Switching from Java to Kotlin

While 'Kotlin is cute' and I strongly agree with it, I faced some obstacles or unexpected results while converting Java source code from my project into Kotlin.

In this talk I showed some of the real examples of unexpected or surprising things I faced while converting the project from Java source code into Kotlin source code, and also what I think is important when you start converting your project from Java to Kotlin, which is reviewing decompiled byte code.

See also:
* Exploring Kotlin’s hidden costs — Part 1 – Christophe Beyls – Medium
https://medium.com/@BladeCoder/exploring-kotlins-hidden-costs-part-1-fbb9935d9b62

* Exploring Kotlin’s hidden costs — Part 2 – Christophe Beyls – Medium
https://medium.com/@BladeCoder/exploring-kotlins-hidden-costs-part-2-324a4a50b70

* Exploring Kotlin’s hidden costs — Part 3 – Christophe Beyls – Medium
https://medium.com/@BladeCoder/exploring-kotlins-hidden-costs-part-3-3bf6e0dbf0a4

* Kotlin's hidden costs - Benchmarks - Renato Athaydes
https://sites.google.com/a/athaydes.com/renato-athaydes/posts/kotlinshiddencosts-benchmarks

Keynote: Sinking Your Teeth Into Bytecode | SkillsCast | 26th October 2017
https://skillsmatter.com/skillscasts/10012-a-talk-by-jake-wharton

