Question

I am interested if there is any framework that implements a collection that would have the following behavior.

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Suppose it initially contains: [1, 2, 3]

• I iterate it (using an iterator) and reach element 2, now I add 4 to the end (the collection will now be [1, 2, 3, 4]).
• now I create a new iterator and iterate the collection, resulting in [1, 2, 3, 4]
• I continue iterating with the first iterator and it will give me just 3 and return
• now resetting the first iterator will give me [1, 2, 3, 4] (similar to creating a new one).

Same should apply for removal of elements. If I remove 3 instead of adding, the second iterator should give me [1, 2] while the first one will still give me 3 and the end.

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So when I get and iterator I want it to give me the collection I had when I created the iterator (even if I iterate it later, on I iterate a bit and continue later), when I reset the iterator, it gets garbage collected it will update to the latest versions and I should be able to have multiple iterator instances created at different times that will give different versions depending on the contents of the array when the iterator was created.

I would need it to work well with multiple threads and preferable have an efficient implementation.

Does anyone know of any implementation of such a collection, or do I have to implement it myself?

Answer 1

What you describe looks very similar to how CopyOnWriteArrayList works:

• once you start iterating, you can change the collection (including from another thread) without affecting the iteration
• if you create a new iterator it will be based on the collection at the creation time
• it is thread safe

Simple example below with the following output:

Iterator 1 – 1
4 has been added
Iterator 2 – 1
Iterator 2 – 2
Iterator 2 – 3
Iterator 2 – 4
Iterator 1 – 2
Iterator 1 – 3

public static void main(String[] args) throws InterruptedException {
    final List<Integer> list = new CopyOnWriteArrayList<Integer>();
    list.addAll(Arrays.asList(1, 2, 3));
    new Thread(new Runnable() {

        @Override
        public void run() {
            for (Integer i : list) {
                System.out.println("Iterator 1 - " + i);
                try {
                    Thread.sleep(10);
                } catch (InterruptedException e) {}
            }
        }
    }).start();
    Thread.sleep(10);
    list.add(4);
    System.out.println("4 has been added");
    for (Integer i : list) {
        System.out.println("Iterator 2 - " + i);
    }

}