Collections.unmodifiableCollection 返回值

Question

List<Integer> integers = Arrays.asList(1, 2);
List<Integer> integers2 = Arrays.asList(1, 2);
Collection<Integer> integers3 = Collections.unmodifiableCollection(integers);
Collections.unmodifiableCollection(integers2);
integers3.add(3);
integers2.add(3); 

我知道执行integers3.add()会抛出 UnsupportedOperationException，但 integers2 没有改变实例。我查看了 Collections.unmodifiableCollection(Collection ..) 的源代码，但是 integers2 的实现也从 AbstractList 抛出了 UnsupportedOperationException。为什么是这样？

Answer 1

这不是因为使用Collections.unmodifiableCollection(integers2);. 这是因为 Arrays.asList() 返回一个无法更改的固定大小数组。这是 asList() 方法文档的前几行。

/**
 * 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}.