Transcript

1. Switching from Java to Kotlin shaunkawano
2. This talk… ‣is introductory ☺ ‣shows some real obstacles I faced when I was converting Java -> Kotlin ‣Android Studio 3.0 / Kotlin: 1.1.51
5. https://speakerdeck.com/ shaunkawano
6. Before release: ~2017.08 ‣☺ 1 Android developer ‣ Develop for initial release!! ‣ Google I/O 2017
7. Before release: ~2017.08 ‣☺ 1 Android developer ‣ Develop for initial release!! ‣ Google I/O 2017 ‣ Fully Java project
8. After Release: 2017.09~ ‣☺ 1 Android developer ‣
9. After Release: 2017.09~ ‣☺ 1 Android developer ‣ Feature Dev Release Java -> kt Feature Dev Release Java -> kt Feature Dev Release Java -> kt Feature Dev Release Java -> kt
11. Code/ ‘Convert Java File to Kotlin File’ (Mac: ⌘ Option Shift K )
13. Things I Struggled
14. Things I Struggled * Or things that may need to be in consideration when converting source code from Java to Kotlin
15. 1. Null Safety
16. e.g. onActivityResult
17. onActivityResult override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent) { super.onActivityResult(requestCode, resultCode, data) ActivityResult.onResult(requestCode, resultCode, data).into(this) }
18. onActivityResult override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent) { super.onActivityResult(requestCode, resultCode, data) ActivityResult.onResult(requestCode, resultCode, data).into(this) }
19. onActivityResult override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent) { super.onActivityResult(requestCode, resultCode, data) ActivityResult.onResult(requestCode, resultCode, data).into(this) }
20. onActivityResult override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) { super.onActivityResult(requestCode, resultCode, data) ActivityResult.onResult(requestCode, resultCode, data).into(this) } ☺
21. e.g. support.v4.app.Fragment
22. Support Library 27.0.0 ‣support.v4.app.Fragment API annotated with @Nullable ✨
23. Support Library 27.0.0 ‣support.v4.app.Fragment API annotated with @Nullable ✨ @Nullable public final Bundle getArguments() { return this.mArguments; } @Nullable public Context getContext() { return this.mHost == null?null:this.mHost.getContext(); } @Nullable public final FragmentActivity getActivity() { return this.mHost == null?null:(FragmentActivity)this.mHost.getActivity(); }
24. Option #1: ‘!!’ fragment.arguments!! fragment.context!! fragment.activity!!
25. import android.support.v4.app.Fragment fun Fragment.argsOrThrow() = arguments ?: throw IllegalStateException("arguments are null") fun Fragment.contextOrThrow() = context ?: throw IllegalStateException("context is null") fun Fragment.activityOrThrow() = activity ?: throw IllegalStateException("activity is null") Option #2: Extension Function
26. import android.support.v4.app.Fragment fun Fragment.argsOrThrow() = arguments ?: throw IllegalStateException("arguments are null") fun Fragment.contextOrThrow() = context ?: throw IllegalStateException("context is null") fun Fragment.activityOrThrow() = activity ?: throw IllegalStateException("activity is null") Option #2: Extension Function Or more?
27. 2. Code Diversion
28. Data Binding private val binding by lazy { DataBindingUtil.setContentView <ActivityHomeBinding>(this, R.layout.activity_home) } binding.button.setOnClickListener { someAction.doAction() }
29. Data Binding private val binding by lazy { DataBindingUtil.setContentView <ActivityHomeBinding>(this, R.layout.activity_home) } binding.button.setOnClickListener { someAction.doAction() }
30. Data Binding <?xml version="1.0" encoding=“utf-8"?> <layout xmlns:app="http://schemas.android.com/apk/res-auto" xmlns:android="http://schemas.android.com/apk/res/android" xmlns:tools="http://schemas.android.com/tools" > <android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:tools="http://schemas.android.com/tools" xmlns:app="http://schemas.android.com/apk/res-auto" android:layout_width="match_parent" android:layout_height="match_parent" > <Button android:layout_width="wrap_content" android:layout_height="wrap_content" android:text=“Hello World!" app:layout_constraintBottom_toBottomOf="parent" app:layout_constraintLeft_toLeftOf="parent" app:layout_constraintRight_toRightOf="parent" app:layout_constraintTop_toTopOf="parent" /> …
31. <?xml version="1.0" encoding=“utf-8"?> <layout xmlns:app="http://schemas.android.com/apk/res-auto" xmlns:android="http://schemas.android.com/apk/res/android" xmlns:tools="http://schemas.android.com/tools" > <android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:tools="http://schemas.android.com/tools" xmlns:app="http://schemas.android.com/apk/res-auto" android:layout_width="match_parent" android:layout_height="match_parent" > <Button android:layout_width="wrap_content" android:layout_height="wrap_content" android:text=“Hello World!" app:layout_constraintBottom_toBottomOf="parent" app:layout_constraintLeft_toLeftOf="parent" app:layout_constraintRight_toRightOf="parent" app:layout_constraintTop_toTopOf="parent" /> … Data Binding activity_home.xml
32. Data Binding private val binding by lazy { DataBindingUtil.setContentView <ActivityHomeBinding>(this, R.layout.activity_home) } binding.button.setOnClickListener { someAction.doAction() }
33. Data Binding public class ActivityHomeBinding extends android.databinding.ViewDataBinding { @Nullable private static final android.databinding.ViewDataBinding.IncludedLayouts sIncludes; @Nullable private static final android.util.SparseIntArray sViewsWithIds; static { sIncludes = null; … } }
34. Data Binding public class ActivityHomeBinding extends android.databinding.ViewDataBinding { @Nullable private static final android.databinding.ViewDataBinding.IncludedLayouts sIncludes; @Nullable private static final android.util.SparseIntArray sViewsWithIds; static { sIncludes = null; … } } ActivityHomeBinding.java
35. Data Binding public class ActivityHomeBinding extends android.databinding.ViewDataBinding { @Nullable private static final android.databinding.ViewDataBinding.IncludedLayouts sIncludes; @Nullable private static final android.util.SparseIntArray sViewsWithIds; static { sIncludes = null; … } } Does anyone know how to jump to .xml file?
36. 3. Method Counts
37. Build/ ‘Analyze APK…’ Check what’s inside classes.dex
38. kotlin package ‣ProGuarded ‣Defined methods: 259 ‣Referenced methods: 293
39. ‣ProGuard Enabled for Release build ‣MultiDex Enabled only for Debug build ‣Json Parser: Moshi

