[ Error 分析] Comparison method violates its general contract!

Posted 2020-10-18 乔桑宁

      public static void main(String[] args) {        

        List<Long> ret = new ArrayList<>();
        
        int n = 103000;
        for(int i=0;i<n;i++){
          ret.add(new Random().nextLong());
        }

        Collections.sort( ret , new Comparator<Long>(){
            @Override
            public int compare(Long o1, Long o2) {
                  return (int) (o2-o1);
            }
        });
    }

 

Exception in thread "main" java.lang.IllegalArgumentException: Comparison method violates its general contract!

at java.util.TimSort.mergeLo(TimSort.java:777)

at java.util.TimSort.mergeAt(TimSort.java:514)

at java.util.TimSort.mergeCollapse(TimSort.java:441)

at java.util.TimSort.sort(TimSort.java:245)

at java.util.Arrays.sort(Arrays.java:1512)

at java.util.ArrayList.sort(ArrayList.java:1454)

at java.util.Collections.sort(Collections.java:175)

 

一直以为没有什么问题，大于，小于，等于情况都考虑了。 原来原因是：integer overflow，看下面的例子就明白了。负数－正数＝正数

o2:-273192312377492627

 

o1:3368764997970232313

 

o2-o1:1167472500

所以，这个compare里面定义的方法不符合compare的约束（传递性，对称性，反对称性）

 

PS:https://docs.oracle.com/javase/7/docs/api/java/util/Comparator.html#compare(T,%20T)

http://svn.python.org/projects/python/trunk/Objects/listsort.txt