归并排序是JDK对象数组的排序算法之一,接下来开始分析JDK的归并排序算法。
/** * 将指定范围的对象数组按自然顺序升序排序 * * Src is the source array that starts at index 0 * Dest is the (possibly larger) array destination with a possible offset * low is the index in dest to start sorting * high is the end index in dest to end sorting * off is the offset to generate corresponding low, high in src * To be removed in a future release. */ @SuppressWarnings({"unchecked", "rawtypes"}) private static void mergeSort(Object[] src, Object[] dest, int low, int high, int off) { int length = high - low; // 规模很小的数组的排序,直接插入排序的效率反而比归并要高 // 规模定在INSERTIONSORT_THRESHOLD = 7之内 // 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; } // 递归排序dest的一半元素并赋值给src // 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); // 如果低子列表中的最高元素小于高子列表中的最低元素,则忽略合并 // 如果需要归并的两端low~(mid-1),mid~high已经有序,即src[mid-1]==src[mid],那么只需要将src的low~high赋值对应的dest即可,无需再归并 // 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; } // 将src的两个部分合并,并赋值给dest // 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++]; } }