40. moshi + kotlin
41. moshi + kotlin
42. moshi + kotlin If you never annotate @Json,
43. moshi + kotlin ✅ ‘com.squareup.moshi:moshi:1.5.0' If you never annotate @Json,
44. moshi + kotlin If you annotate @Json,
45. moshi + kotlin If you annotate @Json, ✅ ‘com.squareup.moshi:moshi-kotlin:1.5.0’
46. ‣ProGuard Enabled for Release build ‣MultiDex Enabled only for Debug build ‣Json Parser: Moshi

47. ‣ProGuard Enabled for Release build ‣MultiDex Enabled only for Debug build ‣Json Parser: Moshi → Add moshi-kotlin extension
48. Build/ ‘Analyze APK…’ Check what’s inside classes.dex
49. kotlin package ‣ProGuarded ‣Defined methods: 9632 ‣Referenced methods: 10396
50. Dex Limit : 65536 Methods
51. Unable to execute dex:method ID not in [0, 0xfffff]: 65536
52. ‘Compare with previous APK…’
53. Things I Care
54. @Metadata( mv = {1, 1, 7}, bv = {1, 0, 2}, k = 1, d1 = {"\u0000\u001a\n\u0002\u0018\u0002\n\u0002\u0018\u0002\n\u0002\b\u0002\n\u0002\u0010\u0002\n\u0000\n\u0002\u0018\u0002\n\u0002\b\u0002\u0018\u0000 \u00072\u00020\u0001:\u0001\u0007B\u0005¢\u0006\u0002\u0010\u0002J\u0012\u0010\u0003\u001a\u00020\u00042\b\u0010\u0005\u001a\u0004\u0018\u00010\u0006H\u0014¨\u0006\b"}, d2 = {"Lcom/shaunkawano/androidkotlinsimpleoverheadsample/MainActivity;", "Landroid/support/v7/app/AppCompatActivity;", "()V", "onCreate", "", "savedInstanceState", "Landroid/os/ Bundle;", "Companion", "production sources for module app"} ) public final class MainActivity extends AppCompatActivity { public static final int SOME_VALUE = 1; public static final MainActivity.Companion Companion = new MainActivity.Companion((DefaultConstructorMarker)null); private HashMap _$_findViewCache; protected void onCreate(@Nullable Bundle savedInstanceState) { super.onCreate(savedInstanceState); this.setContentView(2131296283); String var2 = "" + Companion.getSOME_VALUE(); System.out.println(var2); } public View _$_findCachedViewById(int var1) { if(this._$_findViewCache == null) { this._$_findViewCache = new HashMap(); } View var2 = (View)this._$_findViewCache.get(Integer.valueOf(var1)); if(var2 == null) { var2 = this.findViewById(var1); this._$_findViewCache.put(Integer.valueOf(var1), var2); } return var2; } public void _$_clearFindViewByIdCache() { if(this._$_findViewCache != null) { this._$_findViewCache.clear(); } } @Metadata( mv = {1, 1, 7}, bv = {1, 0, 2}, k = 1, d1 = {"\u0000\u0014\n\u0002\u0018\u0002\n\u0002\u0010\u0000\n\u0002\b\u0002\n\u0002\u0010\b\n\u0002\b\u0003\b\u0086\u0003\u0018\u00002\u00020\u0001B\u0007\b\u0002¢ \u0006\u0002\u0010\u0002R\u0014\u0010\u0003\u001a\u00020\u0004X\u0086D¢\u0006\b\n\u0000\u001a\u0004\b\u0005\u0010\u0006¨\u0006\u0007"}, d2 = {"Lcom/shaunkawano/androidkotlinsimpleoverheadsample/MainActivity$Companion;", "", "()V", "SOME_VALUE", "", "getSOME_VALUE", "()I", "production sources for module app"} ) public static final class Companion { public final int getSOME_VALUE() { return MainActivity.SOME_VALUE; } private Companion() { } // $FF: synthetic method public Companion(DefaultConstructorMarker $constructor_marker) { this(); } } } Decompiled Bytecode
55. e.g. Companion Object
56. class MainActivity : AppCompatActivity() { companion object { val SOME_VALUE = 1 } override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) setContentView(R.layout.activity_main) println("$SOME_VALUE") } }
