Re: Arrays.asList() doesn't work quite like I would think it should

On 03 Oct 2007 08:18:50 GMT, Ken <none@nowhere.com> wrote, quoted or
indirectly quoted someone who said :


Let's peek under the covers at java.util.Collections.shuffle.
Internally it converts to an array to shuffle!

 /**
     * Randomly permute the specified list using the specified source
of
     * randomness. All permutations occur with equal likelihood
     * assuming that the source of randomness is fair.<p>
     *
     * This implementation traverses the list backwards, from the last
element
     * up to the second, repeatedly swapping a randomly selected
element into
     * the "current position". Elements are randomly selected from
the
     * portion of the list that runs from the first element to the
current
     * position, inclusive.<p>
     *
     * This method runs in linear time. If the specified list does
not
     * implement the {@link RandomAccess} interface and is large, this
     * implementation dumps the specified list into an array before
shuffling
     * it, and dumps the shuffled array back into the list. This
avoids the
     * quadratic behavior that would result from shuffling a
"sequential
     * access" list in place.
     *
     * @param list the list to be shuffled.
     * @param rnd the source of randomness to use to shuffle the
list.
     * @throws UnsupportedOperationException if the specified list or
its
     * list-iterator does not support the <tt>set</tt>
operation.
     */
    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
            ListIterator it = list.listIterator();
            for (int i=0; i<arr.length; i++) {
                it.next();
                it.set(arr[i]);
            }
        }
    }
--
Roedy Green Canadian Mind Products
The Java Glossary
http://mindprod.com