Value-based Classes

• the class declares only final instance fields (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 has deprecated any) accessible constructors;
• 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 that declare no instance fields or instance initializers and whose constructors are empty.

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.

Identity-related behavior of value-based classes may change in a future release. For example, synchronization may fail.