A smooth refactor using sealed classes and a factory function

The problem

Lets say we have a contacts app and one of the screens shows the contact’s phone number.

// domain:
class PhoneNumber(val value: String)
class Contact(val phoneNumber: PhoneNumber)
// screen:
class PhoneNumberScreen(
private val phoneNumber: PhoneNumber
) {
fun render() {
println("Phone number: ${phoneNumber.value}")
}
}
// presentation layer:
fun main() {
val contact = Contact(PhoneNumber("12345"))
show(contact.phoneNumber) // "Phone number: 12345"
val contactWithInvalidPhoneNumber = Contact(PhoneNumber(""))
show(contactWithInvalidPhoneNumber.phoneNumber) // "Phone number: "
}
private fun show(phoneNumber: PhoneNumber) {
val phoneNumberScreen = PhoneNumberScreen(phoneNumber)
phoneNumberScreen.render()
}

The problem with that code is that we can easily end up with instances that contain invalid state:

A phone number screen with an empty phone number!

Approach #1:

One way to prevent it is to add some logic in the presentation layer:

Open the phone number screen only if the phone number is not empty

fun main() {
val contact = Contact(PhoneNumber("12345"))
show(contact.phoneNumber) // "Phone number: 12345"
val contactWithInvalidPhoneNumber = Contact(PhoneNumber(""))
show(contactWithInvalidPhoneNumber.phoneNumber) // does not show anything
}
private fun show(phoneNumber: PhoneNumber) {
if (phoneNumber.value.isEmpty()) {
return
}
val phoneNumberScreen = PhoneNumberScreen(phoneNumber)
phoneNumberScreen.render()
}

Unfortunately this approach does not provide an actual solution but a patch. Our main goal is to have a PhoneNumberScreen that handles ONLY valid phone numbers.

Approach #2:

What we need is to move the necessary checks inside the PhoneNumberScreen class.

We could check the number’s validity on render and show some kind of message when there is no phone number.

class PhoneNumberScreen(
private val phoneNumber: PhoneNumber
) {
fun render() {
if (phoneNumber.value.isNotEmpty()) {
println("Phone number: ${phoneNumber.value}")
} else {
println("Invalid phone number")
}
}
}
fun main() {
val contact = Contact(PhoneNumber("12345"))
show(contact.phoneNumber) // "Phone number: 12345"
val contactWithInvalidPhoneNumber = Contact(PhoneNumber(""))
show(contactWithInvalidPhoneNumber.phoneNumber) // "Invalid phone number"
}
private fun show(phoneNumber: PhoneNumber) {
val phoneNumberScreen = PhoneNumberScreen(phoneNumber)
phoneNumberScreen.render()
}

It is quite clear that this solution provides a bad UX. Why open a screen when the user cannot use it? Also, in any additional usage of phoneNumber inside the PhoneNumberScreen we need to make the same check as in render() and handle both of its states.

Approach #3:

What we really need is to make sure that if the screen is created, then it is certain that it has a valid phone number. There are two ways to achieve that. The first one is by checking upon creation that the phone number is valid and throw an exception if it is not.

class PhoneNumberScreen(
private val phoneNumber: PhoneNumber
) {
init {
require(phoneNumber.value.isNotEmpty()) { "cannot handle invalid phone number" }
}
fun render() {
println("Phone number: ${phoneNumber.value}")
}
}
fun main() {
val contact = Contact(PhoneNumber("12345"))
show(contact.phoneNumber) // "Phone number: 12345"
val contactWithInvalidPhoneNumber = Contact(PhoneNumber(""))
show(contactWithInvalidPhoneNumber.phoneNumber) // prints nothing
}
private fun show(phoneNumber: PhoneNumber) {
try {
val phoneNumberScreen = PhoneNumberScreen(phoneNumber)
phoneNumberScreen.render()
} catch (ex: IllegalArgumentException) {
}
}

It works but we need to document it and add a try-catch wherever we create a screen instance.

