当我尝试在操场上运行时,为什么我会在第16行(下面的最后一行)收到错误“join is unavailable：call the joinWithSeparator()”？而且,我该如何解决它？
class Person {
var firstName: String?
var lastName: String?
let gender = "female"
func fullName() -> String {
var parts: [String] = []
if let firstName = self.firstName {
parts += [firstName]
}
if let lastName = self.lastName {
parts += [lastName]
}
return " ".join(parts)
}
}
