The Evolution of Java's Value Classes: A Look at Q-types and v-bytecodes

2023/08/19

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The article explores the evolution of Java's value classes and the introduction of Q-types and v-bytecodes in the Valhalla JVM prototype. These features were initially implemented to support the new kind of class introduced by Valhalla, called value classes. However, recent optimization techniques have shown that struct-like values and their operations can be expressed as normal Java classes without sacrificing performance. As a result, the need for Q-types and v-bytecodes has diminished. The article discusses the removal of these features and the resulting adjusted VM design. This change not only simplifies the Java VM specification but also improves migration compatibility. Existing classes, such as Integer and Optional, can be migrated to value classes without requiring significant changes to client code. This evolution in Java's value classes opens up new possibilities for developers and enhances the language's performance and flexibility.