Why does EK9 reject duplicate properties in records with JSON support?

← Code Quality · Ref: Q829

When a record uses 'default operator' to auto-generate operators including $$ (toJSON), EK9 requires all property names to be unique across the inheritance hierarchy. Duplicate property names would produce ambiguous JSON keys.

THE RULE

If a child record redeclares a parent property AND uses 'default operator':

  ParentRecord
    name as String
    default operator
  ChildRecord extends ParentRecord
    name as String       // ERROR: E02020 — ambiguous JSON key
    default operator

WHY THIS MATTERS

The $$ operator generates JSON like {"name": "value"}. If both parent and child have 'name', the JSON would have duplicate keys — which is undefined behaviour in JSON.

HOW TO FIX

- Use a different name in the child: childName as String
- Or manually implement the $$ operator instead of using default

See Q819 for duplicate property basics. See Q97 for records.

Example

defines module qa.quality.duplicate.json

  defines record

    BaseConfig as open
      label as String: String()
      default operator ?
      default operator $

    ChildConfig extends BaseConfig
      childLabel as String: String()
      default operator ?
      default operator $

  defines program

    DuplicateJsonDemo()
      stdout <- Stdout()
      child <- ChildConfig()
      stdout.println($child)

Common mistakes

E02010 — The parent BaseConfig already has 'label'. Redeclaring it in a child creates a duplicate property. Use a different name. See ek9 -h E02010 for details.

Incorrect:

    ChildConfig extends BaseConfig
      label as String: String()

Correct:

    ChildConfig extends BaseConfig
      childLabel as String: String()

Coming from another language?

Java: Jackson serializes duplicate fields from parent+child creating ambiguous JSON. Python: no enforcement. Rust: no inheritance. Go: embedded struct fields can shadow. EK9: compile-time error prevents ambiguous JSON serialization.

Keywords: property, duplicate, json, E02020, record, serialization, toJson