Kotlin Everywhere Coimbra: Refactoring Legacy Code with Kotlin

Legacy code can be quite the challenge to manage, often resulting from untested scenarios, quick fixes, or less than successful initiatives. With few developers wanting to deal with it, it can end up with little remaining knowledge of its inner workings.

We can take many learnings from Michael Feathers book on "Working Effectively with Legacy Code", but we can also use Kotlin migration as an effective tool to leverage the management, reduction, and removal of legacy code in our applications.

In this session, you'll learn how to get started with Kotlin in your projects, tips and tricks on how to preserve your version control history, some pitfalls when migrating from Java, and what new technologies you can make use of in your journey with Kotlin.

Transcript

1. Refactoring Legacy Code with Kotlin Kotlin Everywhere: Coimbra ! @askashdavies | sli.do/Z169
2. Legacy adj. Denoting or relating to so-ware or hardware that has been superseded but is di4cult to replace because of its wide use. @askashdavies | sli.do/Z169
3. Working Effectively with Legacy Code Michael Feathers » Untested code » Regression tests » Lack of con3dence @askashdavies | sli.do/Z169
4. ! @askashdavies | sli.do/Z169
5. Java @askashdavies | sli.do/Z169
6. JVM != ART @askashdavies | sli.do/Z169
7. twitter.com/QuinnyPig/status/1174434046853599232
8. Java 8 » Lambda expressions » Method references » Default inte8aces » try-with-resources @askashdavies | sli.do/Z169
9. Streams ↝ @askashdavies | sli.do/Z169
10. @askashdavies | sli.do/Z169
11. Kotlin @askashdavies | sli.do/Z169
12. Null ! @askashdavies | sli.do/Z169
13. if (foo == null) { bar(); } @askashdavies | sli.do/Z169
14. Defensive Code < > @askashdavies | sli.do/Z169
15. Offensive Code < > @askashdavies | sli.do/Z169
16. Urgency @askashdavies | sli.do/Z169
17. Kotlin? @askashdavies | sli.do/Z169
18. Kotlin @askashdavies | sli.do/Z169
19. @askashdavies | sli.do/Z169
20. Google IO 2018 @askashdavies | sli.do/Z169
21. ! " @askashdavies | sli.do/Z169
22. Google IO 2019 @askashdavies | sli.do/Z169
23. Kotlin @askashdavies | sli.do/Z169
24. Libraries @askashdavies | sli.do/Z169
25. @askashdavies | sli.do/Z169
26. @askashdavies | sli.do/Z169
27. @askashdavies | sli.do/Z169
28. @askashdavies | sli.do/Z169
29. @askashdavies | sli.do/Z169
30. Rick and Morty, Adult Swim
31. Kotlin @askashdavies | sli.do/Z169
32. Idiomacy @askashdavies | sli.do/Z169
33. @askashdavies | sli.do/Z169
34. Idiomatic adj. Using, containing, or denoting expressions that are natural to a native speaker @askashdavies | sli.do/Z169
35. Idiomatic use of Language Features kotlinlang.org/docs/reference/coding-conventions.html#idiomatic-use-of-language-features
36. Idiomatic Code » Consistent, easier to read » Less cognitive load » Less ambiguity » Function > Style @askashdavies | sli.do/Z169
37. Code Style » kotlinlang.org/docs/reference/coding-conventions.html » android.github.io/kotlin-guides/style.html @askashdavies | sli.do/Z169
38. Refactoring Legacy Code @askashdavies | sli.do/Z169
39. ⌃⌥⇧K @askashdavies | sli.do/Z169
43. public class BadJavaActivity extends Activity { @Inject Dependency dependency; } @askashdavies | sli.do/Z169
44. General Assumptions How I Met Your Mother, CBS
45. class BadKotlinActivity : Activity() { @Inject var dependency: Dependency? = null } @askashdavies | sli.do/Z169
46. class SlightlyLessBadKotlinActivity : Activity() { @Inject internal lateinit var dependency: Dependency } @askashdavies | sli.do/Z169
47. UninitializedPropertyAccessException! @askashdavies | sli.do/Z169
48. lateinit var file: File if (::file.isInitialized) { /* ... */ } @askashdavies | sli.do/Z169
49. Nullability @askashdavies | sli.do/Z169
