Could someone explain to my why this does not work...

# getting-started

[Gustav Elmgren]

Could someone explain to my why this does not work?

class Test<T : Comparable<T?>>(
    val value: T
)

val a = Test<String?>("Hello")
val b = Test<String>("Hello")

//Neither of the above works

I want to create a class that can both handle String and String? as the type argument. And if I understand correct, String is a sub type of String?.

If I instead make it non-nullable:

class Test<T : Comparable<T>>(
    val value: T
)

val a = Test<String?>("Hello")
val b = Test<String>("Hello")

//Now b works atleast, but not a.

[Joffrey]

String is not a Comparable<String?> and String? is not comparable at all

[Gustav Elmgren]

But if I do:

val test: String? = null
val a = test == "hello"

The == operator seems to be the equals method in public class String : Comparable<String>, CharSequence

[Adam S]

Do you want to make T nullable, or the value?

For example:

class Test<T : Comparable<T>>(
  val value: T?
)

val a = Test<String>("Hello")
val b = Test<String>(null)

[Gustav Elmgren]

I want to make T nullable, so Test<String?> should work as well as Test<String>

[Adam S]

can you say more about why you want that? It's not clear to me what that would help with :)

[Gustav Elmgren]

It's a long story... I'll try to make it short. I'm wrapping exposed columns in own made wrapper. So I need to be able to support ExpressionWithColumnType<T> as well as ExpressionWithColumnType<T?> .

[Gustav Elmgren]

For example:

object MyTable : LongIdTable(name = "MyTable") {
    val nonNullable = varchar("nonNullable", 255)
    val nullable = varchar("nullable", 255).nullable()
}

class Test<T : Comparable<T?>>(
    expr: ExpressionWithColumnType<T>
)

val a = Test(MyTable.nonNullable)
val b = Test(MyTable.nullable)

[Gustav Elmgren]

Ah, I see. I looked on how exposed did it and they had two type parameters. Like so:

object MyTable : LongIdTable(name = "MyTable") {
    val nonNullable = varchar("nonNullable", 255)
    val nullable = varchar("nullable", 255).nullable()
}

class Test<T : Comparable<T>, S : T?>(
    expr: ExpressionWithColumnType<in S>
)

val a = Test(MyTable.nonNullable)
val b = Test(MyTable.nullable)