Return `null` from `Converter`

Currently, the Converter’s fromJson method must return Any, which precludes returning null values. However, I think there are legitimate situations in which fromJson should return null, in particular when the converter is used as a field converter.

For example, consider the following JSON:

[
  {
    "name": "Felix",
    "phone": "Nokia"
  },
  {
    "name": "Jake",
    "phone": ""
  }
]

and the following classes:

data class Person(
    val name: String,
    @PhoneAnnotation
    val phone: Phone?
)

data class Phone(val type: String)

In this case, it is not possible to use a field converter for the phone field. A workaround would be to use a converter for the entire Person class, but if Person is a class with, say, 20 fields, this would result in a lot of unnecessary code.

Am I perhaps overseeing another solution to this problem?

[cbeust]

I see. I guess it would make sense for toJson() to be nullable then, I’ll make the change.

[FWDekker]

I need an @JetBrains.com email address to enter the Slack, which I don’t have. I tried signing up here, but I didn’t receive an invite yet.

---

About my usage of the converter, I used the example from the README’s “Field conversion overriding” section. In addition to the code I gave in the issue description, I have the following:

@Target(AnnotationTarget.FIELD)
annotation class PhoneAnnotation

class PhoneConverter : Converter {
    override fun canConvert(cls: Class<*>) = cls == Phone::class.java

    override fun fromJson(jv: JsonValue) = Phone(jv.string!!)

    override fun toJson(value: Any) = "\"${(value as Phone).type}\""
}


fun main(args: Array<String>) {
    val persons = Klaxon()
        .fieldConverter(PhoneAnnotation::class, PhoneConverter())
        .parseArray<Person>(File("persons.json").inputStream())
}

If I change the canConvert function to always return false, nothing changes when I execute the main function.

My goal is to set the phone field of the Person object to null if the JSON’s phone property is an empty string, but because the fromJson function cannot return null it sets the phone field to Phone("") instead.