关于Arrays.asList的注意事项以及源码解析

1.禁止使用Arrays.asList 返回的List进行add/remove/clear操作

    /**
     * 1.remove/add/clear
     * Arrays的内部类并没有实现List，使用remove/add/clear时会抛出java.lang.UnsupportedOperationException异常
     * 对于这用引用传递的方式尤为危险，即使在获取Arrays.asList后没有进行remove/add/clear,
     * 但是并不能确保参数传递到方法中没有使用到这些方法
     */
    @Test
    public void ArraysAsListReferTest(){

        Integer [] arrInt = {1,2,3,4,5};
        List arrList = Arrays.asList(arrInt);
        removeFirst(arrList);
        System.out.println(arrList);//java.lang.UnsupportedOperationException

    }

    public void removeFirst(List list){
        if(list != null && list.size() > 0){
            list.remove(0);
        }
    }

• 源码解析
  Arrays.asList返回的是Arrays内部类ArrayList

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

继承了AbstractList，却没有实现set add remove。

    /**
     * @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);
        }
    }

如果使用了set add remove就会抛出异常

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

2.基本数据类型作为asList参数注意事项

	/**
     * 2.使用基本数据类型作为参数注意事项
     *   public static <T> List<T> asList(T... a)
     *   基本数据类型不属于对象，而T是数组元素的class，asList得到的不是数组本身，而是数组对象，且元素只有一个
     *
     */
    @Test
    public void ArrayAsListBaseTypeTest(){

        int [] arrInt = {1,2,3,4};
        List list = Arrays.asList(arrInt);
        System.out.println("list size:"+list.size());       //返回结果为1
        System.out.println("list get(0):"+list.get(0));     //返回hashCode


    }

3.返回真正ArrayList的正确方式

    /**
     * 3.获得真正的ArrayList
     *      asList作为ArrayList构造方法参数
     */
    @Test
    public void GetArrayListTesty(){

        Integer arrInteger[] = {1,2,3,4,5};
        List<Integer> list = new ArrayList<Integer>(Arrays.asList(arrInteger));
        list.add(6);
        System.out.println("list:"+list);//list:[1, 2, 3, 4, 5, 6]

    }