Build a chained sealed hierarchy: Vehicle permits Car and Truck, Car permits Sedan and Hatchback.

← Sealed Types and Traits · Ref: Q1172

Chain 'allow only' at each level:

  Vehicle allow only Car, Truck, Sedan, Hatchback as open
  Car extends Vehicle allow only Sedan, Hatchback as open
  Sedan extends Car
  Truck extends Vehicle

Each sealed class lists ALL permitted descendants in its own 'allow only'. The compiler enforces at both levels.

See Q1171 for basic sealed class. See Q303 for chained sealing.

Example

defines module qa.sealedtraits.sealedclasshierarchy

  defines class

    Vehicle allow only Car, Truck, Sedan, Hatchback as open
      describe()
        <- rtn as String: "Vehicle"

    Car extends Vehicle allow only Sedan, Hatchback as open
      override describe()
        <- rtn as String: "Car"

    Sedan extends Car
      override describe()
        <- rtn as String: "Sedan"

    Hatchback extends Car
      override describe()
        <- rtn as String: "Hatchback"

    Truck extends Vehicle
      override describe()
        <- rtn as String: "Truck"

  defines program

    SealedClassHierarchyDemo()
      stdout <- Stdout()

      sedan <- Sedan()
      hatchback <- Hatchback()
      truck <- Truck()

      stdout.println(sedan.describe())
      stdout.println(hatchback.describe())
      stdout.println(truck.describe())

Common mistakes

E05240 — The top-level sealed class must list ALL descendants, including indirect ones (Sedan, Hatchback) — not just direct children.

Incorrect:

    Vehicle allow only Car, Truck as open

Correct:

    Vehicle allow only Car, Truck, Sedan, Hatchback as open

Other ways to ask this

Create a multi-level sealed class tree with transitive allow only
I need Vehicle sealed to Car/Truck, and Car itself sealed to Sedan/Hatchback
In Java 17 I'd chain sealed permits. Write the EK9 multi-level sealed hierarchy
Given a two-level hierarchy, seal both levels with their own allow only lists

Coming from another language?

Java 17: sealed class Vehicle permits Car, Truck; sealed class Car extends Vehicle permits Sedan. Kotlin: nested sealed classes. EK9: chained 'allow only' at each level.

Keywords: hierarchy, transitive, chained, allow only, multi-level, sealed