DevFest Ukraine 2020 | Clean Code with Kotlin

Talk at DevFest Ukraine 2020 #DevFest #GDGfun

Bio:
Magda Miu is an enthusiastic and innovative Squad Lead Developer at YOXO.ro (Orange) and GDE for Android with more than 10 years experience in software development.

Abstract:
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.
“Coding is not a sprint, is a marathon” so join my session and let’s exercise together our clean code skills.

Transcript

1. Magda Miu @magdamiu Squad Lead Developer at YOXO.ro (Orange) Android Google Developer Expert Clean Code with Kotlin
2. A. An app that works perfectly but is very difficult to add new features or to change. What would you prefer to have: B. An app that does not contain all the features required by stakeholders but it is easy to extend and change.
3. What is Clean Code? “Express your intentions clearly to the reader of the code. Unreadable code isn’t clever.” Venkat Subramaniam
6. Code quality “measure” Few WTFs Developer Many WTFs Developer WTF/min WTF = What a Terrible Feature
7. MEANINGFUL NAMES
8. 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
9. Examples Types Names Classes and Objects Customer, Account, WikiPage Methods postPayment, deleteAccount, displayPage Solution domain names AccountVisitor Problem domain names churnPerMonth
10. “UNCLEAN” CODE CLEAN CODE
11. 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
12. 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
13. 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”
14. 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”
15. FUNCTIONS
16. Functions are the verbs of the language, and classes are the nouns.
17. Basic Rules: ➔ The functions should be small ➔ The functions should be smaller than that Make it smaller
18. Extra Rules: ➔ No many arguments ➔ No side effects ➔ Indent level max 2 Make it smaller
19. 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") }
20. 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()}") }
21. 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
22. var countUsersYoungerThan30WithSubscriptions = 0 for (user in users) { if (user.isYoungerThan30WithSubscriptions) { countUsersYoungerThan30WithSubscriptions++; } } WARNING: keep the abstraction level consistent and avoid nested code
23. fun sumUpUserPoints(): Int { var sumAllPoints = 0 for (user in users) { sumAllPoints += user.points sendEmail(user) } return sumAllPoints } No side effects Side effect
24. Command Query Separation Command Query Output System System System Changed state Output (result) Changed state Output (result)
25. override fun onSaveInstanceState(outState: Bundle) { super.onSaveInstanceState(outState) outState.putString(KEY_USER_NAME, username) } Command
26. 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
27. CLASSES Dog Cat Baby Dog Pet
28. 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.
29. The newspaper metaphor
30. // 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
31. val listOfYears = listOf(2017, 2018, 2019, 2020) listOfYears.add(2021) Read-only collections
32. 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
33. 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.
34. Cohesion
35. 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.
36. Coupling
37. Best case scenario Cohesion Coupling
38. Principles 1. DRY 2. KISS 3. YAGNI 4. SOLID
39. • Don’t Repeat Yourself • Applicable whenever we copy / paste a piece of code D.R.Y.
40. • Keep It Simple and Stupid • Whenever we want to implement a method to do all things K.I.S.S.
41. • You Ain’t Gonna Need It • Don’t write code which is not yet necessary Y.A.G.N.I.
42. • Single responsibility (SRP) • Open-closed (OCP) • Liskov substitution (LSP) • Interface segregation (ISP) • Dependency inversion (DIP) S.O.L.I.D.
43. OBJECTS AND DATA STRUCTURES
44. 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
45. class Company { var name: String = "" var address: String = "" } DATA STRUCTURE
46. class Company { var name: String = "" get() { if (!field.isEmpty()) { return field.toUpperCase() } return field } var address: String = "" } OBJECT
47. 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.
48. The law of Demeter A module should not know about the internals of the objects it interacts with. Demeter Goddess of the Harvest
49. The law of Demeter Only talk to your friends who share your concerns Demeter Goddess of the Harvest
50. 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
51. 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
52. 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 Note: A friend of a friend is a stranger
53. App “architecture” Model Room Remote Data source Retrofit SQLite REST API Activity / Fragment
54. Recommended app architecture Activity / Fragment Model Room Remote Data source Retrofit SQLite REST API ViewModel LiveData Repository
55. Room.databaseBuilder(context.getApplicationContext(), UserDatabase::class.java, DATABASE_NAME) .addMigrations(MIGRATION_1_2, MIGRATION_2_3) .fallbackToDestructiveMigrationFrom(4) .build() Fluent APIs
56. “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.
57. 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 } } } }
58. class Balance(var value: Double, val alertThreshold: Int) { val isExceedsThreshold: Boolean get() = value >= alertThreshold }
59. class Client { fun alertService(balances: List<Balance>) { for (balance in balances) { if (balance.isExceedsThreshold) { // alert } else { // everything is ok } } } }
60. Objects vs Data Tell, don’t ask. Don’t talk to strangers. Dealing with Objects? Dealing with Data?
61. ERROR HANDLING
62. • 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
63. Types/ Nullable Types (recap) @Nullable Type @NotNull Type Java Type Type? Type Kotlin ?
64. val view = activity!!.findViewById<View>(R.id.menu_add) kotlin.KotlinNullPointerException Not null assertion operator !! OR Bang! Bang! Boom! operator
65. fun computeSqrt(number: Double): Double { if(number >= 0) { return Math.sqrt(number) } else { throw RuntimeException("No negative please") } } Nothing is something...
66. fun getMovie(id: Int): Movie { val movie = movieRepository.findMovie(id) return movie ?: throw RuntimeException("Movie not found") } Throw exceptions
67. 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
68. CODE REVIEW
69. CODE REVIEW
70. 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
71. Reviewer Author <Code>
72. Author
73. • 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
74. The boy scout rule Leave the campground cleaner than you found it.
75. • Add relevant commit comments • Send pull requests often • Have minimum 2 reviewers (one is senior) Before the code review
76. • Be humble • You are on the same side with your reviewer(s) • Know when to unlearn the old habits After the code review
77. Reviewer
78. • I think… • I would… • I believe… • I suggest... Use I… comments
79. • Have you consider using… ? • What do you think about… ? • Have you tried to… ? Ask questions
80. • This code… • This function… • This line of code... It’s about the code, not about the coder
81. 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
82. 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
83. Coding is not a sprint It’s a marathon
84. CREDITS: This presentation template was created by Slidesgo, including icons by Flaticon, and infographics & images by Freepik THANKS Do you have any questions? @magdamiu
85. 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...