CODEWITHSUNDEEP
javacollections
Dharmvir Tiwari
Dharmvir Tiwari

Reputation: 916

Java collection : Map getting updated while using fail-safe iterator

I was working on analyzing fail-safe iterators with Maps and verifying, if we are updating the keyset, will the operation happen on the clone or the actual map

private static void failSafeIterator() {
    ConcurrentHashMap<String, String> map=new ConcurrentHashMap<>();
    map.put("a", "one");
    map.put("b", "two");
    Iterator<String> keyIterator=map.keySet().iterator();
    while(keyIterator.hasNext()){
        String key=keyIterator.next();
        System.out.println(key+":"+map.get(key));
        map.put("c", "three");
        map.put("q", "four");
        map.put("W", "five");

    }
    System.out.println(map.get("q"));

}

As per the above code snippet,

the addition of c,q and w should have happened on the clone and not on the actual collection

But i can see the update happening over the collection.

Also the output is a bit confusing as not all the key value pairs are printed, even though the key is present in the map.

Output:

a:one 
b:two
c:three
W:five
four

Upvotes: 2

Views: 138

Answers (1)

Eugene
Eugene

Reputation: 120938

The problem here is that you don't understand what weakly consistent iterator stands for, when using keySet().iterator(), specifically this part of the documentation:

they are guaranteed to traverse elements as they existed upon construction exactly once, and may (but are not guaranteed to) reflect any modifications subsequent to construction.

Imagine a case like this: you are iterating a ConcurrentHashMap and printing whatever it has. Once you have seen a certain bucket and showed all of it's elements, you move to the next one, also updating the (suppose you add a key-value pair to it) previous one. Updates to that previous one are not going to be shown, although they do exist.

After your loop, you can do :

System.out.println(map);

And see that everything is now present.

Upvotes: 2

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