The implementation of IdentityHashMap is not thread-safe, since its

methods are not synchronized.

The iterators returned by the iterator method of IdentityHashMap

are fail-fast. But the fail-fast behavior of an iterator cannot be

guaranteed.

Since the Iterator is fail-fast, it throws

ConcurrentModificationException

baeldung.com/java-identityhashmap#:~:text=IdentityHashMap%20isn't%20threadsafe%2C%20the%20same%20as%20HashMap.