Value objects, having no identity, cannot be used for synchronization, locking, etc.
An exception should be defined to be thrown in APIs that cannot work with a value object.

Methods can be added to java.util.Objects to have a consistent way of checking for value objects and identity objects. For example,

public static <T> T requireValue(T o) {...}
public static <T> T requireIdentity(T o) {...}

public static <T> T requireValue(T o, String message) {...}
public static <T> T requireIdentity(T o, String message) {...}

Details TBD