparator.compare(a, b) == -1;
  }

  boolean <T> isLessThan(Comparator<T> comparator, T a, T b) {
    return comparator.compare(a, b) < 0;
  }

Even comparisons which are otherwise correct are clearer to other readers of the code if turned into a comparison to 0, e.g.:

  boolean <T> greaterThan(Comparator<T> comparator, T a, T b) {
    return comparator.compare(a, b) >= 1;
  }

  boolean <T> greaterThan(Comparator<T> comparator, T a, T b) {
    return comparator.compare(a, b) > 0;
  }