ConcurentModificationException while adding items in a map

I am facing java.util.ConcurrentModificationException while adding items to an existing list corresponding to a key

Here is my code

final Map<String, List<Apple>> feedToApplesMap = new HashMap<>();
for (final Apple Apple : AppleList) {
    final List<String> feedDocumentIds = Apple.getFeedDocumentIds();
    for (final String feedId : feedDocumentIds) {
        final List<Apple> AppleListForFeed = feedToApplesMap
                .getOrDefault(feedId, new ArrayList<>());
        AppleList.add(Apple);
        feedToApplesMap.put(feedId, AppleListForFeed);
    }
}

I am unable to use iterator to solve this ? Any suggestions ?

It seems like you're storing apples in a wrong place. Shouldn't it be

AppleListForFeed.add(Apple) instead of AppleList.add(Apple) ?

I agree with @andbi that the question is due to a typo. But just for the exception, By doing so for (final Apple Apple : AppleList) you are still using Iterator unintentionally, that's why here AppleList.add(Apple); will throw ConcurrentModificationException, you can do it like this to avoid using Iterator:

final Map<String, List<Apple>> feedToApplesMap = new HashMap<>();
int size = AppleList.size();
for (int i = 0; i < size; i++) {
    final Apple Apple = AppList.get(i);
    final List<String> feedDocumentIds = Apple.getFeedDocumentIds();
    for (final String feedId : feedDocumentIds) {
        final List<Apple> AppleListForFeed = feedToApplesMap
                .getOrDefault(feedId, new ArrayList<>());
        AppleList.add(Apple);
        feedToApplesMap.put(feedId, AppleListForFeed);
    }
}

Please be aware that enhanced for-loop is just a syntatic sugar for for-loop with Iterator, the following two are the same:

    for (Iterator<T> i = c.iterator(); i.hasNext(); )
 
    for (T t : c)
    

Here is the document: The For-Each Loop

This happens when we are iterating a list using for loop or for each and some elements are added in between during iterating so use iterator or iterating a list:

val iterator = yourList.iterator()
    while (iterator.hasNext()) {
        val currentItem = iterator.next()
    }

now add or delete any other element in your list during iterating.