Kotlin Everywhere Hamburg: Refactoring Legacy Code with Kotlin and Coroutines

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 it's 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 & Coroutines
   Kotlin Everywhere: Hamburg
   @askashdavies
   

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2. @askashdavies
   

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3. Java
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4. { }
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5. Streams ↝
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6. @askashdavies
   

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7. Kotlin
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8. Null
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9. if (foo == null) {
   bar();
   }
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10. O"ensive Code < >
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11. Urgency
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12. Kotlin?
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13. Kotlin
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14. @askashdavies
    

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15. Google IO 2018
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16. ! "
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17. Google IO 2019
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18. Kotlin
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19. Libraries
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20. @askashdavies
    

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21. @askashdavies
    

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22. @askashdavies
    

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23. @askashdavies
    

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24. @askashdavies
    

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25. @askashdavies
    

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26. Kotlin
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27. Idiomacy
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28. @askashdavies
    

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29. Idiomatic Code
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30. Idiomatic Code
    • Consistent, easier to read
    • Less cognitive load
    • Less ambiguity
    • Function > Style
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31. Code Style
    • kotlinlang.org/docs/reference/coding-conventions.html
    • android.github.io/kotlin-guides/style.html
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32. Refactoring Legacy Code
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33. ⌃⌥⇧K
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34. @askashdavies
    

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35. @askashdavies
    

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36. public class BadJavaActivity extends Activity {
    @Inject Dependency dependency;
    }
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37. General Assumptions
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38. class BadKotlinActivity : Activity() {
    @Inject var dependency: Dependency? = null
    }
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39. class SlightlyLessBadKotlinActivity : Activity() {
    @Inject internal lateinit var dependency: Dependency
    }
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40. UninitializedPrope.yAccessException!
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41. lateinit var file: File
    if (::file.isInitialized) { /* ... */ }
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42. Data Classes
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43. 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);
    }
    }
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44. 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)
    }
    }
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45. data class User(val firstName: String, val lastName: String)
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46. data class User(val firstName: String, val lastName: String? = null)
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47. @NotNull
    public final User copy(@Nullable String firstName, @Nullable String lastName) {
    return new User(firstName, lastName);
    }
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48. Kotlin
    • Singleton objects
    • String interpolation
    • Elvis operator
    !
    • Destructuring
    • Extension functions
    • Scoping functions
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49. Maintaining History
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50. Maintaining History
    • Change extension .java -> .kt
    • First commit
    • Apply Kotlin conversion
    • Second commit
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51. Maintaining History
    Refactoring
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52. Refactoring
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53. Refactoring
    SOLID
    • Single-responsibility
    • Open-closed
    • Liskov substitution
    • Interface segregation
    • Dependency inversion
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54. Refactoring
    Single Responsibility
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55. Perspective
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56. Asynchronicity
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57. Concurrency is Hard
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58. Thread
    new Thread(() -> {
    foo();
    }).start();
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59. CompletableFuture
    CompletableFuture.supplyAsync(this::findSomeData)
    .thenApply(this:: intReturningMethod)
    .thenAccept(this::notify);
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60. 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)
    );
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61. @askashdavies
    

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62. What If?
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63. Coroutines
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64. fun main() {
    GlobalScope.launch {
    delay(1000L)
    println("World!")
    }
    println("Hello,")
    Thread.sleep(2000L)
    }
    // Hello,
    // World!
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65. fun main() {
    GlobalScope.launch {
    delay(1000L)
    println("World!")
    }
    println("Hello,")
    Thread.sleep(2000L)
    }
    // Hello,
    // World!
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66. fun main() {
    GlobalScope.launch {
    delay(1000L)
    println("World!")
    }
    println("Hello,")
    Thread.sleep(2000L)
    }
    // Hello,
    // World!
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67. ⚖
    Stability
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68. @askashdavies
    

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69. @Annotations
    (
    !
    Here be dragons)
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70. Annotations
    @ExperimentalCoroutinesApi //
    ⚠
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71. Annotations
    @ExperimentalCoroutinesApi //
    ⚠
    @FlowPreview //
    ⚠
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72. Annotations
    @ExperimentalCoroutinesApi //
    ⚠
    @FlowPreview //
    ⚠
    @ObsoleteCoroutinesApi //
    ⚠
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73. Annotations
    @ExperimentalCoroutinesApi //
    ⚠
    @FlowPreview //
    ⚠
    @ObsoleteCoroutinesApi //
    ⚠
    @InternalCoroutinesApi //
    ☠
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74. Annotations
    @Experimental
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75. !
    Coroutines
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76. !
    Native (rst-pa-y library
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77. E"cient
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78. !
    Easy-to-use
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79. !
    suspend fun
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80. !
    Suspend
    fun main() {
    GlobalScope.launch {
    delay(1000L)
    println("World!")
    }
    println("Hello,")
    Thread.sleep(2000L)
    }
    // Hello,
    // World!
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81. !
    Suspend
    fun main() {
    GlobalScope.launch {
    doWorld()
    println("Hello,")
    Thread.sleep(2000L)
    }
    }
    suspend fun doWorld() {
    delay(1000L)
    println("World!")
    }
    // Hello,
    // World!
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82. !
    Suspend
    fun main() {
    GlobalScope.launch {
    doWorld()
    println("Hello,")
    Thread.sleep(2000L)
    }
    }
    suspend fun doWorld() {
    withContext(Dispatchers.IO) {
    delay(1000L)
    println("World!")
    }
    }
    // Hello,
    // World!
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83. Dispatchers
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84. Dispatchers
    • Default
    • IO
    • Main
    • Android (Main Thread Dispatcher)
    • JavaFx (Application Thread Dispatcher)
    • Swing (Event Dispatcher Thread)
    • Unconfined
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85. Testing
    @Test
    fun testFoo() = runBlockingTest {
    val actual = foo()
    // ...
    }
    suspend fun foo() {
    delay(1_000)
    // ...
    }
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86. Fu#her Reading
    • Google Codelab: Refactoring to Kotlin
    https://codelabs.developers.google.com/codelabs/java-to-kotlin/
    • KotlinX Coroutine Test
    https://github.com/Kotlin/kotlinx.coroutines/tree/master/kotlinx-coroutines-test
    • Sean McQuillan: Coroutines + Testing =
    https://www.droidcon.com/media-detail?video=352671106
    • Ash Davies: RxJava & Coroutines: A Practical Analysis v3
    https://speakerdeck.com/ashdavies/rxjava-and-coroutines-a-practical-analysis-
    v3
    @askashdavies
    

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87. Thanks!
    @askashdavies
    

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