How to convert a missing list field into Nil?

[grant-guo]

I have the following demo codes:

case class Person(
  @JsonProperty("first") first:String,
  @JsonProperty("last") last:String,
  @JsonProperty("phones") phones: Seq[String],
  @JsonProperty("address") address:Option[String],
  @JsonProperty("cars") cars: Seq[String]
)

  val mapper = new ObjectMapper()
  mapper.registerModule(DefaultScalaModule)

  val json =
    """
      {
        "first":"grant",
        "last":"guo",
        "phones":["416", "647"],
        "address":"abc"
      }
    """.stripMargin

  val person = mapper.readValue[Person](json, classOf[Person])

In the parsed person instance, cars field is a Java null, is there any way to convert it into Nil directly?

[nbauernfeind]

I'm not sure if there's a solution built-in to jackson that would treat a missing entry as an empty list. Technically a null object, empty list, and non-empty list are different scenarios one might want to detect based on the inputted json.

Work arounds you can try (I'm not sure if all will work):

• rename cars to _cars for the variable and add a getter that converts null's to empty Seq's
• use a JsonCreator that does this once at construction time
• convert the Seq[String] to an Option[Seq[String]] so you know it is optionally provided.

[Phil4]

I have the exact same issue.
It seems strange to want to handle null values in Scala immutable collections.
Anyway, as workaround I used a var instead of a val:

case class Person(
  ...
  @JsonProperty("phones") var phones: Seq[String],
  ...
) {
  phones = if(phones == null) Nil else phones
}

[pjfanning]

merging with #462