How does EK9 detect duplicate names and properties in type hierarchies?

← Code Quality · Ref: Q739

EK9 detects two categories of property naming collisions in type hierarchies at compile time.

DUPLICATE NAME (E02010)

A child type cannot redeclare a property with the same name as a parent property. This prevents field shadowing which causes ambiguity in method resolution and state management.

DUPLICATE PROPERTY FOR JSON (E02020)

When a type hierarchy has two properties with the same name at different levels and uses the JSON serialization operator ($$), the compiler cannot determine which to include. EK9 rejects this at compile time rather than producing ambiguous JSON output.

CORRECT PATTERNS

Use distinct names for properties at each level. If a child needs different data, use a different property name. If the parent property is sufficient, access it through inherited methods.

See Q93 for class definition. See Q97 for records vs classes.

Example

defines module qa.codequality.duplicatenames

  defines class

    Employee
      name <- String()
      role <- String()

      Employee()
        ->
          name as String
          role as String
        this.name: name
        this.role: role

      describe()
        <- rtn as String: `${name} (${role})`

      default operator

  defines program

    DuplicateNamesDemo()
      stdout <- Stdout()

      emp <- Employee("Alice", "Engineer")
      stdout.println(emp.describe())
      stdout.println(`Equal: ${emp == Employee("Alice", "Engineer")}`)

Common mistakes

E50060 — EK9 has no toString() method. Use string interpolation or the class's describe() method. See ek9 -h E50060 for details.

Incorrect:

stdout.println(emp.toString())

Correct:

stdout.println(emp.describe())

E50060 — Employee has no getName() method. Use describe() to get employee information. See ek9 -h E50060 for details.

Incorrect:

emp <- Employee("Alice", "Engineer").getName()

Correct:

emp <- Employee("Alice", "Engineer")

Coming from another language?

Java: field shadowing is allowed but produces warnings. Python: attribute shadowing is implicit. Rust: no inheritance, no shadowing. Go: embedding can shadow fields. Kotlin: property override requires explicit 'override' keyword. EK9: duplicate names rejected at compile time, no shadowing allowed.

Keywords: property, name, quality, E02020, shadow, duplicate, json, collision, hierarchy, E02010