CompareToZero
The result of #compareTo or #compare should only be compared to 0. It is an implementation detail whether a given type returns strictly the values {-1, 0, +1} or others.

Severity
ERROR

The problem

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 directly against 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;
  }

When comparing against 0, 0 should always be on the right-hand side of the operator so that a.compareTo(b) <op> 0 mirrors the relationship a <op> b:

  boolean <T> greaterThan(Comparable<T> a, T b) {
    // Confusing: `<` is used to check that `a` is greater than `b`.
    return 0 < a.compareTo(b);
  }

  boolean <T> greaterThan(Comparable<T> a, T b) {
    return a.compareTo(b) > 0;
  }

Similarly, when switching on the result of compare or compareTo in a switch statement or expression, the selector must be wrapped in Integer.signum() to normalize the result to -1, 0, or 1:

  switch (comparator.compare(a, b)) {
    case -1 -> ...
    case 0 -> ...
    default -> ...
  }

  switch (signum(comparator.compare(a, b))) {
    case -1 -> ...
    case 0 -> ...
    default -> ...
  }

Suppression

Suppress false positives by adding the suppression annotation @SuppressWarnings("CompareToZero") to the enclosing element.