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
   

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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   Denoting or relating to so-ware or hardware that has been superseded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
   

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3. Working Effectively
   with Legacy Code
   Michael Feathers
   » Untested code
   » Regression tests
   » Lack of con3dence
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4. !
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5. Java
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6. JVM != ART
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7. twitter.com/QuinnyPig/status/1174434046853599232
   

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8. Java 8
   » Lambda expressions
   » Method references
   » Default inte8aces
   » try-with-resources
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9. Streams ↝
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10. @askashdavies | sli.do/Z169
    

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11. Kotlin
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12. Null
    !
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13. if (foo == null) {
    bar();
    }
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14. Defensive Code < >
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15. Offensive Code < >
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16. Urgency
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17. Kotlin?
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18. Kotlin
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19. @askashdavies | sli.do/Z169
    

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20. Google IO 2018
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21. ! "
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22. Google IO 2019
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23. Kotlin
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24. Libraries
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25. @askashdavies | sli.do/Z169
    

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26. @askashdavies | sli.do/Z169
    

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27. @askashdavies | sli.do/Z169
    

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28. @askashdavies | sli.do/Z169
    

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29. @askashdavies | sli.do/Z169
    

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30. Rick and Morty, Adult Swim
    

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31. Kotlin
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32. Idiomacy
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33. @askashdavies | sli.do/Z169
    

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34. Idiomatic
    adj.
    Using, containing, or denoting expressions that are natural to a native
    speaker
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35. Idiomatic use of Language Features
    kotlinlang.org/docs/reference/coding-conventions.html#idiomatic-use-of-language-features
    

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36. Idiomatic Code
    » Consistent, easier to read
    » Less cognitive load
    » Less ambiguity
    » Function > Style
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37. Code Style
    » kotlinlang.org/docs/reference/coding-conventions.html
    » android.github.io/kotlin-guides/style.html
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38. Refactoring Legacy Code
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39. ⌃⌥⇧K
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43. public class BadJavaActivity extends Activity {
    @Inject Dependency dependency;
    }
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44. General Assumptions
    How I Met Your Mother, CBS
    

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45. class BadKotlinActivity : Activity() {
    @Inject var dependency: Dependency? = null
    }
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46. class SlightlyLessBadKotlinActivity : Activity() {
    @Inject internal lateinit var dependency: Dependency
    }
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47. UninitializedPropertyAccessException!
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48. lateinit var file: File
    if (::file.isInitialized) { /* ... */ }
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49. Nullability
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50. !!
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51. // Throws NullPointerException
    myNullVal.myMethod()
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52. // Still throws NullPointerException
    myNullVal!!.myMethod()
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53. // Does nothing
    !
    myNullVal?.myMethod()
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54. // Still does nothing
    !
    myNullVal?.also {
    it.myMethod()
    }
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55. // Defensive Code <
    !
    >
    myNullVal?.also {
    it.myMethod()
    }
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56. // Offensive Code <
    !
    >
    myNullVal!!.myMethod()
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57. // throws IllegalArgumentException("Required value was null.")
    requireNotNull(myNullVal).myMethod()
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58. // throws IllegalArgumentException("myNullVaL was null because ...")
    requireNotNull(myNullVal) { "myNullVaL was null because ..." }.myMethod()
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59. class ImpossibleException : IllegalArgumentException()
    // throws ImpossibleException()
    val myNotNullVal = myNullVall ?: throw ImpossibleException()
    myNotNullVal.myMethod()
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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/
    

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61. Data Classes
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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);
    }
    }
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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)
    }
    }
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64. data class User(
    val firstName: String,
    val lastName: String
    )
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65. data class User(
    val firstName: String,
    val lastName: String? = null
    )
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66. @NotNull
    public final User copy(@Nullable String firstName, @Nullable String lastName) {
    return new User(firstName, lastName);
    }
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67. Kotlin: Idioms
    » Singleton objects
    » String interpolation
    » Elvis operator
    !
    » Destructuring
    » Extension functions
    » Scoping functions
    kotlinlang.org/docs/reference/idioms.html
    

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68. Maintaining History
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69. Maintaining History
    » Change extension .java -> .kt
    » First commit
    » Apply Kotlin conversion
    » Second commit
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70. Maintaining History
    Refactoring
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71. Refactoring
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72. Refactoring
    SOLID
    » Single-responsibility
    » Open-closed
    » Liskov substitution
    » Inte9ace segregation
    » Dependency inversion
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73. @askashdavies | sli.do/Z169
    

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74. Refactoring
    Single Responsibility
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75. Perspective
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76. Static
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77. Asynchronicity
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78. Concurrency is Hard
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79. Thread
    new Thread(() -> {
    foo();
    }).start();
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80. CompletableFuture
    CompletableFuture.supplyAsync(this::findSomeData)
    .thenApply(this:: intReturningMethod)
    .thenAccept(this::notify);
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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)
    );
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82. @askashdavies | sli.do/Z169
    

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83. What If?
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84. Coroutines
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85. fun main() {
    GlobalScope.launch {
    delay(1000L)
    println("World!")
    }
    println("Hello,")
    Thread.sleep(2000L)
    }
    // Hello,
    // World!
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86. fun main() {
    GlobalScope.launch {
    delay(1000L)
    println("World!")
    }
    println("Hello,")
    Thread.sleep(2000L)
    }
    // Hello,
    // World!
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87. fun main() {
    GlobalScope.launch {
    delay(1000L)
    println("World!")
    }
    println("Hello,")
    Thread.sleep(2000L)
    }
    // Hello,
    // World!
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88. Stability
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89. Annotations
    @Experimental
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90. Building SDKs - The Kotlin Way
    Kotlin Everywhere: Hamburg - Jossi Wolf
    speakerdeck.com/jossiwolf/building-sdks-the-kotlin-way-ccb4a237-b45c-42e7-8391-640dd058f50c
    

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91. !
    Coroutines
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92. !
    Native library
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93. Efficient
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94. !
    Easy-to-use
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95. !
    suspend fun
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96. !
    Suspend
    fun main() {
    GlobalScope.launch {
    delay(1000L)
    println("World!")
    }
    println("Hello,")
    Thread.sleep(2000L)
    }
    // Hello,
    // World!
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97. !
    Suspend
    fun main() {
    GlobalScope.launch {
    doWorld()
    println("Hello,")
    }
    Thread.sleep(2000L)
    }
    suspend fun doWorld() {
    delay(1000L)
    println("World!")
    }
    // Hello,
    // World!
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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!
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99. Testing
    @Test
    fun testFoo() = runBlockingTest {
    val actual = foo()
    // ...
    }
    suspend fun foo() {
    delay(1_000)
    // ...
    }
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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
     

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101. Thanks!
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102. Questions
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