JDK Collections.shuffle(List list, Random rnd)源碼分析

jdk的源碼以下

public static void shuffle(List<?> list, Random rnd) {
        int size = list.size();
        if (size < SHUFFLE_THRESHOLD || list instanceof RandomAccess) {
            for (int i=size; i>1; i--)
                swap(list, i-1, rnd.nextInt(i));
        } else {
            Object arr[] = list.toArray();

            // Shuffle array
            for (int i=size; i>1; i--)
                swap(arr, i-1, rnd.nextInt(i));

            // Dump array back into list
            // instead of using a raw type here, it's possible to capture
            // the wildcard but it will require a call to a supplementary
            // private method
            ListIterator it = list.listIterator();
            for (int i=0; i<arr.length; i++) {
                it.next();
                it.set(arr[i]);
            }
        }
 }

1、首先是判斷要打亂的list的屬性：list的size和是否實現RandomAccess接口

若是list的size小於SHUFFLE_THRESHOLD(5) 或者 list實現了RandomAccess接口，則直接交換list內元素的位置。具體的交換策略以下：

令list的size爲n, 從n-1位開始，將該位的元素與其前面某一位(隨機獲得)元素交換，直到第1位結束。

使用的函數：

• swap(List<?> list, int i, int j)

public static void swap(List<?> list, int i, int j) {
        // instead of using a raw type here, it's possible to capture
        // the wildcard but it will require a call to a supplementary
        // private method
        final List l = list;
        l.set(i, l.set(j, l.get(i)));   //將j位置的值和i位置的值進行交換
}

• E set(int index, E element)接口

/**
     * Replaces the element at the specified position in this list with the
     * specified element (optional operation).
     * 
     * @param index index of the element to replace
     * @param element element to be stored at the specified position
     */
    E set(int index, E element)

• E set(int index, E element)某一實現

public E set(int index, E element) {
      try {
            ListIterator<E> e = listIterator(index);
            E oldVal = e.next();
            e.set(element);    
            return oldVal;      //將index的值設置爲element，並返回原來的值
        } catch (NoSuchElementException exc) {
            throw new IndexOutOfBoundsException("Index: "+index);
        }
}

2、若是list沒有實現RandomAccess接口且長度大於SHUFFLE_THRESHOLD(5)

這時候首先要作的是將list轉化成數組，這個和RandomAccess有關

/**
 * Marker interface used by <tt>List</tt> implementations to indicate that
 * they support fast (generally constant time) random access.  The primary
 * purpose of this interface is to allow generic algorithms to alter their
 * behavior to provide good performance when applied to either random or
 * sequential access lists.
 *
 * <p>The best algorithms for manipulating random access lists (such as
 * <tt>ArrayList</tt>) can produce quadratic behavior when applied to
 * sequential access lists (such as <tt>LinkedList</tt>).  Generic list
 * algorithms are encouraged to check whether the given list is an
 * <tt>instanceof</tt> this interface before applying an algorithm that would
 * provide poor performance if it were applied to a sequential access list,
 * and to alter their behavior if necessary to guarantee acceptable
 * performance.
 *
......
public interface RandomAccess {
}

RandomAccess用於標識ist的實現類是否支持快速地隨機訪問(通常是O(1)的時間複雜度)，例如ArraryList實現了RandomAccess接口，隨機訪問一個元素(get(i))所花費的時間複雜度是O(1)，而LinkedList卻沒有實現這個接口，因此隨機一個元素的時間複雜度是O(n)(最壞狀況)。因此在遍歷一個list時，能夠先判斷它是否實現了RandomAccess接口，根據數據結構的不一樣先進行相應的處理，避免出現O(n²)的時間複雜度。
如在shuffle()的else代碼段中，就先將沒有實現RandomAccess接口的list轉換成數組，而後在執行交換策略，這樣避免O(n²)的時間複雜度。

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以上內容若有不正確的地方，歡迎支持。