Arrays.asList后对结果进行remove操作导致UnsupportedOperationException的解决方法

 

问题描述：

最近在完成实验的过程中，我想要将一个字符串数组转换为一个List，从而方便后续对其中元素进行检查，删除掉一部分非法元素。简单上网搜索资料以后，我发现了可以使用Arrays.asList的操作，将一个字符串数组转化为List，示例代码如下：

    String[] s = {"aa","bb","cc"};
    List strlist = Arrays.asList(s);

 但是当我使用这个方法完成代码，进行JUnit测试的时候却不能通过，查看Failure Trace时在这段代码处发现了UnsupportedOperationException的报错：

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原因分析:

我们首先查看一下Arrays.asList的源码（按住"Ctrl"键，将鼠标指到"asList"上，即可选择查看其源码）：

    /**
     * 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}.
     *
     * 
This method also provides a convenient way to create a fixed-size
     * list initialized to contain several elements:
     * 
     *     List<String> stooges = Arrays.asList("Larry", "Moe", "Curly");
     * 
     *
     * @param  the class of the objects in the array
     * @param a the array by which the list will be backed
     * @return a list view of the specified array
     */
    @SafeVarargs
    @SuppressWarnings("varargs")
    public static  List asList(T... a) {
        return new ArrayList<>(a);
    }

可以看到该方法返回的是一个ArrayList，继续查看这个ArrayList的源码，发现它并不是java.util.ArrayList的ArrayList，而是java.util.Arrays内部自己所包含的ArrayList。而这个内部类，继承了AbstractList类，没有重写remove方法：

    /**
     * @serial include
     */
    private static class ArrayList extends AbstractList
        implements RandomAccess, java.io.Serializable
    {
        private static final long serialVersionUID = -2764017481108945198L;
        private final E[] a;

        ArrayList(E[] array) {
            a = Objects.requireNonNull(array);
        }

        @Override
        public int size() {
            return a.length;
        }

        @Override
        public Object[] toArray() {
            return a.clone();
        }

        @Override
        @SuppressWarnings("unchecked")
        public  T[] toArray(T[] a) {
            int size = size();
            if (a.length < size)
                return Arrays.copyOf(this.a, size,
                                     (Class) a.getClass());
            System.arraycopy(this.a, 0, a, 0, size);
            if (a.length > size)
                a[size] = null;
            return a;
        }

        @Override
        public E get(int index) {
            return a[index];
        }

        @Override
        public E set(int index, E element) {
            E oldValue = a[index];
            a[index] = element;
            return oldValue;
        }

        @Override
        public int indexOf(Object o) {
            E[] a = this.a;
            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;
        }

        @Override
        public boolean contains(Object o) {
            return indexOf(o) != -1;
        }

        @Override
        public Spliterator spliterator() {
            return Spliterators.spliterator(a, Spliterator.ORDERED);
        }

        @Override
        public void forEach(Consumer action) {
            Objects.requireNonNull(action);
            for (E e : a) {
                action.accept(e);
            }
        }

        @Override
        public void replaceAll(UnaryOperator operator) {
            Objects.requireNonNull(operator);
            E[] a = this.a;
            for (int i = 0; i < a.length; i++) {
                a[i] = operator.apply(a[i]);
            }
        }

        @Override
        public void sort(Comparator c) {
            Arrays.sort(a, c);
        }
    }

再继续查看一下AbstractList类的源码，我们会发现它的remove方法被直接定义为了“throw new UnsupportedOperationException();”：

    /**
     * {@inheritDoc}
     *
     * 
This implementation always throws an
     * {@code UnsupportedOperationException}.
     *
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws IndexOutOfBoundsException     {@inheritDoc}
     */
    public E remove(int index) {
        throw new UnsupportedOperationException();
    }

到这里就完美破案了，进行Arrays.asList的操作得到的是一个java.util.Arrays内部自己所包含的ArrayList，而它的remove方法继承的是AbstractList类中的remove方法，当我们试图对Arrays.asList的结果进行remove操作时，就会抛出UnsupportedOperationException。

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解决方案：

新建一个java.util.ArrayList的ArrayList：

    List inputword = Arrays.asList(input.split(" "));
    List inputwords = new ArrayList<>();
    inputwords.addAll(inputword);