asList詳解

asList(T... a)傳回的是一個固定大小的list集合      

源碼分析：

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

此時的ArrayList為Arrays裡面自定義的一個私有的内部類

private static class ArrayList<E> extends AbstractList<E>
        implements RandomAccess, java.io.Serializable
           

其父類AbstractList中事務方法都被禁止了,使用時需要確定傳回的list不會去修改。若需要修改可以修改為

new ArrayList<>(Arrays.asList(T... a));      

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

    /**
     * {@inheritDoc}
     *
     * <p>This implementation always throws an
     * {@code UnsupportedOperationException}.
     *
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws ClassCastException            {@inheritDoc}
     * @throws NullPointerException          {@inheritDoc}
     * @throws IllegalArgumentException      {@inheritDoc}
     * @throws IndexOutOfBoundsException     {@inheritDoc}
     */
    public void add(int index, E element) {
        throw new UnsupportedOperationException();
    }

    /**
     * {@inheritDoc}
     *
     * <p>This implementation always throws an
     * {@code UnsupportedOperationException}.
     *
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws IndexOutOfBoundsException     {@inheritDoc}
     */
    public E remove(int index) {
        throw new UnsupportedOperationException();
    }