Fail safe iterator - removal during iteration

Question

If fail safe iterator creates a clone of underlying data structure, why 'D' is never printed in program below?

Map<String, String> acMap = new ConcurrentHashMap<String, String>();
acMap.put("A", "Aye");
acMap.put("B", "Bee");
acMap.put("C", "See");
acMap.put("D", "Di");

Iterator<String> itr = acMap.keySet().iterator();
while(itr.hasNext())
{
    acMap.remove("D");
    System.out.println(itr.next());
}

Subhrajyoti Majumder · Answer 1 · 2016-06-03T14:09:45.567

1

As per documentation any operation on map will reflect on keyset and vice versa. And KeySet#iterator() also does not clone the underlying ds.

ConcurrentHashMap#keySet():

Returns a Set view of the keys contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. The set supports element removal, which removes the corresponding mapping from this map, via the Iterator.remove, Set.remove, removeAll, retainAll, and clear operations.

Read more documentation

See openjdk Implementation

edited Jun 03 '16 at 14:09

answered Jun 03 '16 at 13:59

Subhrajyoti Majumder

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It's more specific than the `keySet()`: it's the `keySet().iterator()` which is being used here. – Andy Turner Jun 03 '16 at 14:01
ohk. One sec, what I can see `KeySet#iterator()` returns `KeyIterator` which is also does not clone the underlaying ds too. :) [reference](http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/6-b14/java/util/concurrent/ConcurrentHashMap.java) – Subhrajyoti Majumder Jun 03 '16 at 14:05

score 1 · Accepted Answer · answered Jun 03 '16 at 14:05

1

As it says in the Javadoc for keySet():

The iterator ... may (but is not guaranteed to) reflect any modifications subsequent to construction.

Your iterator is reflecting modifications subsequent to construction.

answered Jun 03 '16 at 14:05

Andy Turner

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Fail safe iterator - removal during iteration

2 Answers2