Method join

guava/src/com/google/common/base/Joiner.java:193–233

Returns a string containing the string representation of each of {@code parts}, using the previously configured separator between each.

(Iterable<?> parts)

   * previously configured separator between each.
   */
  public String join(Iterable<?> parts) {
    /*
     * If we can quickly determine how many elements there are likely to be, then we can use the
     * fastest possible implementation, which delegates to the array overload of String.join.
     *
     * In theory, we can quickly determine the size of any Collection. However, thanks to
     * regrettable implementations like our own Sets.filter, Collection.size() is sometimes a
     * linear-time operation, and it can even have side effects. Thus, we limit the special case to
     * List, which is _even more likely_ to have size() implemented to be fast and side-effect-free.
     *
     * We could consider recognizing specific other collections as safe (like ImmutableCollection,
     * except ContiguousSet!) or as not worth this optimization (CopyOnWriteArrayList?).
     */
    if (parts instanceof List) {
      int size = ((List<?>) parts).size();
      if (size == 0) {
        return "";
      }
      CharSequence[] toJoin = new CharSequence[size];
      int i = 0;
      for (Object part : parts) {
        if (i == toJoin.length) {
          /*
           * We first initialized toJoin to the size of the input collection. However, that size can
           * go out of date (for a collection like CopyOnWriteArrayList, which may have been safely
           * modified concurrently), or it might have been only an estimate to begin with (for a
           * collection like ConcurrentHashMap, which sums up several counters that may not be in
           * sync with one another). We accommodate that by resizing as necessary.
           */
          toJoin = Arrays.copyOf(toJoin, expandedCapacity(toJoin.length, toJoin.length + 1));
        }
        toJoin[i++] = toString(part);
      }
      // We might not have seen the expected number of elements, as discussed above.
      if (i != toJoin.length) {
        toJoin = Arrays.copyOf(toJoin, i);
      }
      return String.join(separator, toJoin);
    }
    return join(parts.iterator());
  }

  /*
   * TODO: b/381289911 - Make the Iterator overload use StringJoiner (including Android or not)—or