Remove a key in hashmap when the value's hashset is Empty

hashmap, hashset, java, set

Solution

In order to avoid `ConcurrentModificationException`, you need to use the `Iterator` interface directly:

Iterator<Map.Entry<String, HashSet<Integer>>> it = stringIDMap.entrySet().iterator();
while (it.hasNext()) {
    Map.Entry<String, HashSet<Integer>> e = it.next();
    String key = e.getKey();
    HashSet<Integer> value = e.getValue();
    if (value.isEmpty()) {
        it.remove();
    }
}

The reason your current code doesn't work is that you are attempting to remove elements from the map while iterating over it. When you call `stringIDMap.remove()`, this invalidates the iterator that the for-each loop uses under the cover, making further iteration impossible.

`it.remove()` solves this problem as it does not invalidate the iterator.

Problem

I have a hashmap that maps strings keys to hashsets values, and I want to remove a key from the hashmap when the hashmaps's hashset value is empty. I'm having trouble approaching this. Here's what I've tried but I'm very stuck: ``` for(Map.Entry<String, HashSet<Integer>> entr : stringIDMap.entrySet()) { String key = entr.getKey(); if (stringIDMap.get(key).isEmpty()) { stringIDMap.remove(key); continue; } //few print statements... } ```

Original source