How to convert Array in Kotlin

问题:

I'm taking my first steps in Kotlin, and trying to write a simple string split function. I started with this:

fun splitCSV(s : String) : Array<String> {
    return s.split(",");
}

Which I guess can be also written like this:

fun splitCSV(s : String) : Array<String> = s.split(",");

But I'm getting a type error, since s.split returns an Array<String?>? and not Array<String>. I couldn't find a simple way to do a cast, so I wrote this function to do the conversion:

fun forceNotNull<T>(a : Array<T?>?) : Array<T> {
    return Array<T>(a!!.size, { i -> a!![i]!! });
}

fun splitCSV(s : String) : Array<String> = forceNotNull(s.split(","));

However, now I'm getting a runtime error:

ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.String

If I change T in forceNotNull to String, then it works, so I guess I'm close to a solution.

Is this the right way to go about it? And if it is, how can I fix forceNotNull to work in the generic case?

回答1:

Not sure it's the best method, but this seems to work:

fun splitCSV(s : String) : Array<String> {
  return ( s.split(",") as? Array<String>? ).sure() ;
}

Although IntelliJ highlights the as? with "This cast can never succeed"... So my initial optimism is fading

Oddly though, it seems to work...

As does:

fun splitCSV(s : String) : Array<String> {
  return s.split(",").sure() as Array<String> ;
}

But with the same warning... I'm getting confused, so I'll stop now :-/

Edit

Of course, you can get it to work with List<String>:

import java.util.List

fun splitCSV(s : String) : List<String> {
  return s.split(",")!!.map<String?,String> { it!! }
}

but that wasn't the question ;-)