Concurrent Modification Exception

[Dariusz Kordonski]

Hi everybody,

I wrote (myself! ) this very sophisticated piece of code:

 
public static void main(String[] args) {
        
        try {
            // Map with custom ordering - access order
            Map<String, String> map2 = new LinkedHashMap<String, String>(16, 0.75f, true);
            map2.put("1", "10");
            map2.put("2", "20");
            map2.put("3", "30");
            map2.put("5", "50");
            System.out.println("THE MAP:");
            for (String key : map2.keySet()) {
                System.out.println(map2.get(key));
            }           
        } catch (Exception e) {
            System.out.println("Exc msg: " + e.getMessage() + ", class: " + e.getClass());
        }
}

Simple as it is, it nevertheless manages to produce exception. The output is:

THE MAP:
10
Exc msg: null, class: class java.util.ConcurrentModificationException

Now can anybody explain to me, how the hell does this happen? Can you spot this concurrent modification the JVM is complaining about??
[ July 18, 2008: Message edited by: Dariusz Kordonski ]

[Henry Wong]

Now can anybody explain to me, how the hell does this happen? Can you spot this concurrent modification the JVM is complaining about??

When an iterator is working through a collection, you are not allowed to change the structural organization of the collection. It is not really a concurrent modification -- it is more of a modification while the iterator is still working on it.

Anyway, to answer your question, you configured you linked hash map to keep access order. This means that an access of any kind can change the structural organization of the map. Your get() method is basically changing the structure of the map, while you are still iterating, hence, concurrent modification exception.

Henry

[Dariusz Kordonski]

Ok thanks a lot, now I get it. So to iterate over keys and values of such a map I would need to use entrySet() instead. That's quite tricky

[Campbell Ritchie]

Welcome to JavaRanch

Using EntrySet isn't that tricky.

You get a Set from the entrySet method. Now your Map is parameterised as <String, String>, so your Set will contain Entries of that type. It will read something like this:

Set<Map.Entry<String, String>> entries = map2.entrySet();
for (Map.Entry<String, String> me : entries)
{
   String key = me.getKey();
   String value = me.getValue();
   . . .
}//end for-each

I think that is how it works. You do not keep the insertion order like this, I am afraid.
[ July 18, 2008: Message edited by: Campbell Ritchie ]

[Dariusz Kordonski]

Hello and thank you for your insights! If anybody's interested in such details: I've just tested the entrySet() iteration and it seems that neither entry.getKey() nor entry.getValue() count as access in this case, so the insertion order remains actually intact.

[Campbell Ritchie]

Well done

[Bill Shirley]

Changing to a different implementation of Map (one that doesn't modify the structure with a get) may also provide a solution.

[Joanne Neal]

Originally posted by Dariusz Kordonski:
I've just tested the entrySet() iteration and it seems that neither entry.getKey() nor entry.getValue() count as access in this case

The put, get and putAll methods are the only ones which will affect the access time.
[ July 22, 2008: Message edited by: Joanne Neal ]