57. class MainActivity : AppCompatActivity() { companion object { val SOME_VALUE = 1 } override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) setContentView(R.layout.activity_main) println("$SOME_VALUE") } } Tools/Kotlin/
 ‘Show Kotlin Bytecode’
58. class MainActivity : AppCompatActivity() { companion object { val SOME_VALUE = 1 } override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) setContentView(R.layout.activity_main) println("$SOME_VALUE") } } Tools/Kotlin/
 ‘Show Kotlin Bytecode’ (Configure own shortcut!)
59. companion object { val SOME_VALUE = 1 } println("$SOME_VALUE")
60. @Metadata( mv = {1, 1, 7}, bv = {1, 0, 2}, k = 1, d1 = {"\u0000\u001a\n\u0002\u0018\u0002\n\u0002\u0018\u0002\n\u0002\b\u0002\n\u0002\u0010\u0002\n\u0000\n\u0002\u0018\u0002\n\u0002\b\u0002\u0018\u0000 \u00072\u00020\u0001:\u0001\u0007B\u0005¢\u0006\u0002\u0010\u0002J\u0012\u0010\u0003\u001a\u00020\u00042\b\u0010\u0005\u001a\u0004\u0018\u00010\u0006H\u0014¨\u0006\b"}, d2 = {"Lcom/shaunkawano/androidkotlinsimpleoverheadsample/MainActivity;", "Landroid/support/v7/app/AppCompatActivity;", "()V", "onCreate", "", "savedInstanceState", "Landroid/os/ Bundle;", "Companion", "production sources for module app"} ) public final class MainActivity extends AppCompatActivity { public static final int SOME_VALUE = 1; public static final MainActivity.Companion Companion = new MainActivity.Companion((DefaultConstructorMarker)null); private HashMap _$_findViewCache; protected void onCreate(@Nullable Bundle savedInstanceState) { super.onCreate(savedInstanceState); this.setContentView(2131296283); String var2 = "" + Companion.getSOME_VALUE(); System.out.println(var2); } public View _$_findCachedViewById(int var1) { if(this._$_findViewCache == null) { this._$_findViewCache = new HashMap(); } View var2 = (View)this._$_findViewCache.get(Integer.valueOf(var1)); if(var2 == null) { var2 = this.findViewById(var1); this._$_findViewCache.put(Integer.valueOf(var1), var2); } return var2; } public void _$_clearFindViewByIdCache() { if(this._$_findViewCache != null) { this._$_findViewCache.clear(); } } @Metadata( mv = {1, 1, 7}, bv = {1, 0, 2}, k = 1, d1 = {"\u0000\u0014\n\u0002\u0018\u0002\n\u0002\u0010\u0000\n\u0002\b\u0002\n\u0002\u0010\b\n\u0002\b\u0003\b\u0086\u0003\u0018\u00002\u00020\u0001B\u0007\b\u0002¢ \u0006\u0002\u0010\u0002R\u0014\u0010\u0003\u001a\u00020\u0004X\u0086D¢\u0006\b\n\u0000\u001a\u0004\b\u0005\u0010\u0006¨\u0006\u0007"}, d2 = {"Lcom/shaunkawano/androidkotlinsimpleoverheadsample/MainActivity$Companion;", "", "()V", "SOME_VALUE", "", "getSOME_VALUE", "()I", "production sources for module app"} ) public static final class Companion { public final int getSOME_VALUE() { return MainActivity.SOME_VALUE; } private Companion() { } // $FF: synthetic method public Companion(DefaultConstructorMarker $constructor_marker) { this(); } } }
61. public final class MainActivity extends AppCompatActivity { public static final int SOME_VALUE = 1; public static final MainActivity.Companion Companion = new MainActivity.Companion((DefaultConstructorMarker)null); private HashMap _$_findViewCache; protected void onCreate(@Nullable Bundle savedInstanceState) { super.onCreate(savedInstanceState); this.setContentView(2131296283); String var2 = "" + Companion.getSOME_VALUE(); System.out.println(var2); } public static final class Companion { public final int getSOME_VALUE() { return MainActivity.SOME_VALUE; } private Companion() { } // $FF: synthetic method public Companion(DefaultConstructorMarker $constructor_marker) { this(); } } }
