Clean Code with Kotlin

With Kotlin we are able to write concise, expressive and safe code. Sounds like clean code, doesn’t it? During this presentation we will recap what clean code is, we will highlight the importance of defining meaningful names and how to write clean functions and classes. Finally we will be able to learn more about the advantages of immutability and how to handle the errors in Kotlin. By the end of the session, you will have a better understanding of what clean code means and you will learn a series of tips and tricks ready to be applied in your code.

Transcript

1. Magda Miu @magdamiu Squad Lead Developer at Orange Android Google Developer Expert Clean Code with Kotlin
2. A. A big software system that works perfectly but is very difficult to change. What would you prefer to have: B. A big software system with some missing features that is very easy to change.
4. What is Clean Code? “Clean code is readable. It tells a story.” Uncle Bob, Clean Code
7. Code quality “measure” Few WTFs Developer Many WTFs Developer WTF/min WTF = What a Terrible Feature
8. MEANINGFUL NAMES
9. A name should reveal intent and should answer at these questions: • Why it exists? • What it does? • How it is used? Use intention-revealing names
10. Examples Types Names Classes and Objects Customer, Account, WikiPage Methods postPayment, deleteAccount, displayPage Solution domain names AccountVisitor Problem domain names churnPerMonth
11. “UNCLEAN” CODE CLEAN CODE
12. if (t % 5 == 0) { val w = t / 5 println("$w week(s)") } else { println("$t days") } TODO: display the number of days/weeks until an offer will expire
13. if (daysUntilOfferEnds % numberOfDaysInAWeek == 0) { val numberOfWeeks = daysUntilOfferEnds / numberOfDaysInAWeek println("$numberOfWeeks week(s)") } else { println("$daysUntilOfferEnds days") } TODO: display the number of days/weeks until an offer will expire
14. class Address { var addressCountry: String? = null var addressCity: String? = null var addressStreety: String? = null var addressStreetNo: Int = 0 } TODO: define a class for geo addresses
15. class Address { var country: String? = null var city: String? = null var street: String? = null var streetNumber: Int = 0 } TODO: define a class for geo addresses
16. users.filter{ it.job == Job.Developer } .map{ it.birthDate.dayOfMonth } .filter{ it <= 10 } .min() Warning: Use explicit argument names and avoid using too often “it”
17. users.filter{ user -> user.job == Job.Developer } .map{ developer -> developer.birthDate.dayOfMonth } .filter { birthDay -> birthDay <= 10 } .min() Warning: Use explicit argument names and avoid using too often “it”
18. FUNCTIONS
19. Functions are the verbs of the language, and classes are the nouns.
20. Uncle Bob Functions should do one thing. They should do it well. They should do it only.
21. Basic Rules: ➔ The functions should be small ➔ The functions should be smaller than that Make it smaller
22. Extra Rules: ➔ No many arguments ➔ No side effects ➔ Indent level max 2 Make it smaller
23. fun parseProduct(response: Response?): Product? { if (response == null) { throw ClientException("Response is null") } val code: Int = response.code() if (code == 200 || code == 201) { return mapToDTO(response.body()) } if (code >= 400 && code <= 499) { throw ClientException("Invalid request") } if (code >= 500 && code <= 599) { throw ClientException("Server error") } throw ClientException("Error $code") }
24. fun parseProduct(response: Response?) = when (response?.code()){ null -> throw ClientException("Response is null") 200, 201 -> mapToDTO(response.body()) in 400..499 -> throw ClientException("Invalid request") in 500..599 -> throw ClientException("Server error") else -> throw ClientException("Error ${response.code()}") }
25. for (user in users) { if(user.subscriptions != null) { if (user.subscriptions.size > 0) { var isYoungerThan30 = user.isYoungerThan30() if (isYoungerThan30) { countUsers++ } } } } WARNING: keep the abstraction level consistent and avoid nested code
26. var countUsersYoungerThan30WithSubscriptions = 0 for (user in users) { if (user.isYoungerThan30WithSubscriptions) { countUsersYoungerThan30WithSubscriptions++; } } WARNING: keep the abstraction level consistent and avoid nested code
27. fun sumUpUserPoints(): Int { var sumAllPoints = 0 for (user in users) { sumAllPoints += user.points sendEmail(user) } return sumAllPoints } No side effects Side effect
28. Command Query Separation Command Query Output System System System Changed state Output (result) Changed state Output (result)
29. override fun onSaveInstanceState(outState: Bundle) { super.onSaveInstanceState(outState) outState.putString(KEY_USER_NAME, username) } Command
30. fun getProfileInfo(account: SignInAccount): UserProfile { var userProfile = UserProfile() if (account != null) { userProfile.name = account.displayName userProfile.email = account.email userProfile.id = account.id } return userProfile } Query
31. CLASSES Dog Cat Baby Dog Pet
32. Class organization • A class should be smaller than small. • It should have only one responsibility and a single reason to change. • A class collaborates with a few others to achieve the desired system behaviors.
