Thursday, November 24, 2011

linked list iteration - Multi-threaded

Mithun,

Thanks for the interview question --

Q: Can you iterate over a LinkedList while another thread adds or removes elements? How might you make it thread safe?

If a long linked list is super busy with a lot of add/remove, then we may never finish iterating -- we get ConcurrentModificationException every time.

I feel solution depends on length of the list, how critical is iteration vs add()/remove(), performance requirement etc.

I feel your initial idea is plausible -- make a copy of the entire linked list before iterating. But when we make a copy, we need to iterate -- chicken and egg problem. How about iteration-free copying? Get a node, copy it, then read the "nextNode" pointer to reach the next node. This way I implement my own loop. Must handle concurrent modification. Not easy, but at least I don't get exceptions.

I also like your 2nd idea of a custom linked list, where add()/remove() methods are customized. This is fine if iteration can finish quickly. However, If add()/remove() are time-critical, then we need other solutions.

If we need to iterate a long linked list that's super busy, then the __design__ is probably wrong. Why not use a database?

Alternatively, split the long linked list into shorter lists. Easier to iterate.

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