50. !! @askashdavies | sli.do/Z169
51. // Throws NullPointerException myNullVal.myMethod() @askashdavies | sli.do/Z169
52. // Still throws NullPointerException myNullVal!!.myMethod() @askashdavies | sli.do/Z169
53. // Does nothing ! myNullVal?.myMethod() @askashdavies | sli.do/Z169
54. // Still does nothing ! myNullVal?.also { it.myMethod() } @askashdavies | sli.do/Z169
55. // Defensive Code < ! > myNullVal?.also { it.myMethod() } @askashdavies | sli.do/Z169
56. // Offensive Code < ! > myNullVal!!.myMethod() @askashdavies | sli.do/Z169
57. // throws IllegalArgumentException("Required value was null.") requireNotNull(myNullVal).myMethod() @askashdavies | sli.do/Z169
58. // throws IllegalArgumentException("myNullVaL was null because ...") requireNotNull(myNullVal) { "myNullVaL was null because ..." }.myMethod() @askashdavies | sli.do/Z169
59. class ImpossibleException : IllegalArgumentException() // throws ImpossibleException() val myNotNullVal = myNullVall ?: throw ImpossibleException() myNotNullVal.myMethod() @askashdavies | sli.do/Z169
60. class ImpossibleException : IllegalArgumentException(""" If you're seeing this, the code is in what I thought was an unreachable state. I could give you advice for what to do, but honestly, why should you trust me? I clearly screwed this up. I'm writing a message that should never appear, yet I know it will probably appear someday. On a deep level, I know I'm not up to this task. I'm so sorry. """) // throws ImpossibleException() val myNotNullVal = myNullVall ?: throw ImpossibleException() myNotNullVal.myMethod() xkcd.com/2200/
61. Data Classes @askashdavies | sli.do/Z169
62. public class User { private String firstName; private String lastName; public User(String firstName, String lastName) { this.firstName = firstName; this.lastName = lastName; } public String getFirstName() { return firstName; } public void setFirstName(String firstName) { this.firstName = firstName; } public String getLastName() { return lastName; } public void setLastName(String lastName) { this.lastName = lastName; } @Override public boolean equals(Object o) { if (this == o) { return true; } if (o == null || getClass() != o.getClass()) { return false; } User user = (User) o; return Objects.equals(firstName, user.firstName) && Objects.equals(lastName, user.lastName); } @Override public int hashCode() { return Objects.hash(firstName, lastName); } } @askashdavies | sli.do/Z169
63. class User(var firstName: String?, var lastName: String?) { override fun equals(o: Any?): Boolean { if (this === o) { return true } if (o == null || javaClass != o.javaClass) { return false } val user = o as User? return firstName == user!!.firstName && lastName == user.lastName } override fun hashCode(): Int { return Objects.hash(firstName, lastName) } } @askashdavies | sli.do/Z169
64. data class User( val firstName: String, val lastName: String ) @askashdavies | sli.do/Z169
65. data class User( val firstName: String, val lastName: String? = null ) @askashdavies | sli.do/Z169
66. @NotNull public final User copy(@Nullable String firstName, @Nullable String lastName) { return new User(firstName, lastName); } @askashdavies | sli.do/Z169
67. Kotlin: Idioms » Singleton objects » String interpolation » Elvis operator ! » Destructuring » Extension functions » Scoping functions kotlinlang.org/docs/reference/idioms.html
68. Maintaining History @askashdavies | sli.do/Z169
69. Maintaining History » Change extension .java -> .kt » First commit » Apply Kotlin conversion » Second commit @askashdavies | sli.do/Z169
70. Maintaining History Refactoring @askashdavies | sli.do/Z169
71. Refactoring @askashdavies | sli.do/Z169
