Comment by vips7L

4 months ago

Yes via sealed classes. It also has pattern matching.

5 comments

vips7L

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

weksf54 3 months ago

You could use Java records to make things more concise:

  record Truck(int loadCapacity) implements Vehicle {}
  record Car(int numberOfSeats, String brandName) implements Vehicle {}
  sealed interface Vehicle permits Car, Truck {}

Tainnor 3 months ago

Scala 3 has:

  enum Vehicle:
    case Truck(loadCapacity: Int)
    case Car(numberOfSets: Int, brandName: String)

vips7L 3 months ago

You implemented this much more verbosely than needed

    sealed interface Vehicle {
        record Truck(int loadCapacity) implements Vehicle {}
        record Car(int numberOfSeats, String brandName) implements Vehicle {}
    }

weksf54 3 months ago

Ah! Thanks, I didn't know that. I should have RTFMD better - https://docs.oracle.com/en/java/javase/21/language/sealed-cl...

Turns out you can do this and not have the annoying inner class e.g. Vehicle.Car too:

  package com.example.vehicles;

  public sealed interface Vehicle
      // The permits clause has been omitted
      // as its permitted classes have been
      // defined in the same file.
  { }
  record Truck(int loadCapacity) implements Vehicle {}
  record Car(int numberOfSeats, String brandName) implements Vehicle {}