Cannot access to BaseColumns provides _ID property in Kotlin

Question

I am defining my Users SQLite database table, and to do so, I have created the following UserContract and UserEntry classes:

class UserContract private constructor(){

    class UserEntry private constructor(): BaseColumns {

        companion object {
            val TABLE_NAME = "users"
            val COLUMN_DISPLAY_NAME = "display_name"
            val COLUMN_EMAIL = "email"
            //...
        }
    }
}

The problem I am facing is that I cannot access _ID property provided by BaseColums implementation:

val columnDisplayName = UserContract.UserEntry.COLUMN_DISPLAY_NAME //It is OK
val columnId = UserContract.UserEntry._ID //Unresolved reference: _ID

The equivalent code in Java works fine, so, does anybody knows what would be the reason or where the mistake is?

Answer 1

You can only access it using BaseColumns._ID since it's a Java interface defining a constant.

On Kotlin, a companion object is an actual object with inheritance, whereas in java a class with static methods does not really behave like an object.

For instance, if you had BaseColumns as a kotlin class instead of a java interface, you could have done something like:

open class KBaseColumns  {
    val _ID = "_id"
}

class UserContract private constructor(){
    class UserEntry private constructor(): BaseColumns {
        companion object : KBaseColumns() {
            val TABLE_NAME = "users"
            val COLUMN_DISPLAY_NAME = "display_name"
            val COLUMN_EMAIL = "email"
        }
    }
}

object Example {
    fun someMethod() {
        val id = UserContract.UserEntry._ID
    }
}

Where _ID is accessible in this case because the companion object is actually a subclass of KBaseColumns