Value-based Classes

# {@index "Value-based Classes"}

Some classes, such as `java.lang.Integer` and `java.time.LocalDate`, are value-based. A value-based class has the following properties:

- 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.