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<Map.Entry<String, Double>> getSortList(String filePath) throws FileNotFoundException{

    BufferedReader reader=null;

    Map<String, Double> pairFeatures = new HashMap<>();

    List<Map.Entry<String, Double>> 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<Map.Entry<String, Double>>() {

@Override
public int compare(Entry<String, Double> en1, Entry<String, Double> 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<high; 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++];
}
}