Value-based Classes

A value-based class has the following properties:

• all instance fields declared in the class and all of its superclasses are final (though these may contain references to mutable objects);
• the class's implementations of equals, hashCode, and toString compute their results solely from the values of the class's instance fields (and the members of the objects they reference), not from the instance's identity;
• the class's methods treat instances as freely substitutable when equal, meaning that interchanging any two instances x and y that are equal according to equals() produces no visible change in the behavior of the class's methods;
• the class performs no synchronization using an instance's monitor;
• the class does not declare, or discourages use of, accessible constructors (because each constructor call promises a unique identity);
• the class does not provide any instance creation mechanism that promises a unique identity on each method call—in particular, any factory method's contract must allow for the possibility that if two independently-produced instances are equal according to equals(), they may also be equal according to ==;
• the class is final, and extends either Object or a hierarchy of abstract classes.

When two instances of a value-based class are equal (according to `equals`), a program should not attempt to distinguish between their identities, whether directly via reference equality == or indirectly via an appeal to synchronization, identity hashing, serialization, or any other identity-sensitive mechanism.

Synchronization on instances of value-based classes is strongly discouraged, because the programmer cannot guarantee exclusive ownership of the associated monitor.

When preview features are enabled, some value-based classes become value classes. Instances of these classes become value objects, which have different identity-related behaviors compared to identity objects:

• Previously distinguishable instances, such as those created from distinct constructor invocations (See JLS §15.9.4^PREVIEW), may be no longer distinguishable via ==.
• The use of value objects for synchronization or with object references results in IdentityException^PREVIEW.