How can I iterate over an object while modifying it in Java?

Question

Possible Duplicates:
Java: Efficient Equivalent to Removing while Iterating a Collection
Removing items from a collection in java while iterating over it

I'm trying to loop through HashMap:

Map<String, Integer> group0 = new HashMap<String, Integer>();

... and extract every element in group0. This is my approach:

// iterate through all Members in group 0 that have not been assigned yet
for (Map.Entry<String, Integer> entry : group0.entrySet()) {

    // determine where to assign 'entry'
    iEntryGroup = hasBeenAccusedByGroup(entry.getKey());
    if (iEntryGroup == 1) {
        assign(entry.getKey(), entry.getValue(), 2);
    } else {
        assign(entry.getKey(), entry.getValue(), 1);
    }
}

The problem here is that each call to assign() will remove elements from group0, thus modifying its size, thus causing the following error:

Exception in thread "main" java.util.ConcurrentModificationException
    at java.util.HashMap$HashIterator.nextEntry(HashMap.java:793)
    at java.util.HashMap$EntryIterator.next(HashMap.java:834)
    at java.util.HashMap$EntryIterator.next(HashMap.java:832)
    at liarliar$Bipartite.bipartition(liarliar.java:463)
    at liarliar$Bipartite.readFile(liarliar.java:216)
    at liarliar.main(liarliar.java:483)

So... how can I loop through the elements in group0 while its dynamically changing?

Thanks.

Answer 1

You need to use the actual iterator and its remove method if you want to modify the collection while looping over it. There isn't really any way to do it with the foreach construct.

If you're trying to remove multiple entries in one iteration, you'll need to loop over something that's not backed by the map.

Set<String> keys = new HashSet<String>(group0.keySet());
for (String key : keys) {
  if (group0.containsKey(key)) {
    Integer value = group0.get(key);
    //your stuff 
  }
}

Answer 2

In this case, how can assign modify group0? More details are needed. Typically you cannot modify a collection while iterating over it. You modify through the Iterator interface.

Answer 3

As my answer here says:

http://stackoverflow.com/questions/223918/java-efficient-equivalent-to-removing-while-iterating-a-collection/223929#223929

Use Iterator.remove()

Answer 4

You could use a ConcurrentHashMap

• Implementation details explained here by Brian Goetz
• More explanation of concurrent collections classes

Answer 5

In your particular case I would not modify the structure of the HashMap but merely null the value you want to remove. Then if you end up visiting a null value just skip it.

In the general case I prefer to use a Stack for things like this because they're particularly easy to visualise and so I tend to have less issues with border conditions (just keeping popping 'till empty).

Answer 6

Others have mentioned the correct solution without actually spelling it out. So here it is:

Iterator<Map.Entry<String, Integer>> iterator = 
    group0.entrySet().iterator();
while (iterator.hasNext()) {
    Map.Entry<String, Integer> entry = iterator.next();

    // determine where to assign 'entry'
    iEntryGroup = hasBeenAccusedByGroup(entry.getKey());

    if (iEntryGroup == 1) {
        assign(entry.getKey(), entry.getValue(), 2);
    } else {
        assign(entry.getKey(), entry.getValue(), 1);
    }

    // I don't know under which conditions you want to remove the entry
    // but here's how you do it
    iterator.remove();
}

Also, if you want to safely change the map in your assign function, you need to pass in the iterator (of which you can only use the remove function and only once) or the entry to change the value.