Adding elements to a collection during iteration

java iterator
grifaton picture grifaton · Jun 14, 2009 · Viewed 136.7k times · Source

Is it possible to add elements to a collection while iterating over it?

More specifically, I would like to iterate over a collection, and if an element satisfies a certain condition I want to add some other elements to the collection, and make sure that these added elements are iterated over as well. (I realise that this could lead to an unterminating loop, but I'm pretty sure it won't in my case.)

The Java Tutorial from Sun suggests this is not possible: "Note that Iterator.remove is the only safe way to modify a collection during iteration; the behavior is unspecified if the underlying collection is modified in any other way while the iteration is in progress."

So if I can't do what I want to do using iterators, what do you suggest I do?

Answer

Avi picture Avi · Jun 14, 2009

How about building a Queue with the elements you want to iterate over; when you want to add elements, enqueue them at the end of the queue, and keep removing elements until the queue is empty. This is how a breadth-first search usually works.

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