Exercise: Conversion and measurement unit creation

Your task is to implement two kinds of extension functions. First, you need functions to transform between User and UserJson classes. User represents our domain object, while UserJson represents the structure that is sent to the client. The conversion should be done according to the following rules:

1. username should be copied as is,
2. email should be converted to/from String in the Email class,
3. registrationDate should be converted to/from String in ISO format. You can use parse and toString fron LocalDateTime for that,
4. height should be converted to/from Int in Centimeters class.

Then implement cm extension property for Int that will create a Centimeters object.

data class User(
    val username: String,
    val email: Email,
    val registrationDate: LocalDateTime,
    val height: Centimeters,
)

data class Email(val value: String)

data class Centimeters(val value: Int)

data class UserJson(
    val username: String,
    val email: String,
    val registrationDate: String,
    val heightCm: Int,
)

Once your solution is ready, the following code should work:

val user = User(
    username = "alex",
    email = Email("[email protected]"),
    registrationDate = LocalDateTime
        .of(1410, 7, 15, 10, 13),
    height = 170.cm,
)
val userJson = user.toUserJson()
println(userJson)
// UserJson(username=alex, [email protected],
// registrationDate=1410-07-15T10:13, heightCm=170)
val user2 = userJson.toUser()
println(user2) // User(username=alex,
// email=Email([email protected]),
// registrationDate=1410-07-15T10:13,
// height=Centimeters(value=170))

This problem can either be solved in the below playground or you can clone kotlin-exercises project and solve it locally. In the project, you can find code template for this exercise in essentials/extensions/User.kt. You can find there example usage and unit tests.

Once you are done with the exercise, you can check your solution here.

Playground

import org.junit.Test
import java.time.LocalDateTime
import kotlin.test.assertEquals

// TODO

data class User(
    val username: String,
    val email: Email,
    val registrationDate: LocalDateTime,
    val height: Centimeters,
)

data class Email(val value: String)

data class Centimeters(val value: Int)

data class UserJson(
    val username: String,
    val email: String,
    val registrationDate: String,
    val heightCm: Int,
)

// TODO: Implement extensions to convert User to UserJson and vice versa,
//  and to create Centimeters from Int

fun main() {
    val user = User(
        username = "alex",
        email = Email("[email protected]"),
        registrationDate = LocalDateTime
            .of(1410, 7, 15, 10, 13),
        height = 170.cm,
    )
    val userJson = user.toUserJson()
    println(userJson)
    // UserJson(username=alex, [email protected],
    // registrationDate=1410-07-15T10:13, heightCm=170)
    val user2 = userJson.toUser()
    println(user2) // User(username=alex,
    // email=Email([email protected]),
    // registrationDate=1410-07-15T10:13,
    // height=Centimeters(value=170))
}

class DataConversionTest {

    @Test
    fun `test User to UserJson conversion`() {
        val user = User(
            username = "alex",
            email = Email("[email protected]"),
            registrationDate = LocalDateTime.now(),
            height = 170.cm
        )

        val userJson = user.toUserJson()

        assertEquals("alex", userJson.username)
        assertEquals("[email protected]", userJson.email)
        assertEquals(user.registrationDate.toString(), userJson.registrationDate)
        assertEquals(user.height.value, userJson.heightCm)
    }

    @Test
    fun `test UserJson to User conversion`() {
        val userJson = UserJson(
            username = "alex",
            email = "[email protected]",
            registrationDate = LocalDateTime.now().toString(),
            heightCm = 170
        )

        val user = userJson.toUser()

        assertEquals("alex", user.username)
        assertEquals("[email protected]", user.email.value)
        assertEquals(userJson.registrationDate, user.registrationDate.toString())
        assertEquals(userJson.heightCm.cm, user.height)
    }

    @Test
    fun `test cm extension property`() {
        val value = 150
        val cm = value.cm

        assertEquals(value, cm.value)
    }
}