Returning one of different object types from single function in kotlin - kotlin

Question

I have the following structure at present:
#Entity
#Table(name = "table_app_settings")
data class AppSetting(
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
#Column(name = "app_setting_id")
val id: Long? = null,
#Column(name = "app_setting_name")
val name: String = "",
#Column(name = "app_setting_value")
var value: String = "",
#Column(name = "app_setting_type")
val type: AppSettingType,
)
enum class AppSettingType {
CHAR,
STRING,
BYTE,
SHORT,
INT,
LONG,
DOUBLE,
FLOAT,
BOOLEAN,
}
This is then saved to the database with the following:
override fun saveAppSetting(setting: AppSetting): DatabaseResult<AppSetting> {
log.info("Saving App Setting ${setting.name} to database.")
return try {
// Attempt to save the entity to the database. If we do not throw an exception, return success.
val savedSetting = appSettingsRepository.save(setting)
DatabaseResult(
code = ResultCode.CREATION_SUCCESS,
entity = savedSetting
)
} catch(exception: DataAccessException) {
log.error("Unable to save App Setting ${setting.name} to database. Reason: ${exception.message}")
DatabaseResult(
code = ResultCode.CREATION_FAILURE
)
}
}
Now, let's say that I wish to save a Char type to database, I figure I would use the following:
override fun saveAppSetting(name: String, value: Char): DatabaseResult<Char> {
val appSettingResult = saveAppSetting(AppSetting(
name = name,
value = value.toString(),
type = AppSettingType.CHAR,
))
return if(appSettingResult.code != ResultCode.CREATION_FAILURE) {
val entity = getAppSetting<Char>(appSettingResult.entity?.name!!).entity.toString().first()
DatabaseResult(
code = appSettingResult.code,
entity = entity
)
} else {
DatabaseResult(
code = ResultCode.CREATION_FAILURE,
)
}
}
I also figured that I would need to do the following in order to retrieve the correct object type:
override fun getAppSetting(name: String): DatabaseResult<Any?> {
log.info("Getting App Setting $name from database.")
val appSetting = appSettingsRepository.findAppSettingByName(name)
return if(appSetting != null) {
log.info("App Setting $name has ID of ${appSetting.id} within the database")
when(appSetting.type) {
AppSettingType.CHAR -> {
DatabaseResult<Char>(
code = ResultCode.FETCH_SUCCESS,
entity = appSetting.value.first(),
)
}
AppSettingType.STRING -> {
DatabaseResult<String>(
code = ResultCode.FETCH_SUCCESS,
entity = appSetting.value,
)
}
AppSettingType.BYTE -> {
DatabaseResult<Byte>(
code = ResultCode.FETCH_SUCCESS,
entity = appSetting.value.toByte(),
)
}
AppSettingType.SHORT -> {
DatabaseResult<Short>(
code = ResultCode.FETCH_SUCCESS,
entity = appSetting.value.toShort(),
)
}
AppSettingType.INT -> {
DatabaseResult<Int>(
code = ResultCode.FETCH_SUCCESS,
entity = appSetting.value.toInt(),
)
}
AppSettingType.LONG -> {
DatabaseResult<Long>(
code = ResultCode.FETCH_SUCCESS,
entity = appSetting.value.toLong(),
)
}
AppSettingType.DOUBLE -> {
DatabaseResult<Double>(
code = ResultCode.FETCH_SUCCESS,
entity = appSetting.value.toDouble(),
)
}
AppSettingType.FLOAT -> {
DatabaseResult<Float>(
code = ResultCode.FETCH_SUCCESS,
entity = appSetting.value.toFloat()
)
}
AppSettingType.BOOLEAN -> {
DatabaseResult<Boolean>(
code = ResultCode.FETCH_SUCCESS,
entity = appSetting.value.toBoolean()
)
}
}
} else {
log.error("App Setting $name does not seem to exist within the database.")
DatabaseResult(
code = ResultCode.FETCH_FAILURE
)
}
However, when I then wish to use said object, I still have to write something like the following:
val newBarcode = getAppSetting("barcode_value").entity.toString().toInt()
Assuming I've "initialised" barcode_value with a value of 177 (for example).
How can I get the function to return what I need without having to do .toString.to...()?

Answer 1

Yes this all possible, here is a simplified demo, firstly
import kotlin.reflect.KClass
data class AppSetting(
val id: Long? = null,
val name: String = "",
var value: String = "",
val type: AppSettingType,
)
enum class AppSettingType(val clazz: KClass<out Any>) {
CHAR(Char::class),
STRING(String::class),
INT(Int::class),
}
So I added a clazz so from the enum we know the Kotlin type
and now a function to simulate your repository fetch
fun findAppSettingByName(name: String): AppSetting? {
return when(name) {
"Char thing" -> AppSetting(value= "C", type = AppSettingType.CHAR)
"String thing" -> AppSetting(value= "Str", type = AppSettingType.STRING)
"Int thing" -> AppSetting(value= "42", type = AppSettingType.INT)
else -> throw IllegalArgumentException()
}
}
Next in the function declaration I have made it generic with T and for the purposes of the demo removed the DatabaseResult container. Then I added a clazz parameter which is the typical Java way of carrying the required class information into the function:
fun <T : Any> getAppSetting(name: String, clazz: KClass<T>): T? {
val appSetting: AppSetting? = findAppSettingByName(name)
return appSetting?.let {
require(clazz == appSetting.type.clazz) {
"appSetting.type=${appSetting.type.clazz} mismatched with requested class=${clazz}"
}
when (appSetting.type) {
AppSettingType.CHAR -> appSetting.value.first()
AppSettingType.STRING -> appSetting.value
AppSettingType.INT -> appSetting.value.toInt()
} as T
}
}
the as T is important to cast the values into the required return type - this is unchecked but the when() clause should be creating the correct types.
Now let's test it:
val c1: Char? = getAppSetting("Char thing", Char::class)
val s1: String? = getAppSetting("String thing", String::class)
val i1: Int? = getAppSetting("Int thing", Int::class)
println("c1=$c1 s1=$s1 i1=$i1")
val c2: Char? = getAppSetting("Char thing")
val s2: String? = getAppSetting("String thing")
val i2: Int? = getAppSetting("Int thing")
println("c2=$c2 s2=$s2 i2=$i2")
}
The output is
c1=C s1=Str i1=42
c2=C s2=Str i2=42
But how do c2/s2/i2 work, the final part is this function
inline fun <reified T : Any> getAppSetting(name: String) = getAppSetting(name, T::class)
This is reified generic parameters... there is no need to pass the clazz because this can be found from the data type of the receiving variable.
There are many articles about this advanced topic, e.g.
https://typealias.com/guides/getting-real-with-reified-type-parameters/
https://medium.com/kotlin-thursdays/introduction-to-kotlin-generics-reified-generic-parameters-7643f53ba513
Now, I didn't completely answer what you wanted because you wanted to receive a DatabaseResult<T> wrapper. What might be possible, is to have a function that returns DatabaseResult<T> and you can obtain the T from it as the "clazz" parameter, but I'll leave that for someone else to improve on :-) but I think that gets you pretty close.