33. The newspaper metaphor
34. // var and val control the variable itself // not the object instance that is assigned val year = 2019 year = 2020 var age = 18 age = 19 Immutable References
35. val listOfYears = listOf(2017, 2018, 2019, 2020) listOfYears.add(2021) Read-only collections
36. data class Movie( val name: String, val studio: String, val rating: Float, val releaseYear: Int = 2020 ) Immutable Data Classes Constructor (assign args to props) Only getters toString() hashCode(), equals() copy() final Default arguments
37. Cohesion • Cohesion is a measure of the degree to which the elements of the class/ module are functionally related. • When classes lose cohesion, split them.
39. Coupling • Coupling is the measure of the degree of interdependence between the modules. • This isolation makes it easier to understand each element of the system.
41. Best case scenario Cohesion Coupling
42. Principles 1. DRY 2. KISS 3. YAGNI 4. SOLID
43. • Don’t Repeat Yourself • Applicable whenever we copy / paste a piece of code D.R.Y.
44. • Keep It Simple and Stupid • Whenever we want to implement a method to do all things K.I.S.S.
45. • You Ain’t Gonna Need It • Don’t write code which is not yet necessary Y.A.G.N.I.
46. • Single responsibility (SRP) • Open-closed (OCP) • Liskov substitution (LSP) • Interface segregation (ISP) • Dependency inversion (DIP) S.O.L.I.D.
47. OBJECTS AND DATA STRUCTURES
48. OBJECTS DATA STRUCTURES ➔ hide their data behind abstractions ➔ expose functions to operate on their data ➔ expose their data ➔ have no meaningful functions Data/object anti-symmetry
49. class Company { var name: String = "" var address: String = "" } DATA STRUCTURE
50. class Company { var name: String = "" get() { if (!field.isEmpty()) { return field.toUpperCase() } return field } var address: String = "" } OBJECT
51. Train wrecks • Chains of calls are generally considered to be sloppy style and should be avoided. • Object interactions spliced into each other can be confusing.
52. fun getColorOfFolder(folder: Folder): String? { return folder.feature.tag.color.name }
53. fun getColorOfFolder(folder: Folder): String? { return tagColorName(tagOfFeatureFolder(folder)) } fun getTagColorName(tag: Tag): String? { return tag.color.name } fun getTagOfFeatureFolder(folder: Folder): Tag { return folder.feature.tag }
54. The law of Demeter A module should not know about the internals of the objects it interacts with. Demeter Goddess of the Harvest
55. The law of Demeter Only talk to your friends who share your concerns Demeter Goddess of the Harvest
56. A method f of a class C should only call the methods of these: 1. C 2. An object created by f 3. An object passed as an argument to f 4. An object held in an instance variable of C LoD - Formal definition
57. Law of Demeter A B B is a friend of A C C is a friend of B Note: A friend of a friend is a stranger Messages from A to B are OK Messages from A to C are discouraged
58. fun displayInfoAboutOwner(car: Car) { var city = car.owner.address.city println("${car.owner.name} lives in $city") } Car Owner Owner is a friend of Car Address Address is a friend of Owner
59. App “architecture” Model Room Remote Data source Retrofit SQLite REST API Activity / Fragment
60. Recommended app architecture Activity / Fragment Model Room Remote Data source Retrofit SQLite REST API ViewModel LiveData Repository
61. Room.databaseBuilder(context.getApplicationContext(), UserDatabase::class.java, DATABASE_NAME) .addMigrations(MIGRATION_1_2, MIGRATION_2_3) .fallbackToDestructiveMigrationFrom(4) .build() Fluent APIs
62. “Tell, Don't Ask” Principle (4) • Object-orientation is about bundling data with the functions that operate on that data. • Rather than asking an object for data and acting on that data, we should instead tell an object what to do.
