Modify map while iterating through it

Question

hi all,
i have some java code iam maintaining and am seeing in many places that
HashMap object's iterator is used to iterate and in the loop, the same map is modified (adding an element). is this valid coding? i didnt get any issues so far.
Thanks.
eg code:
for ( Iterator iterator = myHashMap.keySet().iterator(); iterator.hasNext(); )
//bla bla bla
myHashMap.put(key,value);
iam wondering since the map got modified, how reliable is the iterator used in for loop?
-Madhu

Answer 1

You should not add during iteration. If you must, you should add
into a temporary array. When you're done with the iteration,
you can then add everything from the array into the map.
Or, if you quit the iteration right after adding something, that's okay. (That is, if after you add something, you no longer
touch the iterator, then that's okay)
Furthermore:
First of all, you should not use keySet() since it is very inefficient.
You're supposed to use entrySet() instead.
Second of all, if you use entrySet(), then you CAN delete+modify during iteration (just not "add")
for(Iterator i=map.entrySet(); i.hasNext(); ) {
   Map.Entry e = (Map.Entry)(i.next());
   Object key = e.getKey();
   Object value = e.getValue();
   // You can now remove this entry like this:
   i.remove();
   // Or you can alter the value of this entry, like this:
   e.setValue(someNewValue);
}