Value-based Classes confusion

Question

I\'m seeking some clarification to the definition of Value-based Classes. I can\'t imagine, how is the last bullet point (6) supposed to work together with the first one

Answer 1

You can derive the invalidity of your actions from the specification you’re referring to:

A program may produce unpredictable results if it attempts to distinguish two references to equal values of a value-based class, whether directly via reference equality or indirectly via an appeal to synchronization, identity hashing, serialization, or any other identity-sensitive mechanism. Use of such identity-sensitive operations on instances of value-based classes may have unpredictable effects and should be avoided.

^{(emphasis mine)}

Modifying an object is an identity-sensitive operation, as it only affects the object with the specific identity represented by the reference you are using for the modification.

When you are calling x.get().add(""); you are performing an operation that allows to recognize whether x and y represent the same instance, in other words, you are performing an identity sensitive operation.

Still, I expect that if a future JVM truly tries to substitute value based instances, it has to exclude instances referring to mutable objects, to ensure compatibility. If you perform an operation that produces an Optional followed by extracting the Optional, e.g. … stream. findAny().get(), it would be disastrous/unacceptable if the intermediate operation allowed to substitute the element with another object that happened to be equal at the point of the intermediate Optional use (if the element is not itself a value type)…