java list.toArray 方法？

看一下toArray方法的源码文档：

* If the list fits in the specified array with room to spare (i.e.,
* the array has more elements than the list), the element in the array
* immediately following the end of the list is set to <tt>null</tt>.
* (This is useful in determining the length of the list <i>only</i> if
* the caller knows that the list does not contain any null elements.)

意思就是如果数组的元素比列表多，数组中紧跟在列表末尾后面那一位置的元素设置为null，如果调用者知道列表不包含任何null元素的情况下，方便推断得出列表的真实长度以ArrayList为例，它的实现如下:

@SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
    if (a.length < size)
        // Make a new array of a's runtime type, but my contents:
        return (T[]) Arrays.copyOf(elementData, size, a.getClass());
    System.arraycopy(elementData, 0, a, 0, size);
    if (a.length > size)//如果数组的长度大于列表，把列表后面的第一个位置置为null
        a[size] = null;
    return a;
}

你题目里这个of生成的应该是一个UnmodifiableList? 它的实现也差不多,总之也是只把紧跟着列表后面的那个元素置为null：

@SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
    // We don't pass a to c.toArray, to avoid window of
    // vulnerability wherein an unscrupulous multithreaded client
    // could get his hands on raw (unwrapped) Entries from c.
    Object[] arr = c.toArray(a.length==0 ? a : Arrays.copyOf(a, 0));

    for (int i=0; i<arr.length; i++)
        arr[i] = new UnmodifiableEntry<>((Map.Entry<? extends K, ? extends V>)arr[i]);

    if (arr.length > a.length)
        return (T[])arr;

    System.arraycopy(arr, 0, a, 0, arr.length);
    if (a.length > arr.length)
        a[arr.length] = null;//看这里看这里
    return a;
}