How does the JVM ensure that System.identityHashCode() will never change?

Question

Typically the default implementation of Object.hashCode() is some function of the allocated address of the object in memory (though this is not

Answer 1

Modern JVMs save the value in the object header. I believe the value is typically calculated only on first use in order to keep time spent in object allocation to a minimum (sometimes down to as low as a dozen cycles). The common Sun JVM can be compiled so that the identity hash code is always 1 for all objects.

Multiple objects can have the same identity hash code. That is the nature of hash codes.