droidcon Online: 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. Ash Davies
   GDE Berlin
   @askashdavies
   Refactoring Legacy
   Code with Kotlin
   droidcon Online
   

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2. Legacy
   adj.
   Denoting or relating to software or hardware that has been superseded
   but is difficult to replace because of its wide use.
   

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3. Working Effectively
   with Legacy Code
   Michael Feathers
   ● Untested code
   ● Regression tests
   ● Lack of confidence
   

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4. Java ☕
   

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5. JVM != ART
   

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7. ● Lambda expressions
   ● Method references
   ● Default interfaces
   ● Try-with-resources
   ● Streams ↝
   Java 8
   Released March 18, 2014
   

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8. Kotlin
   noun
   Freakin’ awesome
   

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9. ● Consistently familiar code
   ● Idiomatically behaviour
   ● Looks terse and concise
   ● Aggressive development
   Kotlin
   Freakin’ awesome
   

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10. Nullability.
    The billion dollar mistake
    

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11. “This has led to innumerable errors,
    vulnerabilities, and system crashes,
    which have probably caused a billion
    dollars of pain and damage in the last
    forty years.”
    - Sir Tony Hoare
    

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12. if (foo == null) {
    bar();
    }
    

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13. Defensive Code <>
    

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14. Offensive Code <>
    

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15. Urgency
    

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16. Idiomacy
    

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17. Idiomatic
    adj.
    Using, containing, or denoting expressions that are natural to a native
    speaker.
    

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18. Idiomatic use of
    language features
    kotlinlang.org/docs/reference/coding-conventions.html
    #idiomatic-use-of-language-feature
    

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19. ● Consistent, easier to read
    ● Less cognitive load
    ● Less ambiguity
    ● Function over style
    Idiomatic Code
    Idiomatic use of language features
    

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20. Kotlin style guide
    developer.android.com/kotlin/style-guide
    

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21. ktlint
    github.com/pinterest/ktlint
    

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22. Refactoring legacy code
    

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23. ⌥⇧⌘K
    

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24. Nullability
    

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25. class CoffeeMaker {
    @Inject Heater heater;
    @Inject Pump pump;
    ...
    }
    

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26. class CoffeeMaker {
    @Inject var heater: Heater? = null
    @Inject var pump: Pump? = null
    ...
    }
    

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27. class CoffeeMaker {
    @Inject internal lateinit var heater: Heater
    @Inject internal lateinit var pump: Pump
    ...
    }
    

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28. class CoffeeMaker {
    @Inject internal lateinit var heater: Heater
    @Inject internal lateinit var pump: Pump
    fun brew() {
    heater.heat() // Throws UninitializedPropertyAccessException
    }
    ...
    }
    

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29. class CoffeeMaker {
    @Inject internal lateinit var heater: Heater
    @Inject internal lateinit var pump: Pump
    fun brew() {
    heater!!.heat() // Throws NullPointerException
    }
    ...
    }
    

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30. class CoffeeMaker {
    @Inject internal lateinit var heater: Heater
    @Inject internal lateinit var pump: Pump
    fun brew() {
    if (::file.isInitialized) {
    heater.heat() // Defensive<>
    }
    }
    ...
    }
    

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31. class CoffeeMaker {
    @Inject internal lateinit var heater: Heater
    @Inject internal lateinit var pump: Pump
    fun brew() {
    heater.heat() // Offensive<>
    }
    ...
    }
    

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32. class CoffeeMaker {
    private var heater: Heater? = null
    private var pump: Pump? = null
    fun brew() {
    requireNotNull(heater) {
    "Heater was null because ..."
    }.brew()
    }
    ...
    }
    

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33. class CoffeeMaker {
    private var heater: Heater? = null
    private var pump: Pump? = null
    fun brew() {
    val heater: Heater = heater ?: throw ImpossibleException
    heater.brew()
    }
    ...
    object ImpossibleException : IllegalArgumentException()
    }
    Elvis Operator ?:
    

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34. object 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.
    """)
    xkcd.com/2200/
    

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35. Beans
    

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36. public class User {
    private String firstName;
    private String lastName;
    public User(String firstName, String lastName) { ... }
    public String getFirstName() { ... }
    public void setFirstName(String firstName) { ... }
    public String getLastName() { ... }
    public void setLastName(String lastName) { ... }
    @Override
    public boolean equals(Object o) { ... }
    @Override
    public int hashCode() { ... }
    }
    

