Why do I have to create a copy of the argument?

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I don't understand why do I need to create a copy of the method argument, otherwise I get a weird Concurrent exception! My code that didn't work is this:

public static void removeItemFromMap(HashMap<String, Integer> map) {
    //write your code here
    for (Map.Entry<String, Integer> entry: map.entrySet()) {
        String key = entry.getKey();
        Integer val = entry.getValue();

        if (val < 500) {
            map.remove(key);
        }
    }
}

But when I create a copy of the map argument and use that in my for loop, everything seems to work fine! What the Concurrent exception thing is all about??!!

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Guadalupe Gagnon

Level 37 , Tampa, United States

12 March 2021, 22:49

solution

A ConcurrentModificationException occurs if you remove items from a Collection, such as a map, that you are iterating over; excepting that you use the appropriate Iterator.remove() method. A foreach loop is a short hand technique in Java that allows Iteration over a Collection without creating an Iterator. It is important to know this because a for each loop uses an Iterator behind the scenes, one which you cannot access, and removing items from the Collection being iterated over will result in that exception. So, you found a work around in that you can copy the Collection and then Iterate over the copy, leaving you free to remove items from the main Collection without breaking the rule. This is not the only way to do it though, as just simply using an Iterator and its remove() method would have solved this without having to use the memory to create a copy.

Anonymous #10714753

Level 25

13 March 2021, 16:13

Thanks