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

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

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.

Answer 4

You could use a ConcurrentHashMap

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

Answer 5

As my answer here says:

Iterating through a Collection, avoiding ConcurrentModificationException when removing in loop

Use Iterator.remove()

Answer 6

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).