Arrays.asList(String[]).add(String) 报错

1.异常信息摘要（详细见文末）

2019-11-11 14:24:24,844 ERROR MenuController:114 - java.lang.UnsupportedOperationException
at java.util.AbstractList.add(AbstractList.java:148)
at java.util.AbstractList.add(AbstractList.java:108)
at com.lx.menu.controller.MenuController.delete(MenuController.java:148)

 

2.报错代码

String ids[] = req.getIds();
        if (ids == null || ids.length == 0){
            throw new ServiceException(ErrorCodeEnum.code_2005);
        }
        List<String> idList = Arrays.asList(ids);
        for (String id:ids){
            idList.add(id);
            findChildMenu(id, idList);
        }
        Integer count = menuService.deleteBatch((String[])idList.toArray());
        Map<String, Object> respMap = new HashMap<>();
        respMap.put("count", count);

 

3.原因

　　Arrays.asList 方法返回的是一个  java.util.Arrays.ArrayList  内部类实例，该类虽然跟  java.util.ArrayList  一样继承了  java.util.AbstractList ，却没有重写 add 方法。所以当我们使用 asList 返回的结果集合去调用 add 方法时，实际上只能调用 AbstractList.add 方法。

　　我们首先来看一下  Arrays.asList  方法的实现：

public static <T> List<T> asList(T... a) {
        return new ArrayList<>(a);
    }

 

 

　　这是 AbstractList 的两个 add 方法源码：　

public boolean add(E e) {
        add(size(), e);
        return true;
    }


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

　　

　　注意第二个方法，上述异常就是从这里抛出的。下面是  java.util.Arrays.ArrayList  类的源码：

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);
        }
    }

Arrays.ArrayList源码