java.util.ConcurrentModificationException，但我正在迭代一个副本，而不是我正在修改的集合

Question

Please read the description of my problem before marking duplicate.

private void cleanTrash() {

    HashMap<String, OutfitInfo> tempOutfits = outfits;
    Iterator iter = tempOutfits.entrySet().iterator();
    while (iter.hasNext()) {
        Map.Entry mapElement = (Map.Entry) iter.next();
        String key = (String) mapElement.getKey();
        OutfitInfo info = (OutfitInfo) mapElement.getValue();
        if (info.getParentID().equals(trashID)) {
            outfits.remove(key);
        }
    }
    HashMap<String, ItemInfo> tempItems = items;
    iter = tempItems.entrySet().iterator();
    while (iter.hasNext()) {
        Map.Entry mapElement = (Map.Entry) iter.next();
        String key = (String) mapElement.getKey();
        ItemInfo info = (ItemInfo) mapElement.getValue();
        if (info.getParentID().equals(trashID)) {
            items.remove(key);
        }
    }

}

In the above code I am getting the concurrent modification exception. I understand that this error means that I can't modify a collection while I'm iterating over it. HOWEVER , I made a copy of the collection before assigning the iterator, and the collection being modified is not the one that's being iterated over. So why am I still getting this message, and how do I fix it?

Answer 1

HashMap<String, OutfitInfo> tempOutfits = outfits; does not create a copy. Both varaibles tempOutfits and outfits point to the same instance.

If you actually want to copy you can do:

 HashMap<String, OutfitInfo> tempOutfits = new HashMap<>(outfits);

This line creates a new HashMap and initializes it with the content of outfits

---

As you already have the Iterator , instead of removing the entry directly from the collection. Call iterator.remove(); to remove the item the iterator currently references. No need to copy the collection.

So:

private void cleanTrash() {
    Iterator iter = outfits.entrySet().iterator();
    while (iter.hasNext()) {
        Map.Entry mapElement = (Map.Entry) iter.next();
        String key = (String) mapElement.getKey();
        OutfitInfo info = (OutfitInfo) mapElement.getValue();
        if (info.getParentID().equals(trashID)) {
            iter.remove();
        }
    }
    iter = items.entrySet().iterator();
    while (iter.hasNext()) {
        Map.Entry mapElement = (Map.Entry) iter.next();
        String key = (String) mapElement.getKey();
        ItemInfo info = (ItemInfo) mapElement.getValue();
        if (info.getParentID().equals(trashID)) {
            iter.remove();
        }
    }
}

Answer 2

You can use clone() to create a copy of the HashMap, clone()#Oracle doc

Map<String, String> copy = (Map<String, String>) outfits.clone();

---

    public static void main(String[] args) {
        HashMap<String, String> outfits = new HashMap<>(Map.of("1", "11", "2", "22"));
        Map<String, String> copy = (Map<String, String>) outfits.clone();
        Iterator<Entry<String, String>> iterator = copy.entrySet().iterator();
        while (iterator.hasNext()) {
            Entry<String, String> next = iterator.next();
            if (next.getKey().equals("1"))
                outfits.remove(next.getKey());
        }
    }