The second one is by having a helper function that creates the screen only if the phone number is valid.

class PhoneNumberScreen private constructor(
private val phoneNumber: PhoneNumber
) {
fun render() {
println("Phone number: ${phoneNumber.value}")
}
companion object {
fun create(phoneNumber: PhoneNumber): PhoneNumberScreen? {
return when {
phoneNumber.value.isNotEmpty() > PhoneNumberScreen(phoneNumber)
else > null
}
}
}
}
fun main() {
val contact = Contact(PhoneNumber("12345"))
show(contact.phoneNumber) // "Phone number: 12345"
val contactWithInvalidPhoneNumber = Contact(PhoneNumber(""))
show(contactWithInvalidPhoneNumber.phoneNumber) // prints nothing
}
private fun show(phoneNumber: PhoneNumber) {
val phoneNumberScreen = PhoneNumberScreen.create(phoneNumber)
phoneNumberScreen?.render()
}

This also works as expected but once again we need to document it and on top of that handle any null values returned by the helper function.

Nevertheless this solution seems fine and for a very small code base is quite acceptable. The problem is that it does not scale alongside the code base. Imagine how many helper functions we need to implement every time we have to use a phone number instance in our components if we want to keep them “clean”.

The actual problem

The actual problem lies in the PhoneNumber itself. It represents more than one states and each time we use an instance of it we must “dive” in its value and translate it to that state.

What we really need is a better representation of a valid and invalid phone number.

Final approach

This is where we use sealed classes and separate the two states:

sealed class PhoneNumber
object InvalidPhoneNumber : PhoneNumber()
data class ValidPhoneNumber(val value: String) : PhoneNumber() {
init {
require(value.isNotEmpty()) { "the number cannot be empty" }
}
}

This way we accomplish our main goal: we change the PhoneNumberScreen to accept only instances of ValidPhoneNumber and can now be certain that the screen will be used only with valid data. The code is self documented and any further development in the screen class will not have to consider other states for the phone number:

class PhoneNumberScreen(
private val phoneNumber: ValidPhoneNumber
) {
fun render() {
println("Phone number: ${phoneNumber.value}")
}
}

One big drawback of this change is that every usage of the PhoneNumber class has just broke (see: creation of Contact instances).

Fortunately there is a quick solution for that! Factory function:

A function that has the same name with the previously used class (PhoneNumber) and takes a single string parameter. If the parameter is not empty it returns a ValidPhoneNumber. In any other case it returns an InvalidPhoneNumber:

fun PhoneNumber(value: String): PhoneNumber =
when {
value.isNotEmpty() > ValidPhoneNumber(value)
else > InvalidPhoneNumber
}

The end result is almost the same as the starting point but this time we have clear states and components that can guarantee that they will not crash because of erroneous data:

fun main() {
val contact = Contact(PhoneNumber("12345"))
show(contact.phoneNumber) // "Phone number: 12345"
val contactWithInvalidPhoneNumber = Contact(PhoneNumber(""))
show(contactWithInvalidPhoneNumber.phoneNumber) // prints nothing
}
private fun show(phoneNumber: PhoneNumber) {
if (phoneNumber is ValidPhoneNumber) {
val phoneNumberScreen = PhoneNumberScreen(phoneNumber)
phoneNumberScreen.render()
}
}

A function’s intent should be revealed by its name

And a good way to know if it doesn’t is to read it where it gets used. If, after reading it, you have questions then something is wrong with the name.

A good example is this code that I run into:

if (handleClickItem(customer)) {
    return;
}

When I read this piece of code the very first thing that popped into my head was

How does the click gets handled?!

To figure that out I had to step into the function’s body and start reading it in order to understand what it does and when. It broke my flow and made me change context.

Turned out that this particular function, depending on the customer’s type, opens the contact’s screen and returns true or simply returns false. Having read something like:

if (openContactsScreenWhenCustomerIsCompany(customer)) {
    return;
}

would have been enough for me to simply keep reading and never look back.

Make your code reveal its usage

A small and simple example that shows one of the benefits of having domain objects.

