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