Support interfaces as TypeDeclaration

[mhalbritter]

Right now, TypeDeclaration only supports classes. It would be nice if we could also support interfaces.

[anna-megalou]

I’d like to work on this.

I reviewed the earlier attempt (#1632) and agree this should be a small model-and-writer change across Java, Kotlin, and Groovy, rather than a Java-only tweak that just omits the class keyword.

Proposed approach

Goal

Support interface declarations in generated Java, Kotlin, and Groovy with language-correct inheritance syntax. For example, Java interfaces should use extends for super-interfaces, not implements.

Model

Introduce a TypeKind on TypeDeclaration:

• CLASS - default, preserves current behavior
• INTERFACE - new

For INTERFACE:

• extends is unused and must be null
• the existing implements list is reused to represent declared super-interfaces

This avoids adding new collections to TypeDeclaration and keeps the change backward-compatible for existing callers.

Emission mapping would be:

• Java: interface X extends A, B
• Kotlin: interface X : A, B (dedicated branch: no () after super-types, unlike the class/superclass case)
• Groovy: interface X extends A, B

The name implements is not ideal for interfaces, if needed, a clearer name such as superTypes could be handled in a separate follow-up to keep this change small.

Writers

Update the Java, Kotlin, and Groovy writers to:

• emit interface instead of class when kind == INTERFACE
• map the super-interface list to the correct keyword/syntax for each language
• apply only modifiers that make sense for interfaces, or clearly document the supported constraints
• leave class generation unchanged

For a first iteration, I would keep interface bodies minimal and avoid emitting instance fields on interfaces, or reject them, so unsupported semantics are not modeled by accident.

Tests

Add cases next to the existing writer tests for:

• empty interface
• interface with multiple super-interfaces
• regression coverage showing normal class generation remains unchanged

Planned order

1. TypeDeclaration + TypeKind
2. Java writer + tests
3. Kotlin writer + tests
4. Groovy writer + tests