Collections,sort()實現原理
阿新 • • 發佈:2019-02-13
檢視原始碼可以發現Collections.sort的實現是Arrays.sort,而Arrays.sort的實現是ComparableTimSort.sort,然後才到sort方法和它的定義,檢視排序的主體部分也可以發現一個binarySort的方法,這是排序方法的實現,是通過呼叫Object的CompareTo進行比較的。
每一步程式碼如下:
public class ListSort { public static void main(String[] args){ List<String> list=new ArrayList<String>();list.add("ohjx"); list.add("ghjg"); list.add("hggh"); Collections.sort(list); for (String listString:list){ System.out.println(listString); } } }
Arrays.sort:
public static <T extends Comparable<? super T>> void sort(List<T> list) { Object[] a = list.toArray();ComparableTimSort.sort:Arrays.sort(a); ListIterator<T> i = list.listIterator(); for (int j=0; j<a.length; j++) { i.next(); i.set((T)a[j]); } }
public static void sort(Object[] a) { if (LegacyMergeSort.userRequested) legacyMergeSort(a); else ComparableTimSort.sort(a); }
sort:
static void sort(Object[] a) { sort(a, 0, a.length); } static void sort(Object[] a, int lo, int hi) { rangeCheck(a.length, lo, hi); int nRemaining = hi - lo; if (nRemaining < 2) return; // Arrays of size 0 and 1 are always sorted // If array is small, do a "mini-TimSort" with no merges if (nRemaining < MIN_MERGE) { int initRunLen = countRunAndMakeAscending(a, lo, hi); binarySort(a, lo, hi, lo + initRunLen); return; }
binarySort:
private static void binarySort(Object[] a, int lo, int hi, int start) { assert lo <= start && start <= hi; if (start == lo) start++; for ( ; start < hi; start++) { @SuppressWarnings("unchecked") Comparable<Object> pivot = (Comparable) a[start]; // Set left (and right) to the index where a[start] (pivot) belongs int left = lo; int right = start; assert left <= right; /* * Invariants: * pivot >= all in [lo, left). * pivot < all in [right, start). */ while (left < right) { int mid = (left + right) >>> 1; if (pivot.compareTo(a[mid]) < 0) right = mid; else left = mid + 1; } assert left == right; /* * The invariants still hold: pivot >= all in [lo, left) and * pivot < all in [left, start), so pivot belongs at left. Note * that if there are elements equal to pivot, left points to the * first slot after them -- that's why this sort is stable. * Slide elements over to make room for pivot. */ int n = start - left; // The number of elements to move // Switch is just an optimization for arraycopy in default case switch (n) { case 2: a[left + 2] = a[left + 1]; case 1: a[left + 1] = a[left]; break; default: System.arraycopy(a, left, a, left + 1, n); } a[left] = pivot; } }
這就是層層呼叫的程式碼(按順序)
Collections.sort方法如果只有一個引數,那麼預設採用自然排序的方法對指定的列表(引數)進行排序,也可以傳入兩個引數,一個是待排序的列表,另一個就是指定的排序規則。兩個方法的定義如下: