Exercise: Specify expectations

Implement the notifyUser function that notifies the user using notifier unless:

• The user property is null, in which case do nothing.
• The id property is incorrect according to the checkId function from Notifier, in which case throw an IncorrectId exception.
• user.name or user.surname is null, in which case you should throw IllegalArgumentException.
• The notifier is not initialized, which is an incorrect state, in which case you should throw IllegalStateException.
• notifyPerson does not return true when you send a notification, in which case throw AssertionError.

Starting code:

fun Notifier.notifyUser(user: User?) {
    TODO()
}

data class User(
    val id: Int,
    val name: String?,
    var surname: String?
)

class IncorrectId : Error()

interface Notifier {

    /**
     * Indicate instance readiness to notify users
     */
    val initialized: Boolean

    /**
     * Notifies person
     * @param id Is an id of user we want to notify
     * @return Was the operation successful
     */
    fun notifyPerson(id: Int): Boolean

    /**
     * Checks if we can send message to the following id
     */
    fun checkId(id: Int): Boolean
}

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 effective/safe/NotifyUser.kt. You can find there starting code and unit tests.

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

Playground

import org.junit.Assert.assertThrows
import org.junit.Test
import java.lang.AssertionError
import java.lang.IllegalStateException
import kotlin.test.assertEquals

fun Notifier.notifyUser(user: User?) {
    TODO()
}

data class User(
    val id: Int,
    val name: String?,
    var surname: String?
)

class IncorrectId : Error()

interface Notifier {

    /**
     * Indicate instance readiness to notify users
     */
    val initialized: Boolean

    /**
     * Notifies person
     * @param id Is an id of user we want to notify
     * @return Was the operation successful
     */
    fun notifyPerson(id: Int): Boolean

    /**
     * Checks if we can send message to the following id
     */
    fun checkId(id: Int): Boolean
}

class RequirementsChecks {
    @Test
    fun `Function sends message for correct arguments and state`() {
        val notifier = FakeNotifier(initialized = true)
        val id = 123
        notifier.notifyUser(User(id, "Ben", "Barack"))
        assertEquals(setOf(id), notifier.usersNotified)
    }

    @Test
    fun `Function does nothing when user is null`() {
        val notifier = FakeNotifier(initialized = true)
        notifier.notifyUser(null)
        assertEquals(setOf(), notifier.usersNotified)
    }

    @Test
    fun `Function throws IllegalArgumentException when name is null`() {
        val notifier = FakeNotifier(initialized = true)
        val id = 123
        assertThrows(IllegalArgumentException::class.java) { notifier.notifyUser(User(id, null, "Kowalski")) }
    }

    @Test
    fun `Function throws IllegalArgumentException when surname is null`() {
        val notifier = FakeNotifier(initialized = true)
        val id = 123
        assertThrows(IllegalArgumentException::class.java) { notifier.notifyUser(User(id, "Barack", null)) }
    }

    @Test
    fun `Function throws IllegalStateException when notifier is not initialized`() {
        val notifier = FakeNotifier(initialized = false)
        val id = 123
        assertThrows(IllegalStateException::class.java) { notifier.notifyUser(User(id, "Mike", "Bull")) }
    }

    @Test
    fun `Function throws IncorrectId when id is incorrect`() {
        val notifier = FakeNotifier(initialized = true)
        assertThrows(IncorrectId::class.java) { notifier.notifyUser(User(INCORRECT_ID, "Mike", "Bull")) }
    }

    @Test
    fun `It is expected that notifyPerson will return true to indicate success`() {
        val notifier = FakeNotifier(initialized = true)
        assertThrows(IncorrectId::class.java) { notifier.notifyUser(User(INCORRECT_ID, "Mike", "Bull")) }
    }

    @Test
    fun `Function should check if notifyPerson returns true as expected in correct conditions`() {
        val notifier = FakeNotifier(initialized = true)
        val id = 123
        notifier.incorrectImplementationMakingNotifyPersonReturnFalse = true
        assertThrows(AssertionError::class.java) { notifier.notifyUser(User(id, "Ben", "Barack")) }
    }

    class FakeNotifier(initialized: Boolean = false): Notifier {
        override var initialized = initialized
            private set
        var usersNotified = setOf<Int>()
        var incorrectImplementationMakingNotifyPersonReturnFalse = false

        override fun checkId(id: Int): Boolean = id != INCORRECT_ID

        override fun notifyPerson(id: Int): Boolean {
            if(incorrectImplementationMakingNotifyPersonReturnFalse) return false
            usersNotified = usersNotified + id
            return true
        }
    }

    companion object {
        const val INCORRECT_ID = -100
    }
}