Question

About duplicate

This is NOT a duplicate of How to clone an iterator?

Please do not blindly close this question, all the answers given in so-called duplicate DO NOT work. The OP is in charge of the other problem, and obviously, the answers fitted HIS problem, but not mine.

Not every similar question is a duplicate, there is such feature as “expansion question” on SE, the only way is to ask again on the same subject to get different, working, answers.

Problem

I have iterator. I would like to get copy (duplicate) of it, so then I could proceed with original and copy completely independently.

Important

Copying through reflection or serialization is no-go (performance penalty).

Example

var list = List(1,2,3,4,5)
var it1 = list.iterator
it1.next()

var it2 = it1   // (*)
it2.next()

println(it1.next())

This would make simply reference to it1, so when changing it1, it2 changes as well and vice-versa.

The example above uses List, I am currently struggling with HashMap, but the question is general one — just iterator.

Approach #1

If you edit line (*) and write:

var it2 = it1.toList.iterator

(this was suggested as solution in the linked question) the exception is thrown while executing the program.

Approach #2

“You take the list and…”. No, I don’t. I don’t have a list, I have iterator. In general I don’t know anything about collection which underlies the iterator, the only thing I have is iterator. I have to “fork” it.

Answer 1

You can’t duplicate an iterator without destroying it. The contract for iterator is that it can only be traversed once.

The question you linked to shows how to get two copies in exchange for the one you’ve destroyed. You cannot keep using the original, but you can now run the two new copies forward independently.