62. public final class MainActivity extends AppCompatActivity { public static final int SOME_VALUE = 1; public static final MainActivity.Companion Companion = new MainActivity.Companion((DefaultConstructorMarker)null); private HashMap _$_findViewCache; protected void onCreate(@Nullable Bundle savedInstanceState) { super.onCreate(savedInstanceState); this.setContentView(2131296283); String var2 = "" + Companion.getSOME_VALUE(); System.out.println(var2); } public static final class Companion { public final int getSOME_VALUE() { return MainActivity.SOME_VALUE; } private Companion() { } // $FF: synthetic method public Companion(DefaultConstructorMarker $constructor_marker) { this(); } } }
63. companion object { val SOME_VALUE = 1 } println("$SOME_VALUE")
64. companion object { val SOME_VALUE = 1 } println("$SOME_VALUE") public static final int SOME_VALUE = 1; public static final class Companion { public final int getSOME_VALUE() { return MainActivity.SOME_VALUE; } } … String var2 = "" + Companion.getSOME_VALUE(); System.out.println(var2);
65. companion object { val SOME_VALUE = 1 } println("$SOME_VALUE") public static final int SOME_VALUE = 1; public static final class Companion { public final int getSOME_VALUE() { return MainActivity.SOME_VALUE; } } … String var2 = "" + Companion.getSOME_VALUE(); System.out.println(var2);
66. companion object { val SOME_VALUE = 1 } println("$SOME_VALUE") public static final int SOME_VALUE = 1; public static final class Companion { public final int getSOME_VALUE() { return MainActivity.SOME_VALUE; } } … String var2 = "" + Companion.getSOME_VALUE(); System.out.println(var2);
67. companion object { val SOME_VALUE = 1 } println("$SOME_VALUE") public static final int SOME_VALUE = 1; public static final class Companion { public final int getSOME_VALUE() { return MainActivity.SOME_VALUE; } } … String var2 = "" + Companion.getSOME_VALUE(); System.out.println(var2); ‘const’ keyword
68. ‘const’ keyword ‣Compile-Time-Constants ‣String or Primitive of top-level or members of object ‣No ‘getter’ to be generated
69. companion object { const val SOME_VALUE = 1 } println(“$SOME_VALUE") public static final int SOME_VALUE = 1; public static final class Companion { … } String var2 = "1"; System.out.println(var2);
70. companion object { const val SOME_VALUE = 1 } println(“$SOME_VALUE") public static final int SOME_VALUE = 1; public static final class Companion { … } String var2 = "1"; System.out.println(var2); For non-const property?
71. companion object { const val SOME_VALUE = 1 } println(“$SOME_VALUE") public static final int SOME_VALUE = 1; public static final class Companion { … } String var2 = "1"; System.out.println(var2); @JvmField
72. @JvmField ‣Tell Kotlin compiler not to generate setter/getter ‣Expose property as a field in Java
73. Decompiled Bytecode ‣Shows you how your Kotlin code will look as in Java ‣Enables you to check if you were writing something you would never write in Java ‣There are Pros/Cons; talk to your teammate
74. Conclusion ‣No rush when converting ‣Review your decompiled byte code ‣Always doubt; never blind yourself with the phrase ‘Kotlin is cute’
75. shaunkawano Switching from Java to Kotlin