ExampleShark.swift

class ExampleShark {
    var name:NSString!
    var howSwim:NSString!
    
    init(name:NSString!, howSwim:NSString!) {
        self.name = name
        self.howSwim = howSwim;
    }
    
    func swim() -> String {
        return "\(self.name) swims like '\(self.howSwim)'"
    }
    func dangerousSwim() -> String {
        return "\(self.name.description) swims like '\(self.howSwim.description)'"
    }
    func safeSwim() -> String {
        return "\(self.name?.description) swims like '\(self.howSwim?.description)'"
    }
}
 
let shark1 = ExampleShark(name: "Shark", howSwim: nil)
shark1.swim()           // OK, "Shark swims like '{nil}'"
shark1.dangerousSwim()  // CRASH
shark1.safeSwim()       // OK, "Shark swims like '{nil}'"

let shark2 = ExampleShark(name: nil, howSwim: nil)
shark2.swim()           // OK, ""{nil} swims like '{nil}'"
shark2.dangerousSwim()  // CRASH
shark2.safeSwim()       // OK, ""{nil} swims like '{nil}'"
 
let shark3 = ExampleShark(name: "The Average Shark", howSwim: "Swim Swim Swim Lurk")
shark3.swim()           // OK, "The Average Shark swims like 'Swim Swim Swim Lurk'"
shark3.dangerousSwim()  // OK, "The Average Shark swims like 'Swim Swim Swim Lurk'"
shark3.safeSwim()       // OK, "The Average Shark swims like 'Swim Swim Swim Lurk'"