Arrays.asList返回的有限操作的List
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Arrays.asList返回的有限操作的List
woshixin 发表于5个月前
Arrays.asList返回的有限操作的List
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摘要: Arrays.asList

想把数组转换成List,然后用了Arrays.asList,可是发现这个返回的List中执行操作add与delete中会报

java.lang.UnsupportedOperationException异常。

String [] arrayA = {"123","abc","456"};
for (int i = 0; i < arrayA.length; i++) {
    System.out.println(arrayA[i]);
}
System.out.println("------------------------");
List<String> list = Arrays.asList(arrayA);
for (String a:list) {
    System.out.println(a);
}
System.out.println("------------------------");
System.out.println(list instanceof ArrayList );
System.out.println(list instanceof List );
System.out.println(list.getClass());
list.add("hello");
list.remove(1);

我只想知道为什么会报这样的错误,先看下日志

从上面 我们就意境啊可以知道 这个list 是 arrays中定义的ArrayList,居然还有这一手

下面是java 1.8.0_91 中的源码

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);
    }
}

可以看出支持的操作就上面几种,所以没有我上面报错的那几种操作,这个ArrayList继承了abstractList,而abstractList实现了List接口,在abstractList的设计中,一些没有被子类重写的方法,会爆出UnsupportedOperationException的异常,比如add

/**
 * Appends the specified element to the end of this list (optional
 * operation).
 *
 * <p>Lists that support this operation may place limitations on what
 * elements may be added to this list.  In particular, some
 * lists will refuse to add null elements, and others will impose
 * restrictions on the type of elements that may be added.  List
 * classes should clearly specify in their documentation any restrictions
 * on what elements may be added.
 *
 * <p>This implementation calls {@code add(size(), e)}.
 *
 * <p>Note that this implementation throws an
 * {@code UnsupportedOperationException} unless
 * {@link #add(int, Object) add(int, E)} is overridden.
 *
 * @param e element to be appended to this list
 * @return {@code true} (as specified by {@link Collection#add})
 * @throws UnsupportedOperationException if the {@code add} operation
 *         is not supported by this list
 * @throws ClassCastException if the class of the specified element
 *         prevents it from being added to this list
 * @throws NullPointerException if the specified element is null and this
 *         list does not permit null elements
 * @throws IllegalArgumentException if some property of this element
 *         prevents it from being added to this list
 */
public boolean add(E e) {
    add(size(), e);
    return true;
}

在add(size(),e)中

原来如此,心头小疑惑。

标签: Array.asList
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