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37. class User(var firstName: String?, var lastName: String?) {
    override fun equals(o: Any?): Boolean { ... }
    override fun hashCode(): Int { ... }
    }
    

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38. data class User(
    val firstName: String,
    val lastName: String
    )
    

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39. Extensions
    

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40. sharedPreferences
    .edit()
    .putBoolean("key", value)
    .apply()
    

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41. inline fun SharedPreferences.edit(
    commit: Boolean = false,
    action: Editor.() -> Unit
    ) {
    val editor = edit()
    action(editor)
    if (commit) {
    editor.commit()
    } else {
    editor.apply()
    }
    }
    

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42. sharedPreferences.edit {
    putBoolean("key", value)
    }
    

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43. ● Default values
    ● Singleton objects
    ● String interpolation
    ● Destructuring
    ● Scope functions
    Kotlin Idioms
    kotlinlang.org/docs/reference/idioms.html
    

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44. Maintaining History
    

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45. ● Rename .java -> .kt
    ● First commit/merge
    ● Kotlin conversion
    ● Second commit/merge
    Maintaining History
    Git
    

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46. Maintaining History
    Refactoring
    

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47. ● Single Responsibility Principle
    ● Open/Closed Principle
    ● Liskov Substitution Principle
    ● Interface Segregation Principle
    ● Dependency Inversion Principle
    Refactoring Legacy Code
    SOLID Design principles
    

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49. ● Perspective
    ● Better understanding
    ● Static ‍♂
    ● Asynchronicity
    Refactoring Legacy Code
    SOLID Design principles
    

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50. Refactoring Legacy
    Code with Kotlin
    Concurrency
    

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51. new Thread(() -> {
    foo();
    }).start();
    

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52. CompletableFuture.supplyAsync(this::findSomeData)
    .thenApply(this:: intReturningMethod)
    .thenAccept(this::notify);
    

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53. 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, getIntent()))
    .observeOn(AndroidSchedulers.mainThread())
    .subscribeOn(Schedulers.io())
    .subscribe(
    resource -> repository.setResource(resourceId, resource, provisionalResourceId),
    resourceUploadError(resourceId, resourceDraft, provisionalResourceId)
    );
    Well that escalated quickly...
    

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54. Coroutines
    

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55. fun main() {
    GlobalScope.launch {
    delay(1000L)
    println("World!")
    }
    println("Hello,")
    Thread.sleep(2000L)
    }
    Hello,
    World!
    

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56. Kotlin Coroutines
    Stability
    

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57. Building SDKs - The
    Kotlin Way: Jossi Wolf
    bit.ly/sdks-the-kotlin-way
    

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58. ● Native library
    ● Efficient
    ● Easy-to-use
    ● suspend fun
    Refactoring Legacy Code
    Kotlin Coroutines
    

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59. fun main() {
    GlobalScope.launch {
    delay(1000L)
    println("World!")
    }
    println("Hello,")
    Thread.sleep(2000L)
    }
    Hello,
    World!
    

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60. fun main() {
    GlobalScope.launch {
    doWorld()
    }
    println("Hello,")
    Thread.sleep(2000L)
    }
    suspend fun doWorld() {
    delay(1000L)
    println("World!")
    }
    Hello,
    World!
    

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61. 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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62. @Test
    fun testFoo() = runBlockingTest {
    val actual = foo()
    ...
    }
    suspend fun foo() {
    delay(1_000L)
    ...
    }
    Easy to test!
    

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63. Coroutine Testing
    kotlinx-coroutines-test
    org.jetbrains.kotlinx:kotlinx-coroutines-test:1.3.3
    

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64. ● codelabs.developers.google.com/codelabs/java-to-kotlin/
    ● github.com/Kotlin/kotlinx.coroutines/tree/master/kotlinx-coroutines-test
    ● droidcon.com/media-detail?video=352671106
    ● speakerdeck.com/ashdavies/mdevcamp-rxjava-and-coroutines-a-practical-analysis
    Refactoring Legacy Code
    Further Reading
    

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65. Ash Davies
    GDE Berlin
    @askashdavies
    Thank You!
    

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