How and when to use Kotlin sealed classes in Java?

Question

Thinking about misusing sealed classes, let's look at the following design.

There are two modules:

• parser (Kotlin) - is responsible for making instances from String
• processor (Java) - pumps over raw incoming data into a strongly typed storage (i.e. relational tables)

1. String comes from external source to processor
2. processor delegates its recognition to parser
3. parser makes instances of different types [Banana, Nail, Shoe] based on some rules X
4. processor persists each instance into an appropriate table based on some rules Y

Is it appropriate to use sealed classes here like this in parser and after that in processor make decisions base on concrete type of each instance?

// parser module exposes Item and its subclasses

sealed interface Item {
    class Banana(/*state 1*/) : Item
    class Nail(/*state 2*/) : Item
    class Shoe(/*state 3*/) : Item
}

fun parse(value: String, rule: ParseRule): Item {
    return when (true) {
        rule.canParseBanana(value) -> rule.makeBananaFrom(value)
        rule.canParseNail(value) -> rule.makeNailFrom(value)
        rule.canParseShoe(value) -> rule.makeShoeFrom(value)
        else -> throw RuntimeException("cannot parse")
    }
}    

// processor module makes decisions based on class 

void process(String value){
  Item item = parser.parse(value);

  if (item instance of Item.Banana){
    persistBanana((Item.Banana) item)
  } else if ( ... )
    // etc         
  } else {
     throw new RuntimeException("Unknown subclass of Item : " + item.getClass())
  }
}

I see that something is wrong in this approach, because growing number of Item's subclasses could lead to a design catastrophe, but cannot figure out whether is there a "canonical" use case of sealed classes sufficiently different from this one.

What is the limit of sealed classes applicability, when system designer should prefer something "less typed" like the following:

class Item{
  Object marker; // String or Enum
  Map<String, Object> attributes;
}

// basically it is the same, but without dancing with types
void process(String value){
    Item item = parser.parse(value);

    if ("BANANA".equals(item.marker)){
      persistBanana(item.attributes)
    } else if (...){
      // etc
    }
}

Answer 1

You can use the visitor pattern to provide a when..is style approach to Java.

// Kotlin
abstract class ItemVisitor<OUT> {
    operator fun invoke(item: Item) = when (item) {
        is Banana -> visitBanana(item)
        is Shoe -> visitShoe(item)
        is Nail -> visitNail(item)
    }
    abstract fun visitBanana(item: Banana): OUT
    abstract fun visitShoe(item: Shoe): OUT
    abstract fun visitNail(item: Nail): OUT
}

Because Item is sealed you don't need the else case in that when and then Java code can create a visitor instead of doing its own instanceof checks with an else, and if you add a variant, you add a method and get reminded that your Java implementations of the visitor need the new method.

Answer 2

If you choose to use sealed classes you take it upon yourself to extend the hierarchy whenever a new subtype is introduced. Given that you have a subtype specific rules and persistence I am not sure how much you can do to avoid this.

Having said that, it's a shame to do instanceOf after you already determined the item type. The code below is more or less visitor pattern(this is disputed, see comments). I assume the separation to Java and Kotlin is required.

// Kotlin
sealed interface Item {
    class Banana(/*state 1*/) : Item
    class Nail(/*state 2*/) : Item
    class Shoe(/*state 3*/) : Item
}

class Parser(val persistor: Persistor, val rule: ParseRule) {
    fun parse(value: String): Item =
        when (true) {
            rule.canParseBanana(value) -> rule.makeBananaFrom(value).also { persistor.persist(it) }
            rule.canParseNail(value) -> rule.makeNailFrom(value).also { persistor.persist(it) }
            rule.canParseShoe(value) -> rule.makeShoeFrom(value).also { persistor.persist(it) }
            else -> throw RuntimeException("cannot parse: $value")
        }
}

class ParseRule {
    fun canParseBanana(value: String): Boolean = ...
    fun makeBananaFrom(value: String): Item.Banana = ...
    ...
}

// Java
public class Persistor {
    void persist(Item.Banana item) {
        System.out.println("persisting " + item);
    }
    void persist(Item.Shoe item) {
        System.out.println("persisting " + item);
    }
    void persist(Item.Nail item) {
        System.out.println("persisting " + item);
    }
}


public class Processor {
    private final Parser parser;

    public Processor(Parser parser) {
        this.parser = parser;
    }

    void process(String value) {
        parser.parse(value);
    }
    
    public static void main(String[] args) {
        Parser parser = new Parser(new Persistor(), new ParseRule());
        Processor p = new Processor(parser);
        p.process("banana");
        p.process("nail");
        p.process("zap");
    }
}