Check if current is available within Iterator without moving the iterator about sdk HOT 1 OPEN

The example here is a little misleading. The join was written deliberately to not allocate a buffer if the iterable has only one element, so the extra lines at the beginning are there by choice.

Otherwise it would likely have been:

String join([String separator = ""]) {
  Iterator<E> iterator = this.iterator;
  if (!iterator.moveNext()) return "";
  if (separator.isNotEmpty) {
    while (true) {
      buffer.write(iterator.current);
      if (iterator.moveNext()) {
        buffer.write(separator);
      } else {
        break;
      }
    }
  } else {
    do {
      buffer.write(iterator.current);
    } while (iterator.moveNext());
  }
  return buffer.toString();
}

That has iterator.moveNext() at every point where a branch is needed, everywhere the two directions need to behave differently.

I have generally found that the moveNext() returning a bool and the current returning the value allows the branches to be put exactly where they are needed. It fits the algorithms I write.

Caching the last value of moveNext() is something you can always do (and possibly write wrapper class to help with), but most of the time it's not needed.
(It's likely true that every Iterator implementation has the necessary information to answer whether there is a current value, so it's also not impossible to add. It's just that it would be breaking for all existing implementations of Iterator, and I don't expect the benefit to be all that great.)

from sdk.