[Solved] I'm testing `java.nio` in Kotlin and I ha...

# announcements

[frank]

[Solved] I'm testing java.nio in Kotlin and I have a question:

Why don't work member references to function is this case? *I know that with Files.isDirectory(it) work i*t

val stream = Files.list(folder) 
val files  = stream.filter {  it != null && Files::isDirectory } // error in Files::isDirectory
                   .map    { it!! }

Error Msg:

Type mismatch.

Required: Boolean

Found: KFunction2<Path!, Array<(out) LinkOption!>!, Boolean>

PD: Sorry, if this topic doesn't have to be in this channel, but I didn't find JVM channel.

[jbnizet]

Because in an expression a && b, both a and b must be booleans. it != null is a boolean expression, but Files::isDirectory is not: it’s a function reference.

[jbnizet]

You could pass such a function reference to filter, because that’s what it expects, but that’s not what you’re doing. I.e. fileStream.filter(Files::isDirectory) would be corrrect.

[frank]

fileStream.filter(Files::isDirectory) would be corrrect.

@jbnizet , Work it this but I can't obtain it in the scope of filter() function if I add Predicate inside of filter I can obtain it but fail Files::isDirectory .

val stream = Files.list(folder)
val files =  stream.filter(Files::isDirectory)//I cant obtain it
                    map   { it!! }.toList() 
//Files::isDirectory fail in predicate
files =  stream.filter(Predicate { it != null && Files::isDirectory  })
                               .map    { it!! }.toList()

[jbnizet]

Well, you’re still trying to combine, with &&, a boolean and a function. Files::isDirectory is not a boolean. It’s a function which returns a boolean. To get a boolean, you need to call the function: it != null && Files.isDirectory(it). When using stream.filter(Files::isDirectory), it’s correct because filter expects a function which takes a Path and returns a boolean as argument, and Files::isDirectory is such a function.

[jbnizet]

Note that testing if a file returned by Files.list() is null is useless: this methods returns instances of Path. Why would there be a null in there?

[frank]

Files.list() returns me Stream<Path!> and Path! means Path or Path?

[jbnizet]

Yes. So you need to then read the documentation of the method (and think about what that function does), to know if there is a chance to have a null in that stream. Why would a method that returns the files in a directory would add nulls into that list?

[frank]

@jbnizet Ok, Thx for you help. I checked it