The Journey From Legacy Code to Idiomatic Kotlin

We are often faced with large legacy codebases in Java that need to be converted to Kotlin.

Sure, you can let Android Studio do the conversion, but then you end up with a bunch of !!s and inefficient Kotlin code. So where do you begin?

This talk walks you through key steps in converting your Java codebase to efficient, idiomatic Kotlin. After working with multiple clients on legacy projects, I have learned lessons on doing this conversion efficiently, and ensuring that you are taking full advantage of Kotlin. We will walk through the process, along with tips on major patterns that generally require attention. After all, shouldn't every developer's goal be to leave the codebase better than you found it?

Transcript

1. The Journey From Legacy Code to Idiomatic Kotlin Seetha Annamraju @seetha_a
2. Every Android project written in Java is now a legacy project.
3. I
4. ◂ What is idiomatic Kotlin? ◂ Convert common patterns into idiomatic Kotlin ◂ Ideas and suggestions Outline
5. ◂ You know basic Kotlin syntax. ◂ You believe code should be left better than you found it. Assumptions
6. Legacy
7. Step 1: Take a step back and look at your project.
8. ◂ How much time do I have? ◂ What are the priority items? ◂ Are there components that need fixes? Ask yourself:
9. Step 2: Start converting.
10. ◂ Rewrite the file in Kotlin from scratch. Option 1: Rewrite
11. ◂ Rewrite the file in Kotlin from scratch. ◂ Works well for large files or files with many dependencies. Option 1: Rewrite
12. ◂ "Convert Java file to Kotlin file” Option 2: Pull off the bandaid
13. ◂ 1. Compiler errors
14. ◂ 1. Compiler errors ◂ 2. Warnings and suggestions
15. ◂ 1. Compiler errors ◂ 2. Warnings and suggestions ◂ 3. Idiomatic Kotlin stuff
16. Step 3: Ensure Idiomatic code.
17. ◂ Null Safety ◂ Immutability ◂ Being expressive and concise ◂ Easy Interoperability What is idiomatic Kotlin?
18. Item #1: Null Safety Getting rid of !!s
19. ◂ ? —“maybe it’s null?” ◂ !! —“this is definitely NOT null!!” Null Safety
20. fun glDrawFrame(viewProjectionMatrix: FloatArray, eyeType: Int) { // ... if (frameAvailable.compareAndSet(true, false)) { displayTexture!!.updateTexImage() checkGlError() } displayMesh!!.glDraw(viewProjectionMatrix, eyeType) if (videoUiView != null) { canvasQuad!!.glDraw(viewProjectionMatrix, videoUiView.alpha) } reticle.glDraw(viewProjectionMatrix, controllerOrientationMatrix) } fun glShutdown() { if (displayMesh != null) { displayMesh!!.glShutdown() } // ... } @BinderThread @Synchronized fun setControllerOrientation(currentOrientation: Orientation) { this.controllerOrientation = currentOrientation controllerOrientation!!.toRotationMatrix(controllerOrientationMatrix) } @MainThread fun handleClick() { // ... val clickTarget = CanvasQuad.translateClick(controllerOrientation!!) // ... // The actual processing of the synthetic event needs to happen in the UI thread. uiHandler!!.post { // ... } } Code Sample
21. fun glDrawFrame(viewProjectionMatrix: FloatArray, eyeType: Int) { // ... if (frameAvailable.compareAndSet(true, false)) { displayTexture!!.updateTexImage() checkGlError() } displayMesh!!.glDraw(viewProjectionMatrix, eyeType) if (videoUiView != null) { canvasQuad!!.glDraw(viewProjectionMatrix, videoUiView.alpha) } reticle.glDraw(viewProjectionMatrix, controllerOrientationMatrix) } fun glShutdown() { if (displayMesh != null) { displayMesh!!.glShutdown() } // ... } @BinderThread @Synchronized fun setControllerOrientation(currentOrientation: Orientation) { this.controllerOrientation = currentOrientation controllerOrientation!!.toRotationMatrix(controllerOrientationMatrix) } @MainThread fun handleClick() { // ... val clickTarget = CanvasQuad.translateClick(controllerOrientation!!) // ... // The actual processing of the synthetic event needs to happen in the UI thread. uiHandler!!.post { // ... } } Code Sample !! !! !! !! !! !! !!
