is there a way to impl a custom interface for an final class say "String"?

You mean make the existing

String

class implement your custom interface? This is not possible at the moment

OK. I just want to pass String to a fun that accept a custom Interface as param

Value classes will get you close, but it's not exactly what you want

Then maybe you can wrap that string into a custom class (possibly value class as Sam mentioned) that implements that interface. What does the interface look like?

and I found a discuss 'https://discuss.kotlinlang.org/t/implementing-an-interface-for-an-existing-type/20967/13' here

I don't quite understand your point. If you're defining the parameter type to be this interface (and not string), isn't it the whole point to forbid other types like

String

?

Main idea is I want accept

Selectable

interface as param. Assume Class

Column

impl

Selectable

, so I can pass Column to the fun. Like SQL

SELECT Selectable

. so I want to let

String

Impl

Selectable

for some more common way to select.

For those cases you could probably create your own value class to wrap a string. It could even use an

init

block to perform validation on the string, because surely not any arbitrary string should be considered a

Selectable

, right?

Yes. @Joffrey you're right. I will try value class. and do more validation. correct is more important than simple.

It will still be pretty simple to call, btw:

yourFunction(Sql("SELECT Selectable"))

(if we define

value class Sql(val text: String) : Selectable

)

I would have thought

Selectable

is anything that can be a parameter to

SELECT

- such as a column name or, indeed, any String literal. After all, both

SELECT mycolumn FROM mytable

and

SELECT 'any arbitrary string'

are both valid statements.

My bad, when King River Lee said "Like SQL `SELECT Selectable`" I didn't think twice and just took the whole thing at face value. But you're right, most likely only the part after

SELECT

is the string that is passed to the function. Nevertheless, I still believe it's worth using a type-safe column/selectable type for this, and maybe a simple string wrapper for a column name:

yourFunction(ColumnName("Selectable"))

@Klitos Kyriacou is my main idea. that any thing impl

Selectable

can be param of

myFun(selectable: Selectable)