Comment by tonyedgecombe

So they are there, but ugly to define:

    public abstract sealed class Vehicle permits Car, Truck {
      public Vehicle() {}
    }

    public final class Truck extends Vehicle implements Service {
      public final int loadCapacity;

      public Truck(int loadCapacity) {
        this.loadCapacity = loadCapacity;
      }
    }

    public non-sealed class Car extends Vehicle implements Service {
      public final int numberOfSeats;
      public final String brandName;

      public Car(int numberOfSeats, String brandName) {
        this.numberOfSeats = numberOfSeats;
        this.brandName = brandName;
      }
    }

In Kotlin it's a bit better, but nothing beats the ML-like langs (and Rust/ReScript/etc):

    type Truck = { loadCapacity: int }
    type Car = { numberOfSeats: int, brandName: string }
    type Vehicle = Truck | Car