java list.toArray 方法？-灵析社区
sssssjkl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 null. * (This is useful in determining the length of the list only 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[] toArray(T[] a) { 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[] 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((Map.Entry)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; }