RFR: 8305774: String.join(CharSequence, Iterable) can be optimized if Iterable is a Collection [v2]

[Chen Liang]

On Sun, 9 Apr 2023 02:28:37 GMT, Tingjun Yuan <duke at openjdk.org> wrote:

>> In the current implementation of `String.join(CharSequence, Iterable)`, the temp array `elems` is always initialized with a length of 8. It will cause many array recreations when the `Iterable` contains more than 8 elements. Furthermore, it's very common that an `Iterable` is also a `Collection`. So if the `Iterable` is an instance of `Collection`, the initial length of the array can be `((Collection<?>)elements).size()`. It will not change the current behavior even if the `Collection` is modified asynchronously.
>> 
>> I don't know whether this change requires a CSR request.
>
> Tingjun Yuan has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Add benchmark

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

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PR Comment: https://git.openjdk.org/jdk/pull/13383#issuecomment-1501351704