Java HashMap add new entry while iterating

Question

In a HashMap

map = new HashMap<String,String>();

it = map.entrySet().iterator();
while (it.hasNext())
{
    entry = it.next();
    it.remove(); //safely remove a entry
    entry.setValue("new value"); //safely update current value
    //how to put new entry set inside this map
    //map.put(s1,s2); it throws a concurrent access exception

}

When i trying to add a new entry to map it throws ConcurrentModificationException. For remove and update iterator has safely removing methods. How to add new entry while iterating?

Create a new `Map foo` instance and set the desired values there. At the end of your process, assign this map to your old map by using `map = foo;`. — Luiggi Mendoza, Jan 03 '15 at 08:56
You could do it by using `ConcurrentHashMap` but that seems overkill. Also, these clean-up tasks are not good to do using an iterator. — Luiggi Mendoza, Jan 03 '15 at 09:05

Dunes · Answer 1 · 2015-01-03T09:17:47.093

You need to consider what it means to put a value to a Map whilst iterating. HashMap defines no order over which its entries will be iterated over. So when you put a new entry, should the entry be returned by the iterator later or not. Consistency of behaviour is important. However, whichever way you decide you'll get inconsistent behaviour when you put a new value to a preexisting key. If the key has already been iterated over then the change won't appear and will appear if the key has yet to be produced by the iterator.

A simple way to overcome this problem is to create a temporary Map of the new key-value pairs and add the temporary Map to the main Map at the end of your iteration.

Map<String,String> values = ...

Map<String,String> temp = new HashMap<>();
for (Entry<String,String> entry : values.entrySet()) {
    if ("some value".equals(entry.getValue()) {
        temp.put(entry.getValue(), "another value");
    }
}
values.putAll(temp);

Consistent behavior is not always needed, especially with probabilistic algorithms. — luke1985, Apr 25 '16 at 11:19

score 3 · Answer 2 · answered Jan 03 '15 at 08:53

3

You need to use ConcurrentHashMap to add elements while iterating the collection. HashMap uses fail-fast iterator, which throws ConcurrentModificationException when the collection is updated while iterating. Whereas ConcurrentHashMap uses fail-safe iterator, which basically works on the clone of the underlying collection and hence allows modification while iterating.

answered Jan 03 '15 at 08:53

Juned Ahsan

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I think using `ConcurrentHashMap` in this case may be overkill. – Luiggi Mendoza Jan 03 '15 at 08:57

score 2 · Answer 3 · answered Oct 21 '15 at 15:57

How about:

map = new HashMap<String,String>();

it = map.entrySet().iterator();
while (it.hasNext())
{
    entry = it.next();
    entry.setValue("new value"); // update current value
}

I checked the HashMap implementation, it does not change its modification count when updating an entry like this. I also see no reason why this shouldn't be allowed. Nothing is removed, nothing is added and no keys are changed.

score 0 · Answer 4 · answered May 07 '20 at 15:02

What I did was to create an array with the current elements and then iterate through the array:

Feature[] initialFeatures = featureMap.values().toArray(new Feature[featureMap.values().size()]);

for (Feature feature : initialFeatures)
{/* Code that can safely add to the featureMap */}

Java HashMap add new entry while iterating

4 Answers4

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