Arrays.asList回来ArrayList并非java.util.ArrayList，调用报 UnsupportedOperationException

Arrays.asList返回ArrayList并非java.util.ArrayList，调用报 UnsupportedOperationException
调用代码：

String[] tmpIds = ids.split(",");rt = Arrays.asList(tmpIds);rt.add(String.valueOf(userId));

/**     * Returns a fixed-size list backed by the specified array.  (Changes to     * the returned list "write through" to the array.)  This method acts     * as bridge between array-based and collection-based APIs, in     * combination with {@link Collection#toArray}.  The returned list is     * serializable and implements {@link RandomAccess}.     *     *      *     * <p>This method also provides a convenient way to create a fixed-size     * list initialized to contain several elements:     * <pre>     *     List&lt;String&gt; stooges = Arrays.asList("Larry", "Moe", "Curly");     * </pre>     *     * @param a the array by which the list will be backed     * @return a list view of the specified array     */    public static <T> List<T> asList(T... a) {        return new ArrayList<T>(a);    }

 public void add(int index, E element) {     throw new UnsupportedOperationException(); }

public class Arrays {//...............//...............    public static <T> List<T> asList(T... a) {        return new ArrayList<T>(a);    }//...............//...............    private static class ArrayList<E> extends AbstractList<E>implements RandomAccess, java.io.Serializable    {        private static final long serialVersionUID = -2764017481108945198L;private final E[] a;ArrayList(E[] array) {            if (array==null)                throw new NullPointerException();    a = array;}public int size() {    return a.length;}public Object[] toArray() {    return a.clone();}public <T> T[] toArray(T[] a) {    int size = size();    if (a.length < size)return Arrays.copyOf(this.a, size,     (Class<? extends T[]>) a.getClass());    System.arraycopy(this.a, 0, a, 0, size);    if (a.length > size)a[size] = null;    return a;}public E get(int index) {    return a[index];}public E set(int index, E element) {    E oldValue = a[index];    a[index] = element;    return oldValue;}        public int indexOf(Object o) {            if (o==null) {                for (int i=0; i<a.length; i++)                    if (a[i]==null)                        return i;            } else {                for (int i=0; i<a.length; i++)                    if (o.equals(a[i]))                        return i;            }            return -1;        }        public boolean contains(Object o) {            return indexOf(o) != -1;        }    }//...............//...............}