How to remove and add elements to TreeMap while iterating?

Question

I want to write code like this -

for (Map.Entry<Long, Integer> e : map.entrySet()){
    map.remove(k);
    map.put(x, value);
}

but I got java.util.ConcurrentModificationException I tried to use Iterator also but I got the same Exception

Answer 1

Explanation why it caused ConcurrentModificationException

map.remove(k);
map.put(x, value);

for-each loop also internally create a iterator of the entrySet of map. While iterating over map you have modified the structure of the map by putting the value again to the map (map.put(x,value)) which cause this ConcurrentModificationException.

It is even well explained in documentation -

The iterators returned by all of this class's "collection view methods" are fail-fast: if the map is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove method, the iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.

How to solve this -

you must change the change the structure of this map while iterating, you can insert this values later, like keep a temporary map and add this to the main map once iteration is finished his job.

Map<Long, Integer> tempMap = new HashMap<>();
for (Map.Entry<Long, Integer> e : map.entrySet()){
    map.remove(k);
    tempMap.put(x, value);
}
map.putAll(tempMap);