Project Valhalla brings value classes to Java JDK 28

Project Valhalla addresses the 'object tax' in Java, which refers to the overhead associated with every non-primitive object, including an object header, reference semantics, and mandatory heap allocation.

• Value classes mitigate this by eliminating object identity, removing the need for object headers, and allowing their data to be stored directly inline within arrays or other objects, rather than through references.
• The core optimizations for value objects are:
  • Heap Flattening: This technique stores value objects contiguously in memory, reducing memory footprint, improving data locality, and enhancing CPU cache efficiency.
  • Scalarization: A JIT compiler optimization where, for certain short-lived value objects, the compiler can eliminate the object allocation entirely and instead operate directly on the individual fields of the value.
• Value classes are declared using the value modifier (e.g., value class Point { int x; int y; }).
• For value objects, the == operator performs a state-wise comparison, checking if all corresponding field values are equal, as opposed to comparing memory addresses for identity objects.
• Value classes are designed to be immutable (their fields are implicitly final) and cannot support synchronization or traditional inheritance in the same manner as identity classes.
• Future phases of Project Valhalla aim to introduce 'primitive classes' to allow existing primitive wrapper types (e.g., java.lang.Integer) to be expressed as value classes, and 'universal generics' to enable generic types to work seamlessly with both primitive and value types without the performance penalty of boxing.