java - AbstractCollection 的 toArray 方法实现中代码的用途是什么

Question

public Object[] toArray() {
    // Estimate size of array; be prepared to see more or fewer elements
    Object[] r = new Object[size()];
    Iterator<E> it = iterator();
    for (int i = 0; i < r.length; i++) {
        if (! it.hasNext()) // fewer elements than expected
            return Arrays.copyOf(r, i);
        r[i] = it.next();
    }
    return it.hasNext() ? finishToArray(r, it) : r;
}

这里是AbstractCollection.toArray方法的实现代码。

if (! it.hasNext()) // fewer elements than expected
    return Arrays.copyOf(r, i);

我不明白上面代码的用法。我怀疑代码用于避免在调用方法时更改大小。所以我有两个问题:

1. 我怀疑的是对还是错？如果错了，这段代码有什么用？
2. 如果为真，在调用该方法时什么情况下会导致尺寸发生变化？

Answer 1

好吧，方法的 javadoc 说明了一切:

 /**
     * {@inheritDoc}
     *
     * <p>This implementation returns an array containing all the elements
     * returned by this collection's iterator, in the same order, stored in
     * consecutive elements of the array, starting with index {@code 0}.
     * The length of the returned array is equal to the number of elements
     * returned by the iterator, even if the size of this collection changes
     * during iteration, as might happen if the collection permits
     * concurrent modification during iteration.  The {@code size} method is
     * called only as an optimization hint; the correct result is returned
     * even if the iterator returns a different number of elements.
     *
     * <p>This method is equivalent to:
     *
     *  <pre> {@code
     * List<E> list = new ArrayList<E>(size());
     * for (E e : this)
     *     list.add(e);
     * return list.toArray();
     * }</pre>
     */

我发现这里有两件有趣的事情要提:

1. 是的，您是对的，正如 javadoc 所说，即使同时修改了 Collection，此方法也准备好正确返回。这就是为什么初始大小只是一个提示。迭代器的使用还确保避免“并发修改”异常。

2. 很容易想象一种多线程情况，其中一个线程从集合中添加/删除元素，而另一个线程调用集合中的“toArray”方法。在这种情况下，如果 Collection 不是线程安全的(比如通过 Collections.synchronizedCollection(...) 方法获得，或者通过手动创建同步访问代码)，你会遇到这样的情况它被同时修改和 toArray-ed。