实现Comparable接口不严谨导致Comparison method violates its general contract!
2017-12-21 14:53
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因为业务需要,自己的model实现了Comparable接口,在Service层使用Collections.sort()方法对model对象的list进行排序。但是在运行的时候,却报下面的异常。
经过搜索得知,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。而我的代码中,因为有对象属性为null的判断,所以有以下模式的代码
这种是有不满足对称性的结果的。因此只能完善对象属性为null的判断
这一点是JDK6和JDK7+以上不兼容的点。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。
参考:http://iamyida.iteye.com/blog/2255804
java.lang.IllegalArgumentException: Comparison method violates its general contract! at java.util.ComparableTimSort.mergeHi(ComparableTimSort.java:866) at java.util.ComparableTimSort.mergeAt(ComparableTimSort.java:483) at java.util.ComparableTimSort.mergeCollapse(ComparableTimSort.java:404) at java.util.ComparableTimSort.sort(ComparableTimSort.java:213) at java.util.Arrays.sort(Arrays.java:1312) at java.util.Arrays.sort(Arrays.java:1506) at java.util.ArrayList.sort(ArrayList.java:1454) at java.util.Collections.sort(Collections.java:141)
经过搜索得知,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。而我的代码中,因为有对象属性为null的判断,所以有以下模式的代码
if (this.xxx== null || o.xxx== null) { return 0; } int xxxFirst = this.xxx.compareTo(o.xxx); if (xxxFirst != 0) { return xxxFirst; } ......
这种是有不满足对称性的结果的。因此只能完善对象属性为null的判断
if (this.xxx == null && o.xxx == null) { return 0; } if (this.xxx == null) { return 1; } if (o.xxx == null) { return -1; }
这一点是JDK6和JDK7+以上不兼容的点。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++]; } }
参考:http://iamyida.iteye.com/blog/2255804
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