72. Refactoring SOLID » Single-responsibility » Open-closed » Liskov substitution » Inte9ace segregation » Dependency inversion @askashdavies | sli.do/Z169
73. @askashdavies | sli.do/Z169
74. Refactoring Single Responsibility @askashdavies | sli.do/Z169
75. Perspective @askashdavies | sli.do/Z169
76. Static @askashdavies | sli.do/Z169
77. Asynchronicity @askashdavies | sli.do/Z169
78. Concurrency is Hard @askashdavies | sli.do/Z169
79. Thread new Thread(() -> { foo(); }).start(); @askashdavies | sli.do/Z169
80. CompletableFuture CompletableFuture.supplyAsync(this::findSomeData) .thenApply(this:: intReturningMethod) .thenAccept(this::notify); @askashdavies | sli.do/Z169
81. RxJava Observable .fromIterable(resourceDraft.getResources()) .flatMap(resourceServiceApiClient::createUploadContainer) .zipWith(Observable.fromIterable(resourceDraft.getResources()), Pair::create) .flatMap(uploadResources()) .toList() .toObservable() .flatMapMaybe(resourceCache.getResourceCachedItem()) .defaultIfEmpty(Resource.getDefaultItem()) .flatMap(postResource(resourceId, resourceDraft.getText(), currentUser, resourceDraft.getIntent())) .observeOn(AndroidSchedulers.mainThread()) .subscribeOn(Schedulers.io()) .subscribe( resource -> repository.setResource(resourceId, resource, provisionalResourceId), resourceUploadError(resourceId, resourceDraft, provisionalResourceId) ); @askashdavies | sli.do/Z169
82. @askashdavies | sli.do/Z169
83. What If? @askashdavies | sli.do/Z169
84. Coroutines @askashdavies | sli.do/Z169
85. fun main() { GlobalScope.launch { delay(1000L) println("World!") } println("Hello,") Thread.sleep(2000L) } // Hello, // World! @askashdavies | sli.do/Z169
86. fun main() { GlobalScope.launch { delay(1000L) println("World!") } println("Hello,") Thread.sleep(2000L) } // Hello, // World! @askashdavies | sli.do/Z169
87. fun main() { GlobalScope.launch { delay(1000L) println("World!") } println("Hello,") Thread.sleep(2000L) } // Hello, // World! @askashdavies | sli.do/Z169
88. Stability @askashdavies | sli.do/Z169
89. Annotations @Experimental @askashdavies | sli.do/Z169
90. Building SDKs - The Kotlin Way Kotlin Everywhere: Hamburg - Jossi Wolf speakerdeck.com/jossiwolf/building-sdks-the-kotlin-way-ccb4a237-b45c-42e7-8391-640dd058f50c
91. ! Coroutines @askashdavies | sli.do/Z169
92. ! Native library @askashdavies | sli.do/Z169
93. Efficient @askashdavies | sli.do/Z169
94. ! Easy-to-use @askashdavies | sli.do/Z169
95. ! suspend fun @askashdavies | sli.do/Z169
96. ! Suspend fun main() { GlobalScope.launch { delay(1000L) println("World!") } println("Hello,") Thread.sleep(2000L) } // Hello, // World! @askashdavies | sli.do/Z169
97. ! Suspend fun main() { GlobalScope.launch { doWorld() println("Hello,") } Thread.sleep(2000L) } suspend fun doWorld() { delay(1000L) println("World!") } // Hello, // World! @askashdavies | sli.do/Z169
98. ! Suspend fun main() { GlobalScope.launch { launch { doWorld() } println("Hello,") } Thread.sleep(2000L) } suspend fun doWorld() { withContext(Dispatchers.IO) { delay(1000L) println("World!") } } // Hello, // World! @askashdavies | sli.do/Z169
99. Testing @Test fun testFoo() = runBlockingTest { val actual = foo() // ... } suspend fun foo() { delay(1_000) // ... } @askashdavies | sli.do/Z169
100. Further Reading ! » Google Codelab: Refactoring to Kotlin codelabs.developers.google.com/codelabs/java-to-kotlin/ » KotlinX Coroutine Test github.com/Kotlin/kotlinx.coroutines/tree/master/kotlinx-coroutines-test » Sean McQuillan: Coroutines + Testing = droidcon.com/media-detail?video=352671106 » Ash Davies: RxJava & Coroutines: A Practical Analysis v3 speakerdeck.com/ashdavies/rxjava-and-coroutines-a-practical-analysis-v3 @askashdavies | sli.do/Z169
101. Thanks! @askashdavies | sli.do/Z169
102. Questions sli.do/Z169