22. Safe-call operator (?) fun destroy() { mediaPlayer?.stop() mediaPlayer?.release() mediaPlayer = null isDestroyed = true }
23. Safe-call operator (?) fun destroy() { mediaPlayer?.stop() mediaPlayer?.release() mediaPlayer = null isDestroyed = true }
24. Use Elvis operator val result = name.toString ?: "Unknown"
25. Can you use lateinit var? @Inject lateinit var viewModelFactory:ViewModelProvider.Factory
26. Item #2: Immutability By lazy, local captures
27. Can you use by lazy? private MediaLoader mediaLoader; public void initialize(){ mediaLoader = new MediaLoader(getContext()); ... }
28. Can you use by lazy? private val mediaLoader: MediaLoader by lazy { MediaLoader(context) } private MediaLoader mediaLoader; public void initialize(){ mediaLoader = new MediaLoader(getContext()); ... }
29. Local captures of values
30. private var mediaPlayer: MediaPlayer? = null if (mediaPlayer != null && sceneRenderer != null) { displaySurface = sceneRenderer?.createDisplay( mediaPlayer!!.videoWidth, mediaPlayer!!.videoHeight, mesh) mediaPlayer!!.apply { setSurface(displaySurface) isLooping = true start() } } Local captures of values
31. private var mediaPlayer: MediaPlayer? = null if (mediaPlayer != null && sceneRenderer != null) { displaySurface = sceneRenderer?.createDisplay( mediaPlayer!!.videoWidth, mediaPlayer!!.videoHeight, mesh) mediaPlayer!!.apply { setSurface(displaySurface) isLooping = true start() } } Local captures of values
32. Local captures of values val player = mediaPlayer val renderer = sceneRenderer if (player != null && renderer != null) { displaySurface = renderer.createDisplay( player.videoWidth, player.videoHeight, mesh ) with(player) { setSurface(displaySurface) isLooping = true start() } }
33. Local captures of values val player = mediaPlayer val renderer = sceneRenderer if (player != null && renderer != null) { displaySurface = renderer.createDisplay( player.videoWidth, player.videoHeight, mesh ) with(player) { setSurface(displaySurface) isLooping = true start() } }
34. Item #3: Being more concise and expressive Using Kotlin’s Standard Library
35. Over 70% of your time is spent reading code, rather than writing code. -Unknown
36. Java if-statements if (detailResponse != null && detailResponse.getStudent() != null && !Helper.isListEmpty(detailResponse.getStudent() .getRegisteredClasses()) && detailResponse.getStudent() .getRegisteredClasses().get(0) != null && !Helper.isListEmpty(detailResponse.getStudent() .getRegisteredClasses() .get(0).getSubjectAssignments()) && detailResponse.getStudent().getRegisteredClasses().get(0) .getSubjectAssignments().get(0) != null){ //do something }
37. try{ detailResponse.getStudent().getRegisteredClasses() .get(0).getSubjectAssignments().get(0) } catch (NullPointerException e){} Java shorter statement
38. Kotlin’s Let detailResponse.getStudent?.getRegisteredClasses()? .get(0)?.getSubjectAssignments()?.get(0)?.let { //do something }
39. if (detailResponse != null && detailResponse.getStudent() != null && ! Helper.isListEmpty(detailResponse.getStudent() .getRegisteredClasses()) && detailResponse.getStudent() .getRegisteredClasses().get(0) != null && ! Helper.isListEmpty(detailResponse.getStudent() .getRegisteredClasses() .get(0).getSubjectAssignments()) && detailResponse.getStudent().getRegisteredClasses() .get(0) .getSubjectAssignments().get(0) != null){ //do something } Kotlin’s Let