Lets assume we need to make a request for some kind of a token and the flow for doing so goes like this:

  1. Validate an email address
  2. With the validated address validate a password
  3. With both the validated values request for the token

The not so revealing way

fun validateEmailAddress(value: String) {
// does some validation
}
fun validatePassword(validEmailAddress: String, password: String) {
// does some validation
}
fun requestToken(validEmailAddress: String, validPassword: String): String {
return "some kind of token"
}
view raw not-revealing-way.kt hosted with ❤ by GitHub

Q: Can the consumer of this API, without knowing the aforementioned flow, figure it out just by looking at the functions’ signatures?

A: I guess she could if the parameters’ names were like that. If not she needs to read the bodies of those functions to see that there is some kind of order that needs to be followed.

Q: Can the creator of this code be 100% certain that the functions will be used as expected and the passed parameters will be valid? For example, will requestToken always be called last and with valid addresses and passwords?

A: No! There is nothing that can guarantee that so, just to be safe, we check in every function that the provided values are valid and make the code flexible enough that, for example, each function could be used on its own. That will, potentially, lead to code duplication or unnecessary abstractions.

Q: What about errors and invalid values? Can the consumer of this API predict the code’s behavior on erroneous inputs without reading a documentation?

A: No. Just no.

The revealing way

First lets enrich our API with some, much needed, domain objects:

sealed class EmailAddress
data class ValidEmailAddress(val value: String) : EmailAddress()
data class InvalidEmailAddress(val value: String, val error: String): EmailAddress()
sealed class Password
data class ValidPassword(val value: String) : Password()
data class InvalidPassword(val value: String, val error: String): Password()
data class Token(val value: String)
view raw revealing-way.kt hosted with ❤ by GitHub

Having those constructs we can change the functions and make them reveal both the order they can be used and their behavior:

fun validateEmailAddress(value: String): EmailAddress {
// does some validation and
// if the validation fails it returns InvalidEmailAddress
// otherwise a ValidEmailAddress
}
fun validatePassword(emailAddress: ValidEmailAddress, password: String): Password {
// does some validation against the valid email address
// if the validation fails it returns an InvalidPassword
// otherwise a ValidPassword
}
fun requestToken(emailAddress: ValidEmailAddress, password: ValidPassword): Token {
// having only valid values makes the code simple
// just make the request and return the token
return Token("some kind of token")
}
view raw revealing-way-2.kt hosted with ❤ by GitHub

So lets pretend that we are the consumer of this API and all we have is the code.

Our main goal is to request for a token. By just looking at the functions’ signatures we see that there is a requestToken. Nice! Our task is half way done! What do we need for calling this function? A valid email address and a valid password. Ok. How do we get one of each?

Looking again at the signatures we see validateEmailAddress and validatePassword that return an EmailAddress and Password respectively. Lets check what those are. These are abstractions and each of them has been extended to a valid and an invalid state. The invalid one carries the error that occurred too! So, back to the functions.

We see that validating a password needs to be fed with a valid email address so we first need to call validateEmailAddress, then validatePassword and finally requestToken.

That’s it. We didn’t read the code of the functions and we didn’t have to worry about our inputs.

Use sealed classes for better domain representation

Lets start with the business logic.

We have a task. A task can be unassigned OR it can be assigned either to a user OR a group.

First implementation: the ugly way

One way to implement this is by putting all the logic in the task:

class UglyTask(
  val name: String,
  val assignedUser: User? = null,
  val assignedGroup: Group? = null
) {
  init {
  if (assignedUser != null && assignedGroup != null) {
  throw IllegalArgumentException("a task can be assigned to either a user OR a group.")
  }
  }
}
view raw uglytask.kt hosted with ❤ by GitHub

By default the task is unassigned and if we want an assigned task we provide a user or a group. The or factor is being enforced by a check in the constructor.

This implementation not only relies on nulls to represent the business logic but it also hides the logic from the developer who has to read the code to understand how to create an assigned task.

