Caused by: java.lang.UnsupportedOperationException

对Arrays.asList()返回的List进行操作之后报错Caused by: java.lang.UnsupportedOperationException

让我们来看一下Arrays.asList的源码:

/**
     * Returns a {@code List} of the objects in the specified array. The size of the
     * {@code List} cannot be modified, i.e. adding and removing are unsupported, but
     * the elements can be set. Setting an element modifies the underlying
     * array.
     *
     * @param array
     *            the array.
     * @return a {@code List} of the elements of the specified array.
     */
    @SafeVarargs
    public static <T> List<T> asList(T... array) {
        return new ArrayList<T>(array);
    }

private static class ArrayList<E> extends AbstractList<E> implements
            List<E>, Serializable, RandomAccess {

        private static final long serialVersionUID = -2764017481108945198L;

        private final E[] a;

        ArrayList(E[] storage) {
            if (storage == null) {
                throw new NullPointerException("storage == null");
            }
            a = storage;
        }

        @Override
        public boolean contains(Object object) {
            if (object != null) {
                for (E element : a) {
                    if (object.equals(element)) {
                        return true;
                    }
                }
            } else {
                for (E element : a) {
                    if (element == null) {
                        return true;
                    }
                }
            }
            return false;
        }

        @Override
        public E get(int location) {
            try {
                return a[location];
            } catch (ArrayIndexOutOfBoundsException e) {
                throw java.util.ArrayList.throwIndexOutOfBoundsException(location, a.length);
            }
        }

        @Override
        public int indexOf(Object object) {
            if (object != null) {
                for (int i = 0; i < a.length; i++) {
                    if (object.equals(a[i])) {
                        return i;
                    }
                }
            } else {
                for (int i = 0; i < a.length; i++) {
                    if (a[i] == null) {
                        return i;
                    }
                }
            }
            return -1;
        }

        @Override
        public int lastIndexOf(Object object) {
            if (object != null) {
                for (int i = a.length - 1; i >= 0; i--) {
                    if (object.equals(a[i])) {
                        return i;
                    }
                }
            } else {
                for (int i = a.length - 1; i >= 0; i--) {
                    if (a[i] == null) {
                        return i;
                    }
                }
            }
            return -1;
        }

        @Override
        public E set(int location, E object) {
            E result = a[location];
            a[location] = object;
            return result;
        }

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

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

        @Override
        @SuppressWarnings({"unchecked", "SuspiciousSystemArraycopy"})
        public <T> T[] toArray(T[] contents) {
            int size = size();
            if (size > contents.length) {
                Class<?> ct = contents.getClass().getComponentType();
                contents = (T[]) Array.newInstance(ct, size);
            }
            System.arraycopy(a, 0, contents, 0, size);
            if (size < contents.length) {
                contents[size] = null;
            }
            return contents;
        }
    }

索噶，原来返回的是Arrays类的静态内部类！这样能会报错也就正常咯。