Collections.sort报错，大神求分析、求答案

Question

 Collections.sort(lcd1, new Comparator<CouponDto>(){                     @Override                     public int compare(CouponDto o1, CouponDto o2) {                             if(o1.getCoupon().getListOrder2() > o2.getCoupon().getListOrder2()){                                 return -1;                             }                             if(o1.getCoupon().getListOrder2() == o2.getCoupon().getListOrder2()){                                 return 0;                             }                             return  1;                     };                 });     报Comparison method violates its general contract异常             改成     Collections.sort(lcd1, new Comparator<CouponDto>(){                     @Override                     public int compare(CouponDto o1, CouponDto o2) {                             return -(o1.getCoupon().getListOrder2() - o2.getCoupon().getListOrder2());                     };                 });             就不再报错；             各位大神，这个是怎么回事

Answer 1

你的代码虽然满足了自反性，但是不满足偏序。jdk 1.7还要求偏序性。
比如a = 10 b = 8 c = 4,那么你的代码出现 comp(a,b) == comp(a, c)，sort会假设b=c但是实际上是b>c