CODEWITHSUNDEEP
kotlin
Rohan Prabhu
Rohan Prabhu

Reputation: 7302

Kotlin generate constructor that sets default values to nulled arguments

Let's take the class of a data class:

data class User(
    val userNumber: Int = -1,
    val name: String,
    val userGroups; List<String> = emptyList(),
    val screenName: String = "new-user"
)

When calling this function from Kotlin, it is pretty straightforward. I can simply use the named-argument syntax to do so. Calling from Java, I have to specify all values, or use the @JvmOverloads annotation, which generates the following constructors (in addition to the constructor that kotlin generates with the bit-mask for default values):

User(int userNumber, @NotNull String name, @NotNull List userGroups,
     @NotNull String screenName)
User(int userNumber, @NotNull String name, @NotNull List userGroups)
User(int userNumber, @NotNull String name)
User(@NotNull String name)

Now, if I want to create a User object in Java equivalent to User(name="John Doe", userGroups=listOf("admin", "super") I can't do it with the above constructors. I CAN however do it if I put val userNumber: Int = -1 at the end in the data class declaration (the generation of constructors seems to depend on the order the optional arguments are defined in). Which is fine, because expecting kotlin to generate all permutations is going to heavily bloat some classes.

The biggest problem that tools like Jackson simply don't work as they have no idea which constructor to use (and not like I can annotate one of the generated ones specially).

So, is there a way to generate a (single) constructor like:

User(Integer userNumber, String name, List<String> userGroups, String screenName) {
    this.userNumber = (userNumber == null) ? -1 : userNumber;
    this.userGroups = (userGroups == null) ? Collections.emptyList() : userGroups;
    //...
}

Currently I am using the above approach, but manually defining the constructors where I need them.

EDIT

I should clarify, creating a similar constructor doesn't work, obviously because both the signatures would clash on the JVM. This is what it would like in my case:

data class User(
    val userNumber: Int = -1,
    val name: String,
    val userGroups; List<String> = emptyList(),
    val screenName: String = "new-user"
) {
    companion object {
        @JvmStatic
        @JsonCreator
        fun constructionSupport(
            @JsonProperty("userNumber") userNumber : Int?,
            @JsonProperty("name") name : String,
            @JsonProperty("userGroups") userGroups : List<String>?,
            @JsonProperty("screenName") screenName : String?
        ) = User(
            userNumber = userNumber ?: -1,
            name = name,
            userGroups = userGroups ?: emptyList(),
            screenName = screenName ?: "new-user"
        )
    }
}

Also note the redundancy where I have to write the default values for the properties twice. I Now that I look at it, I doubt there exists a solution for this. Maybe this is a good use-case for a kapt based side-project of mine :)

Upvotes: 11

Views: 3419

Answers (2)

kurt
kurt

Reputation: 1550

Better solution is to add possibility to library understand Kotlin functional. For example, for Jackson exists jackson-module-kotlin. With this library we can use default arguments in data classes.

Example:

data class User(
        val userNumber: Int = -1,
        val name: String,
        val userGroups: List<String> = emptyList(),
        val screenName: String = "new-user"
)

fun main(args: Array<String>) {
    val objectMapper = ObjectMapper()
            .registerModule(KotlinModule())

    val testUser = User(userNumber = 5, name = "someName")

    val stringUser = objectMapper.writeValueAsString(testUser)
    println(stringUser)

    val parsedUser = objectMapper.readValue<User>(stringUser)
    println(parsedUser)

    assert(testUser == parsedUser) {
        println("something goes wrong")
    }
}

Upvotes: 1

smallstepstoday
smallstepstoday

Reputation: 180

After kicking this around for a minute, I think I found a solution that may work well here. Simply define a top level function in the same source file, that will build the object. Perhaps like so:

fun build_user(userNumber: Int?, name: String, userGroups: List<String>?, screenName: String?) : User {
  return User(if(userNumber !== null) userNumber else -1, name, if(userGroups !== null) userGroups else emptyList(),
    if(screenName !== null) screenName else "new-user")
}

Then when you need it, you simply call it from Java:

User user = UserKt.build_user(null, "Hello", null, "Porterhouse Steak");
System.out.println(user);

Output from the example:

User(userNumber=-1, name=Hello, userGroups=[], screenName=Porterhouse Steak)

The method is somewhere between a constructor and a builder. It beats hammering out a full-blown Builder object, and avoids cluttering your data class with unnecessary Java-interop glue code messiness.

See Package Level Functions for more information.

Upvotes: 0

Related Questions