On Mon, 10 Apr 2023 04:28:34 GMT, Chen Liang <[email protected]> wrote:
>> Nah. I mean like:
>>
>> public static String join(CharSequence delimiter,
>> Iterable<? extends CharSequence> elements) {
>> Objects.requireNonNull(delimiter);
>> Objects.requireNonNull(elements);
>> var delim = delimiter.toString();
>> Object[] elems;
>> final int size;
>> if (elements instanceof Collection<?> c) {
>> elems = c.toArray();
>> size = elems.length;
>> for (int i = 0; i < size; i++) {
>> elems[i] = String.valueOf(elems[i]);
>> }
>> } else {
>> elems = new String[elements instanceof Collection<?> c ?
>> c.size() : 8]; // or whatever spliterator you decide to use
>> size = 0;
>> for (CharSequence cs: elements) {
>> if (size >= elems.length) {
>> elems = Arrays.copyOf(elems, elems.length << 1);
>> }
>> elems[size++] = String.valueOf(cs);
>> }
>> }
>> return join("", "", delim, elems, size);
>> }
>> // ...
>> static String join(String prefix, String suffix, String delimiter, Object[]
>> elements, int size) { // change array type to Object[], cast on access
>> elements
>
>> @liach I don't think it's safe because `c.toArray()` can return an array
>> that may be modified by others. In fact, although the specification of
>> `Collection.toArray()` indicates that the returned array is free to be
>> modified, nobody actually trusts this contract. They only trust `ArrayList`
>> which is most widely-used.
>
> It's safe. We don't keep the array in another object, and it's discarded
> after we finish the join. The most risky scenario is that the array is shared
> to another thread that can execute in parallel with the malicious Collection,
> which can swap out the String elements in the array we've obtained with
> irrelevant items.
>
> Then you might go for
> https://github.com/openjdk/jdk/pull/13383#issuecomment-1501340839 but it
> involves one extra array copy.
But @liach 's sample modifies the returned array directly. It cause problems
with this situation:
class BadList implements List<Object> {
private Object[] array;
public Object get(int i) {
return array[i];
}
public Object[] toArray() {
return array; // no clone
}
// ...
}
Modifying the array will accidentally modify the collection implemented as
above. That's why we should not trust that the array is *safe to be modified*.
-------------
PR Comment: https://git.openjdk.org/jdk/pull/13383#issuecomment-1501387583
