target-java.md

Java Target (--java)

Transpiles Oak/Magnolia source to Java 17+.

Usage

magnolia build --entry src/main.oak --output OakBundle.java --java

Output

A single OakBundle.java file containing:

• Runtime helpers — __oak_eq, __oak_push, __oak_acc, __as_oak_string, module system (__oak_modularize, __oak_module_import)
• Transpiled AST — Oak expressions mapped to Java equivalents inside public static void main

Design Decisions

• All values are Object: Oak is dynamically typed; Java uses Object with runtime casts.
• Functions as Function<Object[], Object>: Oak functions become Java lambdas accepting an Object[] and returning Object.
• Maps for objects: Oak objects become HashMap<String, Object>.
• Lists: Oak lists become ArrayList<Object>.
• Pattern matching via if-chains: Oak if expressions with pattern branches translate to if/else if chains comparing with __oak_eq.
• Identifier mangling: Java reserved words are prefixed with __oak_java_ (e.g., class → __oak_java_class). ? → _qm, ! → _exclam.

Limitations

• No type inference — everything is Object with casts.
• Arithmetic requires Number casts at runtime.
• Closures work via Java lambda capture but mutable state requires workarounds.
• String concatenation uses __as_oak_string helper for type coercion.
• Large Oak programs may hit Java's method size limit; future work may split into multiple methods.

Requirements

• Java 17+ (uses Map.of, enhanced switch, text blocks-compatible style)
• No external dependencies