Name: auR10023 Date: 06/22/2001



HashMap.entrySet.iterator() does not raise ConcurrentModificationException
if content of the entry set changes after iterator has been created.

The javadoc for HashMap says:

....
The iterators returned by all of this class's "collection view methods" are
fail-fast: if the map is structurally modified at any time after the iterator is
created, in any way except through the iterator's own remove or add methods, the
iterator will throw a ConcurrentModificationException. Thus, in the face of
concurrent modification, the iterator fails quickly and cleanly, rather than
risking arbitrary, non-deterministic behavior at an undetermined time in the
future.
...
        
Here is the example:
        
class t {
    public static void main(String args[]) {
        HashMap map = new HashMap();
        Iterator iter = null;
        HashMap map1 = new HashMap();
        
        map.put("key", "value");
        iter = map1.entrySet().iterator();
        map1.putAll(map);
            
        try {
            iter.next();
            System.out.println("ConcurrentModificationException was not " +
                               "thrown after changing of " +
                               "the HashMap by putAll() method");
            return;
        } catch (ConcurrentModificationException e) {
        } catch (NoSuchElementException e) {
            System.out.println("NoSuchElementException was thrown instead of " +
                               "ConcurrentModificationException ");
            return;
        }
    }
}

#java -version

Java(TM) 2 Runtime Environment, Standard Edition (build 1.4.0-beta-b65)
Java HotSpot(TM) Client VM (build 1.4.0-beta-b65, mixed mode)

#java t

NoSuchElementException was thrown instead of ConcurrentModificationException

Following methods have the same troubles:

HashMap.keySet().iterator()
HashMap.values().iterator()

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