Exception in thread "main" java.lang.UnsupportedOperationException

今天写代码时把 String[] 数组使用Arrays.asList方法直接转成List，然后想在list.add一个字符结果却报

Exception in thread "main" java.lang.UnsupportedOperationExceptionat java.util.AbstractList.add(AbstractList.java:148)at java.util.AbstractList.add(AbstractList.java:108)......

查阅源码发现

Arrays.asList方法生成的list是List是使用的是类内部的构建ArrayList类而不是我们常用的java.util.ArrayList

它的内部类根本就没有重写List的add方法而是调用的java.util.AbstractList.add的add方法

而AbstractList的add方法会直接抛出UnsupportedOperationException异常

也就是我们看见的异常

贴上java.util.Arrays类的部分源码

public static <T> List<T> asList(T... a) {        return new ArrayList<>(a);    }    /**     * @serial include     */    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);        }    }

解决方法就是把Arrays.ArrayList转换成java.util.arraylist类

List arrayList = new ArrayList(a);

参考

When you call Arrays.asList it does not return a java.util.ArrayList. It returns a java.util.Arrays$ArrayList which is an immutable list. You cannot add to it and you cannot remove from it.

If you want a mutable list built from your array you will have to loop over the array yourself and add each element into the list in turn.

Even then your code won't work because you'll get an IndexOutOfBoundsException as you remove the elements from the list in the for loop. There are two options: use an Iterator which allows you to remove from the list as you iterate over it (my recommendation as it makes the code easier to maintain) or loop backwards over the loop removing from the last one downwards (harder to read). 

You are using AbstractList. ArrayList and Arrays$ArrayList are both types of AbstractList. That's why you get UnsupportedOperationException: Arrays$ArrayList does not override remove(int) so the method is called on the superclass, AbstractList, which is what is throwing the exception because this method is not implemented on that class (the reason being to allow you to build immutable subclasses).