实现Comparable接口报错：Comparison method violates its general contract!

1、代码（利用Collections.sort()方法对list进行排序）

    public static List> getSortList(String filePath) throws FileNotFoundException{
    BufferedReader reader=null;
    Map pairFeatures = new HashMap<>();
    List> list =null;
    try {
    reader=new BufferedReader(new FileReader(new File(filePath)));
String line=null;
while((line=reader.readLine())!=null){
String []arrs=line.split(":");
pairFeatures.put(arrs[0].trim(), Double.valueOf(arrs[1]));
}
list = new ArrayList<>(pairFeatures.entrySet());
Collections.sort(list, new Comparator>() {


@Override
public int compare(Entry en1, Entry en2) {
return (int) (en2.getValue() * 10000 - en1.getValue() * 10000);
}
});
return list;
} catch (IOException e) {
e.printStackTrace();
}finally{
try {
reader.close();
} catch (IOException e) {
e.printStackTrace();
}
}
    return list;
    }

2、报错：

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:439)
at java.util.TimSort.sort(TimSort.java:245)
at java.util.Arrays.sort(Arrays.java:1512)
at java.util.ArrayList.sort(ArrayList.java:1462)
at java.util.Collections.sort(Collections.java:175)
at com.applabel.utils.SortList.getSortList(SortList.java:38)
at com.applabel.utils.SortList.main(SortList.java:22)

3、分析原因：
在JDK7以后，实现Comparable接口后，要满足一下三个特性：

1） 自反性：x，y 的比较结果和 y，x 的比较结果相反。

2） 传递性：x>y,y>z,则 x>z。

3） 对称性：x=y,则 x,z 比较结果和 y，z 比较结果相同。

可以通过在代码中，System.setProperty("java.util.Arrays.useLegacyMergeSort", "true");  

或者设置JVM参数-Djava.util.Arrays.useLegacyMergeSort=true来兼容JDK6的代码。

这是因为JDK在函数Arrays.sort中，使用了一个if判断来决定使用什么排序算法来对数组进行排序。

public static void sort(Object[] a, int fromIndex, int toIndex) {  
    if (LegacyMergeSort.userRequested)  
        legacyMergeSort(a, fromIndex, toIndex);  
    else  
        ComparableTimSort.sort(a, fromIndex, toIndex);  

不设置就使用ComparableTimSort实现的TimSort算法来进行排序。

下面是JDK1.8.0_102中legacyMergeSort的主要代码

/**
 * 已经将src数组复制了一份到dest数组
 * low和high是mergesort在数组中的下标
 * off为low相对于下标0的偏移量
 */
private static void mergeSort(Object[] src,
                              Object[] dest,
                              int low,
                              int high,
                              int off) {
    int length = high - low;

    //一段精简的插入排序代码，INSERTIONSORT_THRESHOLD = 7
    // Insertion sort on smallest arrays
    if (length < INSERTIONSORT_THRESHOLD) {
        for (int i=low; i)
            for (int j=i; j>low &&
                     ((Comparable) dest[j-1]).compareTo(dest[j])>0; j--)
                swap(dest, j, j-1);
        return;
    }

    // Recursively sort halves of dest into src
    int destLow  = low;
    int destHigh = high;
    low  += off;
    high += off;
    int mid = (low + high) >>> 1;
    mergeSort(dest, src, low, mid, -off);
    mergeSort(dest, src, mid, high, -off);

    //判断整个数组是否已经有序
    // If list is already sorted, just copy from src to dest.  This is an
    // optimization that results in faster sorts for nearly ordered lists.
    if (((Comparable)src[mid-1]).compareTo(src[mid]) <= 0) {
        System.arraycopy(src, low, dest, destLow, length);
        return;
    }

    //归并
    // Merge sorted halves (now in src) into dest
    for(int i = destLow, p = low, q = mid; i < destHigh; i++) {
        if (q >= high || p < mid && ((Comparable)src[p]).compareTo(src[q])<=0)
            dest[i] = src[p++];
        else
            dest[i] = src[q++];
    }
}