Short answer
Subtraction overflows for large or negative int values, so a comparison like a.value - b.value can return a positive number when a is actually smaller, silently corrupting sort order and breaking TreeMap and TreeSet invariants.
Explanation
The correct approach is Integer.compare(a.value, b.value), or for multiple fields Comparator.comparingInt(...).thenComparing(...). The same overflow trap is why you never hand-roll subtraction comparators; the built-in compare methods and Comparator combinators are safe and clearer.
Common mistakes
- Using subtraction in compareTo, which overflows for large values.
- Not using Integer.compare or Comparator.comparing.