Exercise: Using scope functions
See the below implementation of StudentService. Modify it to use scope functions. As a result, all the methods should be single-expression functions.
class StudentService(
private val studentRepository: StudentRepository,
private val studentFactory: StudentFactory,
private val logger: Logger,
) {
fun addStudent(addStudentRequest: AddStudentRequest):Student?{
val student = studentFactory
.produceStudent(addStudentRequest)
?: return null
studentRepository.addStudent(student)
return student
}
fun getStudent(studentId: String): ExposedStudent? {
val student = studentRepository.getStudent(studentId)
?: return null
logger.log("Student found: $student")
return studentFactory.produceExposed(student)
}
fun getStudents(semester: String): List<ExposedStudent> {
val request = produceGetStudentsRequest(semester)
val students = studentRepository.getStudents(request)
logger.log("${students.size} students in $semester")
return students
.map { studentFactory.produceExposed(it) }
}
private fun produceGetStudentsRequest(
semester: String,
): GetStudentsRequest {
val request = GetStudentsRequest()
request.expectedSemester = semester
request.minResult = 3.0
return request
}
}
Beware! The form after transformation is shorter but not necessarily better. It is less readable for less experienced Kotlin developers, and it is harder to debug. Outside this exercise, use scope functions with caution.
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 functional/scope/Scope.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 junit.framework.TestCase.assertEquals
import org.junit.After
import org.junit.Test
import java.time.LocalDateTime
import java.util.*
class StudentService(
private val studentRepository: StudentRepository,
private val studentFactory: StudentFactory,
private val logger: Logger,
) {
fun addStudent(addStudentRequest: AddStudentRequest):Student?{
val student = studentFactory
.produceStudent(addStudentRequest)
?: return null
studentRepository.addStudent(student)
return student
}
fun getStudent(studentId: String): ExposedStudent? {
val student = studentRepository.getStudent(studentId)
?: return null
logger.log("Student found: $student")
return studentFactory.produceExposed(student)
}
fun getStudents(semester: String): List<ExposedStudent> {
val request = produceGetStudentsRequest(semester)
val students = studentRepository.getStudents(request)
logger.log("${students.size} students in $semester")
return students
.map { studentFactory.produceExposed(it) }
}
private fun produceGetStudentsRequest(
semester: String,
): GetStudentsRequest {
val request = GetStudentsRequest()
request.expectedSemester = semester
request.minResult = 3.0
return request
}
}
class StudentFactory(
private val timeProvider: TimeProvider,
private val uuidGenerator: UuidGenerator,
) {
fun produceExposed(student: Student): ExposedStudent = ExposedStudent(
name = student.name,
)
fun produceStudent(student: AddStudentRequest): Student? = Student(
name = student.name,
semester = student.semester,
result = student.result,
creationTime = timeProvider.now(),
id = uuidGenerator.generate(),
)
}
class StudentServiceTest {
private val logger = FakeLogger()
private val userRepository = FakeStudentRepository()
private val timeProvider = FakeTimeProvider()
private val uuidGenerator = FakeUuidGenerator()
private val studentFactory = StudentFactory(timeProvider, uuidGenerator)
private val studentService = StudentService(userRepository, studentFactory, logger)
@After
fun cleanup() {
logger.cleanup()
userRepository.cleanup()
timeProvider.cleanup()
uuidGenerator.cleanup()
}
@Test
fun `should add student`() {
// Given
val time1 = LocalDateTime.of(2023, 5, 6, 7, 8)
timeProvider.now = time1
val studentId = "1234"
uuidGenerator.constantUuid = studentId
val addStudentRequest = AddStudentRequest(
name = "Marc",
semester = "1",
result = 4.0,
)
// When
val result = studentService.addStudent(addStudentRequest)
// Then
val expected = Student(
name = "Marc",
semester = "1",
result = 4.0,
creationTime = time1,
id = studentId,
)
assertEquals(expected, result)
assertEquals(expected, userRepository.getStudent(studentId))
}
@Test
fun `should get student`() {
// Given
val studentId = "1234"
uuidGenerator.constantUuid = studentId
val addStudentRequest = AddStudentRequest(
name = "Marc",
semester = "1",
result = 4.0,
)
studentService.addStudent(addStudentRequest)
// When
val result = studentService.getStudent(studentId)
// Then
val expected = ExposedStudent(
