ArrayList源码

[qq_28938099]

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)
a[size] = null;//为什么要置为空
return a;
}
这是Arraylist中的一个方法，为什么当传入的数组长度大于arraylist中元素个数时，传入数组index为size的元素要置null
我试着将这句代码拿掉后，并没有什么显见的影响，这句代码是何用途，请大牛指教？

[ps45221]

同学，遇到问题先去看看API。

    /**
     * <p>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 collection 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.)
     */

[qq_28938099]

引用 1 楼 Sun1956 的回复:

同学，遇到问题先去看看API。

    /**
     * <p>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 collection 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.)
     */

谢谢提醒，我一定记住您的话

[qq_28938099]

引用 1 楼 Sun1956 的回复:

同学，遇到问题先去看看API。

    /**
     * <p>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 collection 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.)
     */

道友，我想了一下，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. 这句话的意思是【仅 在调用者知道列表中不包含任何 null 元素时才能用此方法确定列表长度】，我有点不理解【这里的 列表长度指的是arraylist的长度吗，这是如何确定的】，希望道友能进一步说明一下，原谅我的愚钝

[ps45221]

引用 3 楼 qq_28938099 的回复:

[quote=引用 1 楼 Sun1956 的回复:] 同学，遇到问题先去看看API。

    /**
     * <p>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 collection 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.)
     */

道友，我想了一下，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. 这句话的意思是【仅 在调用者知道列表中不包含任何 null 元素时才能用此方法确定列表长度】，我有点不理解【这里的 列表长度指的是arraylist的长度吗，这是如何确定的】，希望道友能进一步说明一下，原谅我的愚钝[/quote] 就是你原来的那个list中没有null元素，这样它a[size] = null；赋值后，在确定list长度时才是有用的。如果之前的list当中有null元素，就没办法判断哪个是真的长度了。