63. class Balance(var value: Double) class Client { fun alertService(balances: List<Balance>) { for (balance in balances) { if (balance.value == 0.0) { // alert } else { // everything is ok } } } }
64. class Balance(var value: Double, val alertThreshold: Int) { val isExceedsThreshold: Boolean get() = value >= alertThreshold }
65. class Client { fun alertService(balances: List<Balance>) { for (balance in balances) { if (balance.isExceedsThreshold) { // alert } else { // everything is ok } } } }
66. Objects vs Data Tell, don’t ask. Don’t talk to strangers. Dealing with Objects? Dealing with Data?
67. ERROR HANDLING
68. • Prefer exceptions to returning error codes. • Error handling is important, but if it obscures logic, it’s wrong. • In Kotlin we have only unchecked exceptions. • Define dedicated exception classes. • Don’t return or pass null. Error handling
69. Types/ Nullable Types (recap) @Nullable Type @NotNull Type Java Type Type? Type Kotlin ?
70. Nullability in Kotlin String? “Clean Code” null String “Clean Code”
71. val name: String = if (nullableValue == null) "default" else nullableValue Smart-cast
72. val view = activity!!.findViewById<View>(R.id.menu_add) kotlin.KotlinNullPointerException Not null assertion operator !! OR Bang! Bang! Boom! operator
73. fun computeSqrt(number: Double): Double { if(number >= 0) { return Math.sqrt(number) } else { throw RuntimeException("No negative please") } } Nothing is something...
74. fun getMovie(id: Int): Movie { val movie = movieRepository.findMovie(id) return movie ?: throw RuntimeException("Movie not found") } Throw exceptions
75. sealed class MovieSearchResult data class MovieFound(val movie: Movie) : MovieSearchResult() object MovieNotFound : MovieSearchResult() object DatabaseOffline : MovieSearchResult() fun getMovie(id: Int): MovieSearchResult { val movie = movieRepository.findMovie(id) return if (movie == null) { MovieNotFound } else { MovieFound(movie) } } Return result class
76. inputStream.use { outputStream.use { // do something with the streams outputStream.write(inputStream.read()) } } “try-with-resources” in Kotlin - initial solution
77. arrayOf(inputStream, outputStream).use { // do something with the streams outputStream.write(inputStream.read()) } “try-with-resources” in Kotlin - improved solution
78. private inline fun <T : Closeable?> Array<T>.use(block: ()->Unit) { // implementation } use implementation
79. CODE REVIEW
80. CODE REVIEW
81. 1. Help to improve the quality and consistency of the code 2. Exchange of knowledge and best practices 3. Learn the code base 4. New perspective(s) on your code 5. Learn new tips and tricks about writing code Code Review Advantages
82. Reviewer Author <Code>
83. Author
84. • Make sure you understand your task • Refactor the code if it’s unreadable • Write tests and follow the team conventions • Format your code before commit it Writing the code
85. The boy scout rule Leave the campground cleaner than you found it.
86. • Add relevant commit comments • Send pull requests often • Have minimum 2 reviewers (one is senior) Before the code review
87. • Be humble • You are on the same side with your reviewer(s) • Know when to unlearn the old habits After the code review
88. Reviewer
89. • I think… • I would… • I believe… • I suggest... Use I… comments
90. • Have you consider using… ? • What do you think about… ? • Have you tried to… ? Ask questions
91. • This code… • This function… • This line of code... It’s about the code, not about the coder
92. Feedback equation* Observation of a behavior Impact of the behavior Question or Request I observed this function has 60 lines. This makes it difficult for me to understand the logic. I suggest extracting a part of the code into other functions and give them relevant names. * Defined by Lara Hogan
93. 1. Define with your team a set of conventions 2. Justify technology use 3. Enforce good practices (XP) 4. Question until you understand 5. Criticize ideas, not people 6. Testing, testing, testing 7. Integrate early, integrate often 8. Emphasize collective ownership of code 9. Prioritize and actively evaluate trade-offs 10. Listen to users My summary
94. Coding is not a sprint It’s a marathon
95. CREDITS: This presentation template was created by Slidesgo, including icons by Flaticon, and infographics & images by Freepik THANKS Do you have any questions? @magdamiu
96. 1. Article “CommandQuerySeparation” by Martin Fowler 2. “Law of Demeter: Principle of Least Knowledge” by Professor Karl, Northeastern University 3. Article “Breaking the Law of Demeter is Like Looking for a Needle in the Haystack” by Miško Hevery 4. Article “TellDontAsk” by Martin Fowler 5. Article “One Assertion Per Test” by Dave Astels 6. The Tragedy Of Checked Exceptions by Howard Lewis Ship 7. Exceptions are Bad by Jed Wesley-Smith 8. Checked exceptions I love you, but you have to go by Miško Hevery If you want to learn more...