Summary

Enhance the Java language with value classes and objects.

Problem

The JEP provides a high-level discussion motivating the feature.

In --enable-preview mode, the Java language needs the following new capabilities:

• A way to express, and validate, that a given class (abstract or concrete) has no dependency on identity
• Constraints on instance creation of these classes, so that final fields can never be observed to mutate
• Identity-free semantics for operations on reference types, including the == and != operators and the synchronized statement
• Automatically interpreting certain JDK classes to have no identity at compile time

Solution

Our solution enhances the Java language as follows:

• Introduce the value modifier for classes (not interfaces or enums, but including records); a value class is either abstract or implicitly final, and may not extend an identity class (other than Object); instance fields are implicitly final, and instance methods may not be synchronized
• Require that the instance fields of a value class be assigned before the super() constructor call; where this call is implicit, place it at the end, not the start, of a value class constructor body
• Specify when two value class instances are "the same", and so will successfully pass a == test; specify that a synchronized statement will throw when operating on a value class instance, and reject programs with synchronized statements operating on final value class types
• Designate certain JDK classes as value classes when in --enable-preview mode

Any class files that declare or use value classes are marked as preview-versioned class files. (This includes all classes that use, e.g., Integer or Optional.)

javac also adds new checks to its -Xlint:serial warnings feature, identifying value classes that implement Serializable but that serialization cannot support (see also JDK-8317279).

The javax.lang.model API and javadoc tool include trivial enhancements to surface the value modifier.

A more comprehensive outline of the solution can be found in the JDK-8317277 description.

Specification

JLS changes are included in the attached spec change document bundle. See the file specs/value-objects-jls.html.