Creating an email – first attempt

So, at 10 A.M., I plan to drink a coffee in my local cafe. But I also want to contact my manager, since my payslip didn't arrive yesterday. I attempt to create my first email like so:

val mail = Mail("manager@company.com", // TO
    null,   // CC
    null,   // BCC
    "Ping", // Title
    null    // Message)

This may have worked in Java, but in Kotlin this wouldn't compile, since we cannot pass null to List<String>. Null-safety is very important in Kotlin:

val mail = Mail("manager@company.com", // TO
    listOf(),  // CC
    listOf(),  // BCC
    "Ping",    // Title
    null       // Message) 

Note that since our constructor receives a lot of arguments, I had to put in some comments, so I wouldn't get lost.

The Kotlin compiler is smart enough to infer the type of list that we pass. Since our constructor receives List<String>, it's enough to pass listOf() for an empty list. We don't need to specify the type like so: listOf<String>(). In Java, Diamond Operator serves the same purpose.

Oh, but I forgot about attachments. Let's change our constructor:

data class Mail(val to: String, 
           val cc: List<String>, 
           val bcc: List<String>,
           val title: String?,
           val message: String?,
           val attachments: List<java.io.File>)

But then our instantiation stops compiling again:

val mail = Mail("manager@company.com", // TO
    listOf(), listOf(),
    "Ping",
    null) // Compilation error, No value passed for for parameter 'attachments'

This clearly becomes a mess.