How to instantiate an object using default constructor values ​​in Kotlin?

Question

I have a data class with default values.

data class Project(
    val code: String,
    val name: String,
    val categories: List<String> = emptyList())

Java reflection cannot create an instance of a class if some values ​​are null. I get an exception

java.lang.IllegalArgumentException: Parameter specified as non-null is null: method Project.<init>, parameter categories

This is because the method java.lang.reflect.Constructor<T>.instantiateClassexpects non-null arguments.

I have type information, I have a constructor definition, but I'm not sure how to call the constructor to use the default values ​​(the values ​​come from the database, categoriesmaybe null), is there a way to achieve this in Kotlin?

Answer 1

Kotlin Java, , , Kotlin , , , .

.

• . , callBy , .

• @JvmOverloads , Java.

  @JvmOverloads
  data class Project(
      val code: String,
      val name: String,
      val categories: List<String> = emptyList()
  )
  

Answer 2

null , . https://kotlinlang.org/docs/reference/functions.html#default-arguments

, NULL. a ? , , , , .

https://kotlinlang.org/docs/reference/null-safety.html

Answer 3

Kotlin:

, . , . ,

data class Bird (val name: String = "peacock", val gender: String = "male")

Bird(), Bird ( "" ) Bird (gender = "female" ).

, ? . ,

data class Project(val code: String,
                   val name: String,
                   val categories: List<String>?)

emptyList() . emptyList, , null , ,

val project = if(categories == null)
       {
          Project(code,name)
       }
       else
       {
          Project(code,name,categories)
       }

kotlin.

JAVA:

java, @Hotkey , , kotlin , .

, java, @JvmOverloads, , @Hotkey ,

data class Project @JvmOverloads constructor(val code: String,
                                             val name: String,
                                             val categories: List<String>? = emptyList())