Java ConcurrentModificationException while editing value inside a list streaming a map

Question

I have a Map<String, ClassA> results.

When I do this, I get a ConcurrentModificationException:

results.entrySet().stream().map((entry) -> {
    ClassA objA = entry.getValue();
    if(objA.getList() != null) {
        objA.getList().forEach(x -> {
            if(x.getAttr() != null && x.getAttr.containsKey(key)) {
                List<String> y = x.getAttr().get(key);
                y.replaceAll(ClassB::transformationFunc);
            }
        });
    }
})

Basically what I am trying to do is if my results, has a value of ClassA, check for each element of the list if there is any Attribute with a given key. If so, replace the values of the key with a given transformation function.

public static String transformationFunc(String input) {
    try {
        JSONObject inputJson = (JSONObject) jsonParser.parse(input);
        return String.format("%s_%s", inputJson.get(key1), inputJson.get(key2));
    } catch (ParseException e) {
        //log
        //throw
    }
}

Possible duplicate of Java Concurrent Modification Exception Error — Raju Sharma
– Raju Sharma, Commented Nov 7, 2017 at 11:26
Interesting - assuming you are using a JDK implementation of List, replaceAll will flag your list as mutated after all its elements have been replaced. But I cannot see from your code where and when the co-modification occurs. Do you have the stack trace associated to the ConcurrentModificationException? — Alexandre Dupriez
– Alexandre Dupriez, Commented Nov 7, 2017 at 16:08

Piotr Wilkin · Accepted Answer · 2017-11-07 11:26:56Z

4

Basically, you get a ConcurrentModificationException whenever you modify a collection that you are iterating over.

The only way in which you are allowed to modify a collection that you are iterating over is by explicitly using an Iterator. If you want to do work on a collection while you're iterating over it, you have to do it on a working copy of the collection instead.

answered Nov 7, 2017 at 11:26

Piotr Wilkin

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7 Comments

user1692342 Over a year ago

Would I need to replace the stream as well? Or do I just need to modify the foreach to use an iterator?

Piotr Wilkin Over a year ago

Yes, using a stream is also implicitly iterating over a collection (same as using forEach() or a foreach loop). You have to stream over a copy of the map.

Piotr Wilkin Over a year ago

To clarify: you only have to make a copy if your code is actually modifying the collection you're iterating over. By "modifying the collection" I mean "adding, removing or replacing elements" - unless you use some sort of weird observable collection, modifying the internals of objects that are parts of a collection does not constitute modifying the collection.

Piotr Wilkin Over a year ago

@Eugene: that's obviously hard to answer without seeing the internals of the methods in question or seeing a stacktrace.

Eugene Over a year ago

@user1692342 it's not. your problem is elsewhere or you are not showing us everything. stacktrace.... also that exception would be thrown when you change the size of the collection you are streaming from, not the contents.

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