40. if (detailResponse != null && detailResponse.getStudent() != null && ! Helper.isListEmpty(detailResponse.getStudent() .getRegisteredClasses()) && detailResponse.getStudent() .getRegisteredClasses().get(0) != null && ! Helper.isListEmpty(detailResponse.getStudent() .getRegisteredClasses() .get(0).getSubjectAssignments()) && detailResponse.getStudent().getRegisteredClasses() .get(0) .getSubjectAssignments().get(0) != null){ //do something } Kotlin’s Let detailResponse.getStudent?.getRegisteredClasses()? .get(0)?.getSubjectAssignments()?.get(0)?.let { //do something }
41. Apply mediaPlayer = (MediaPlayer()).apply { setDisplay(surfaceHolder) setDataSource(context, uri) prepare() setOnPreparedListener(preparedListener) }
42. Apply mediaPlayer = (MediaPlayer()).apply { setDisplay(surfaceHolder) setDataSource(context, uri) prepare() setOnPreparedListener(preparedListener) }
43. With fun getCourse(syllabus: Syllabus): Course { return with(syllabus) { Course(title, professorName, syllabus) } }
44. With fun getCourse(syllabus: Syllabus): Course { return with(syllabus) { Course(title, professorName, syllabus) } }
45. Transforming Collections
46. Transforming Collections Students -> StudentViewItems
47. Transforming Collections val studentsViewItems = studentsList .mapIndexed { index, student -> Pair(student.major, StudentViewItem(student.name, student.imgURL)) } .filter { (studentMajor, _) -> studentMajor == COMPUTER_SCIENCE }.map { (_, studentViewItem) -> studentViewItem }
48. Transforming Collections val studentsViewItems = studentsList .mapIndexed { index, student -> Pair(student.major, StudentViewItem(student.name, student.imgURL)) } .filter { (studentMajor, _) -> studentMajor == COMPUTER_SCIENCE }.map { (_, studentViewItem) -> studentViewItem }
49. Transforming Collections val studentsViewItems = studentsList .mapIndexed { index, student -> Pair(student.major, StudentViewItem(student.name, student.imgURL)) } .filter { (studentMajor, _) -> studentMajor == COMPUTER_SCIENCE }.map { (_, studentViewItem) -> studentViewItem }
50. Transforming Collections val studentsViewItems = studentsList .mapIndexed { index, student -> Pair(student.major, StudentViewItem(student.name, student.imgURL)) } .filter { (studentMajor, _) -> studentMajor == COMPUTER_SCIENCE }.map { (_, studentViewItem) -> studentViewItem }
51. Item #4: Easy interop Models and Utility Classes
52. Annotate your Java APIs @NonNull @Nullable
54. 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; } } Convert your Java Objects
55. 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; } } Convert your Java Objects data class User(var firstName: String?, var lastName: String?)
56. Extensions for utility classes public class ExtraStringUtils { public ExtraStringUtils(){} public static String toOppositeCase(String str) { // ... } }
57. StringExt.kt @JvmMultifileClass @file:JvmName("ExtraStringUtils") package com.example.androidtest // ... fun String.toOppositeCase(): String { // ... }
58. FAQ Common Questions
59. How do I do a large refactoring into Kotlin?
60. ◂ Commit #1: Convert file to Kotlin Refactoring to Kotlin
61. ◂ Commit #1: Convert file to Kotlin ◂ Commit #2: Fix compiler errors Refactoring to Kotlin
62. ◂ Commit #1: Convert file to Kotlin ◂ Commit #2: Fix compiler errors ◂ Commit #3: Fix warnings Refactoring to Kotlin
63. ◂ Commit #1: Convert file to Kotlin ◂ Commit #2: Fix compiler errors ◂ Commit #3: Fix warnings ◂ Commit #4: Idiomatic changes Refactoring to Kotlin
64. How do I ensure idiomatic code?
65. ◂ Code reviews ◂ Learn from each other ◂ https://kotlinlang.org/docs/reference/ idioms.html Ensuring Idiomatic Kotlin
66. What’s next?
67. ◂ Lambdas ◂ Sealed classes ◂ Standard library functions What’s next?
68. ◂ Step back and take a look at the project. ◂ Look for areas to apply null safety, immutability, readability, better interop ◂ Keep your team on the same page. Recap
69. What challenges do you face? @seetha_a / seetha.io