java.util.Arrays$ArrayList addAll报错

执行下面代码时报错:

List<String> centerList = WebConstants.SUPPORT_BIG_CENTERS_LIST;   // WebConstants.SUPPORT_BIG_CENTERS_LIST是Arrays.asList转化来的
centerList.removeAll(WebConstants.SUPPORT_CENTERS_LIST);
centerList.addAll(WebConstants.SUPPORT_CENTERS_LIST);

最后一行报错：

java.lang.UnsupportedOperationException: null
    at java.util.AbstractList.add(AbstractList.java:148) ~[na:1.8.0_101]
    at java.util.AbstractList.add(AbstractList.java:108) ~[na:1.8.0_101]
    at java.util.AbstractCollection.addAll(AbstractCollection.java:344) ~[na:1.8.0_101]

查看Arrays$ArrayList的源代码，如下(1.8版本)：

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) {
            a = Objects.requireNonNull(array);
        }

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

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

        @Override
        @SuppressWarnings("unchecked")
        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;
        }

        @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<E> spliterator() {
            return Spliterators.spliterator(a, Spliterator.ORDERED);
        }

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

        @Override
        public void replaceAll(UnaryOperator<E> 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<? super E> c) {
            Arrays.sort(a, c);
        }
    }

进入父类找下addAll方法:

public boolean addAll(int index, Collection<? extends E> c) {
        rangeCheckForAdd(index);
        boolean modified = false;
        for (E e : c) {
            add(index++, e);
            modified = true;
        }
        return modified;
    }

实现只存在于父类中，如下所示：

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

父类AbstractList add方法直接抛出异常。

所以问题就在这里，我们改下代码，如下就不报错了：

List<String> centerList = new ArrayList<>();

if (null != WebConstants.SUPPORT_BIG_CENTERS_LIST) {  //addAll的目标是null会报错
centerList.addAll(WebConstants.SUPPORT_BIG_CENTERS_LIST);
}
if (null != WebConstants.SUPPORT_CENTERS_LIST) {
centerList.removeAll(WebConstants.SUPPORT_CENTERS_LIST);
centerList.addAll(WebConstants.SUPPORT_CENTERS_LIST);
}

它调用的就是arrayList的addAll方法了