IndentityHashMap in Java implements Map interface. But it is not a

general purpose implementation. It violates the general contract of

Map interface by a different implementation of equals() method.

In an IdentityHashMap, two keys k1 and k2 are equal if and only if

(k1==k2). (In a normal Map implementation (like HashMap) two

keys k1 and k2 are considered equal if and only if (k1==null ?

k2==null : k1.equals(k2)).)

It implements the Map interface with a hash table, using referenceequality in place of object-equality when comparing keys (and

values).

https://www.javatpoint.com/identityhashmap-class-in-java#:~:text=Difference%20Between%20IdentityHashMap%20and%20HashMap,is%20faster%20than%20the%20HashMap.