Creating an email – the Kotlin way – second attempt

Let's try a fluent setter approach, instead. We'll have only mandatory fields in our constructor, and all of the others will become setters. So to create a new email, we no longer need to do the following:

   val mail = Mail("manager@company.com") 
   mail.title("Ping") 
   mail.cc(listOf<String>()) 

Instead, we will do the following:

Mail("manager@company.com").title("Ping").cc(listOf<String>())

Fluent setters allow us to chain one set call to another.

That's a lot nicer for a couple of reasons:

• The order of fields can now be arbitrary, unlike the order used with the constructor.
• It's clearer which field is being set; no more need for comments.
• Optional fields don't need to be set at all. As an example, the CC field is set while the BCC field is omitted.

Let's look at one way of implementing this approach. There are other convenient ways to do this, which we'll discuss in Chapter 10, Idioms and Anti-Patterns:

   data class Mail(// Stays the same 
                   private var _message: String = "", 
                   // ...) { 
       fun message(message: String) : Mail { 
           _message = message 
return this } 
       // Pattern repeats for every other variable 
   } 

Using underscores for private variables is a common convention in Kotlin. It allows us to avoid repeating the phrase this.message = message and mistakes, such as message = message.

This is nice and is very similar to what we may achieve in Java, although we did have to make our message mutable.

We can also implement a full-blown builder design pattern, of course:

class MailBuilder(val to: String) { 
    private var mail: Mail = Mail(to) 
    fun title(title: String): MailBuilder { 
        mail.title = title 
        return this 
    }
    // Repeated for other properties 
    fun build(): Mail { 
        return mail 
    } 
} 

You can use it to create your email in the following way:

val email = MailBuilder("hello@hello.com").title("What's up?").build()

But Kotlin provides two other ways that you may find even more useful.