name = "Marc",
)
assertEquals(expected, result)
}
@Test
fun `should log when getting student`() {
// Given
val time1 = LocalDateTime.of(2023, 5, 6, 7, 8)
timeProvider.now = time1
val studentId = "1234"
uuidGenerator.constantUuid = studentId
val addStudentRequest = AddStudentRequest(
name = "Marc",
semester = "1",
result = 4.0,
)
studentService.addStudent(addStudentRequest)
// When
val result = studentService.getStudent(studentId)
// Then
val expected = listOf(
"Student found: Student(id=1234, name=Marc, creationTime=2023-05-06T07:08, semester=1, result=4.0)"
)
assertEquals(expected, logger.messages)
}
@Test
fun `should return null when getting non-existing student`() {
// Given
val studentId = "1234"
uuidGenerator.constantUuid = studentId
val addStudentRequest = AddStudentRequest(
name = "Marc",
semester = "1",
result = 4.0,
)
studentService.addStudent(addStudentRequest)
// When
val result = studentService.getStudent("1")
// Then
assertEquals(null, result)
}
@Test
fun `should get students list and log its size`() {
// Given
listOf(
AddStudentRequest("Alex", "Semester1", 4.0),
AddStudentRequest("Bob", "Semester2", 4.0),
AddStudentRequest("Celine", "Semester1", 2.0),
AddStudentRequest("Diana", "Semester1", 5.0),
AddStudentRequest("Ester", "Semester2", 3.0),
AddStudentRequest("Fiona", "Semester1", 4.0),
AddStudentRequest("Gina", "Semester2", 2.9),
AddStudentRequest("Helen", "Semester1", 3.0),
AddStudentRequest("Irene", "Semester1", 4.0),
AddStudentRequest("Jack", "Semester2", 4.0),
AddStudentRequest("Kate", "Semester1", 5.0),
AddStudentRequest("Linda", "Semester1", 3.0),
AddStudentRequest("Marc", "Semester1", 2.0),
).forEach { studentService.addStudent(it) }
// When
val result1 = studentService.getStudents("Semester1")
// Then should receive students from the same semester and with result >= 3.0
assertEquals(
listOf(
ExposedStudent("Alex"),
ExposedStudent("Diana"),
ExposedStudent("Fiona"),
ExposedStudent("Helen"),
ExposedStudent("Irene"),
ExposedStudent("Kate"),
ExposedStudent("Linda"),
), result1
)
assertEquals(listOf("7 students in Semester1"), logger.messages)
// Given
logger.cleanup()
// When
val result = studentService.getStudents("Semester2")
// Then should receive students from the semester and with result >= 3.0
assertEquals(
listOf(
ExposedStudent("Bob"),
ExposedStudent("Ester"),
ExposedStudent("Jack"),
), result
)
assertEquals(listOf("3 students in Semester2"), logger.messages)
}
}
interface StudentRepository {
fun addStudent(student: Student)
fun getStudent(id: String): Student?
fun getStudents(request: GetStudentsRequest): List<Student>
}
class GetStudentsRequest {
var minResult: Double? = null
var expectedSemester: String? = null
}
data class AddStudentRequest(
val name: String,
val semester: String,
val result: Double,
)
data class Student(
val id: String,
val name: String,
val creationTime: LocalDateTime,
val semester: String,
val result: Double,
)
data class ExposedStudent(
val name: String,
)
interface Logger {
fun log(message: String)
}
interface TimeProvider {
fun now(): LocalDateTime
}
class RealTimeProvider : TimeProvider {
override fun now(): LocalDateTime = LocalDateTime.now()
}
class FakeTimeProvider : TimeProvider {
var now: LocalDateTime = INIT_TIME
fun cleanup() {
now = INIT_TIME
}
override fun now(): LocalDateTime = now
companion object {
val INIT_TIME = LocalDateTime.of(2020, 1, 1, 0, 0)
}
}
interface UuidGenerator {
fun generate(): String
}
class RealUuidGenerator : UuidGenerator {
override fun generate(): String = UUID.randomUUID().toString()
}
class FakeUuidGenerator : UuidGenerator {
var constantUuid: String? = null
private var counter: Int = 0
fun cleanup() {
counter = 0
constantUuid = null
}
override fun generate(): String = constantUuid
?: (counter++).toString()
}
class FakeLogger : Logger {
var messages: List<String> = emptyList()
fun cleanup() {
messages = emptyList()
}
override fun log(message: String) {
this.messages += message
}
}
class FakeStudentRepository() : StudentRepository {
private var students: List<Student> = emptyList()
fun cleanup() {
students = emptyList()
}
override fun addStudent(student: Student) {
students += student
}
override fun getStudent(id: String): Student? = students
.find { it.id == id }
override fun getStudents(request: GetStudentsRequest): List<Student> =
students
.filter { request.expectedSemester == null || it.semester == request.expectedSemester }
.filter { request.minResult == null || it.result >= request.minResult!! }
}
