简易做法

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String id = "a,b,c";
List<String> ids = Arrays.asList(id.split(","));

在使用前，可以先尝试看一下 Arrays.asList() 的源码，这有助于防范一些比较明显的错误。

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/**
 * Returns a fixed-size list backed by the specified array.  (Changes to
 * the returned list "write through" to the array.)  This method acts
 * as bridge between array-based and collection-based APIs, in
 * combination with {@link Collection#toArray}.  The returned list is
 * serializable and implements {@link RandomAccess}.
 *
 * <p>This method also provides a convenient way to create a fixed-size
 * list initialized to contain several elements:
 * <pre>
 *     List&lt;String&gt; stooges = Arrays.asList("Larry", "Moe", "Curly");
 * </pre>
 *
 * @param <T> the class of the objects in the array
 * @param a the array by which the list will be backed
 * @return a list view of the specified array
 */
@SafeVarargs
@SuppressWarnings("varargs")
public static <T> List<T> asList(T... a) {
    return new ArrayList<>(a);
}

在这个方法中，文档一开始就告诉我们，这个方法返回的是一个定长的数列。所以当我们添加或删除当中的元素时是会抛出异常的，所以我们需要将它进行转化一下
当然，如果并不需要添加或删除当中的元素，是可以不用转化的。

转化

这里使用构造函数直接构造一个相同内容、不定长度的 ArraysList。
构造函数如下:

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/**
 * Constructs a list containing the elements of the specified
 * collection, in the order they are returned by the collection's
 * iterator.
 *
 * @param c the collection whose elements are to be placed into this list
 * @throws NullPointerException if the specified collection is null
 */
public ArrayList(Collection<? extends E> c) {
    elementData = c.toArray();
    if ((size = elementData.length) != 0) {
        // c.toArray might (incorrectly) not return Object[] (see 6260652)
        if (elementData.getClass() != Object[].class)
            elementData = Arrays.copyOf(elementData, size, Object[].class);
    } else {
        // replace with empty array.
        this.elementData = EMPTY_ELEMENTDATA;
    }
}

文档中提到，该构造函数用于生成包含指定元素的 ArraysList ，也就是说并没有长度限制。

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String id = "a,b,c";
List<String> ids = Arrays.asList(id.split(","));
List<String> ids2 = new ArraysList<>(ids);

简化一下就是

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String id = "a,b,c";
List<String> ids = new ArraysList<>(Arrays.asList(id.split(",")));

这样子就完成了一个数组转不定 List 的方法。