The null checking comes also up when we want to figure out if and where a task is assigned which is tedious and can easily lead to bugs. As an example lets consider a simple function that prints the task’s assignment state:

fun UglyTask.printAssignment() {
when {
assignedGroup == null && assignedUser == null > println("\"$name\" is assigned to no one")
assignedGroup != null > println("\"$name\" is assigned to to group: ${assignedGroup.name}")
assignedUser != null > println("\"$name\" is assigned to to user: ${assignedUser.name}")
}
}

Finally two points that we should not neglect are readability and scalability. When using UglyTask if we want our code to be readable, in all cases, we have to pass the arguments by their names (thank you Kotlin 🙂 ):

val le0nidas = User("le0nidas")
val kotlinEnthusiasts = Group("kotlin enthusiasts")
UglyTask("buy milk").printAssignment()
UglyTask("write post", assignedUser = le0nidas).printAssignment()
// here we pass the parameter by name to avoid the usage of null
// which makes it less readable
UglyTask("write kotlin", assignedGroup = kotlinEnthusiasts).printAssignment()
UglyTask("write kotlin", null, kotlinEnthusiasts).printAssignment()
view raw uglytask_usage.kt hosted with ❤ by GitHub

As far as scalability, consider how many changes we need to do to add a new assigned entity. One to the constructor, one to the init function to enforce our business logic and one in every function that we use the task’s state (see: printAssignment()) which adds even more null checks.

Second implementation: The less ugly way

Another way is by having multiple constructors, each for every valid assignment:

class LessUglyTask private constructor(
val name: String,
val assignedUser: User?,
val assignedGroup: Group?
) {
constructor(name: String) : this(name, null, null) // assigned to no one
constructor(name: String, assignedUser: User) : this(name, assignedUser, null) // assigned to a user
constructor(name: String, assignedGroup: Group) : this(name, null, assignedGroup) // assigned to a group
}
view raw lessunglytask.kt hosted with ❤ by GitHub

This implementation also puts all the logic in the task but it removes those null checks and makes it easier for the developer to understand it:

With that said, all other drawbacks in readability and scalability remain the same:

fun LessUglyTask.printAssignment() {
when {
assignedGroup == null && assignedUser == null > println("\"$name\" is assigned to no one")
assignedGroup != null > println("\"$name\" is assigned to to group: ${assignedGroup.name}")
assignedUser != null > println("\"$name\" is assigned to to user: ${assignedUser.name}")
}
}
val le0nidas = User("le0nidas")
val kotlinEnthusiasts = Group("kotlin enthusiasts")
LessUglyTask("buy milk").printAssignment()
LessUglyTask("write post", assignedUser = le0nidas).printAssignment()
LessUglyTask("write kotlin", assignedGroup = kotlinEnthusiasts).printAssignment()

Final implementation: the sealed classes way 🙂

The best way to implement the business logic is by using Kotlin’s sealed classes. This way we can represent our business logic straight into our code and also keep our code clean, readable and scalable:

sealed class AssignedTo
object AssignedToNoOne : AssignedTo()
data class AssignedToUser(val user: User) : AssignedTo()
data class AssignedToGroup(val group: Group) : AssignedTo()
class Task(val name: String, val assignedTo: AssignedTo)
view raw task.kt hosted with ❤ by GitHub

Now, printAssignment() leverages all of Kotlin’s powers, including smart cast, making it easier to the eye:

fun Task.printAssignment() {
when (assignedTo) {
is AssignedToNoOne > println("\"$name\" is assigned to no one")
is AssignedToGroup > println("\"$name\" is assigned to ${assignedTo.group.name}")
is AssignedToUser > println("\"$name\" is assigned to ${assignedTo.user.name}")
}
}

and the rest of the code does not need any extra help like named arguments:

val le0nidas = User("le0nidas")
val kotlinEnthusiasts = Group("kotlin enthusiasts")
Task("buy milk", AssignedToNoOne).printAssignment()
Task("write post", AssignedToUser(le0nidas)).printAssignment()
Task("write kotlin", AssignedToGroup(kotlinEnthusiasts)).printAssignment()
view raw task_usage.kt hosted with ❤ by GitHub

As for scalability, when we want to add a new way of assignment we just extend AssignedTo and we are good to go.