Why Incorrect Comparator(Java) Does not throw Exception "Comparison method violates general contract"

Question

I think below implementation of a comprator is wrong. It will got give me a sorted order of Strings. But when i use this implementation to sort a List it does not throw the below exception : "java.lang.IllegalArgumentException: Comparison method violates its general contract!"

Question : Why above Exception is not thrown by Collections.sort(list, new WrongComparator()) method call ?

 public class WrongComparator implements Comparator<String> {

   public int compare(String o1, String o2){
     return 1;
   }
 }

It does or does not throw the exception? And what is your question? — Robin Topper, May 01 '17 at 11:45
Why should it throw the exception? How does it violate any "general contract"? — Hovercraft Full Of Eels, May 01 '17 at 11:52
Because following is violated -> instance.compare("A", "B") = - instance.compare("B", "A") — HakunaMatata, May 01 '17 at 11:59
The problem i am facing is that on "Production" Environment java throws exception for such comparator but on other environments no such error comes. — HakunaMatata, May 01 '17 at 12:03
[Similar question](http://stackoverflow.com/questions/41910304/comparison-method-violates-its-general-contract-is-thrown-only-in-certain-case) — Hovercraft Full Of Eels, May 01 '17 at 12:20

Feedforward · Answer 1 · 2017-05-01T14:24:05.663

Because IllegalArgumentException is thrown if it's detected. It means, that Collections.sort(..) doesn't make special checks on comparator, but if something goes wrong during the sorting (because of a bad comparator), this exception is thrown. Collections.sort() use Tim sort algorithm and comparator check is performed at the end of the merge phase.

Why don't you get this exception in this particular case?
Because your collection is already sorted, according to you comparator. Tim's algorithm detects that it's already sorted and doesnt' perform any merge.

Why Incorrect Comparator(Java) Does not throw Exception "Comparison method violates general contract"

1 Answers1