Do you dream of "value types" in Java? So do I! Hence I was thrilled to see that Project Valhalla is slowly coming out of exploration with two draft JEPs. Here's what they currently propose. 🧵👇🏾 (If you prefer video: https:/www.youtube.com/watch?v=WBvTilbh8S0&t=344s) 1/10

First, why value types? To bridge the divide between * primitives (fast, no memory overhead) and * reference types (abstraction, safety, identity) As is, we sometimes have to choose between performance and design. And we often choose wrongly. 2/10

Draft JEP openjdk.java.net/jeps/8251554 proposes new terminology and JVM rules: 1. Interface `IdentityObject` (the boring part): * for reference types * called "identity classes" * instances are "identity objects" For identity classes / reference types everything stays as is. 3/10

2. Interface `PrimitiveObject` (the exciting part): * for a new kind of type * called "primitive classes" * instances are "primitive objects" "JVMs may freely duplicate and re-encode them in an effort to improve computation time, memory footprint, and garbage collection" 4/10

Primitive classes want to make good on Valhalla's promise: "Codes like a class, works like an int." See strawman code attached. Details: * class and fields are implicitly final * no circular dependency on itself * no synchronized methods * more in the JEP 5/10

A bit more terminology on primitive classes: * `Point p` is a "primitive value" * `Point.ref p` is a "primitive reference" Primitive values will be like today's primitives (no `null`, no memory indirection/overhead), primitive references like their wrapper types. 6/10

But this doesn't cleanly map to today's situation because if `Integer` were a primitive class, `Integer i` would be a non-nullable primitive value and `Integer.ref i` the primitive reference. To fix that mismatch, the JEP proposes "reference-favoring primitive classes"… 7/10

@nipafx This feels better and more natural than what comes in the next tweet.