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Switching from Java
to Kotlin
shaunkawano
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This talk…
‣is introductory ☺
‣shows some real obstacles I faced
when I was converting Java ->
Kotlin
‣Android Studio 3.0 / Kotlin: 1.1.51
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https://speakerdeck.com/
shaunkawano
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Before release: ~2017.08
‣☺ 1 Android developer
‣ Develop for initial release!!
‣ Google I/O 2017
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Before release: ~2017.08
‣☺ 1 Android developer
‣ Develop for initial release!!
‣ Google I/O 2017
‣ Fully Java project
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After Release: 2017.09~
‣☺ 1 Android developer
‣
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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
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Code/
‘Convert Java File to Kotlin File’
(Mac: ⌘ Option Shift K )
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Things I Struggled
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Things I Struggled
* Or things that may need to be in consideration
when converting source code from Java to Kotlin
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1. Null Safety
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e.g. onActivityResult
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onActivityResult
override fun onActivityResult(requestCode: Int,
resultCode: Int, data: Intent) {
super.onActivityResult(requestCode, resultCode,
data)
ActivityResult.onResult(requestCode, resultCode,
data).into(this)
}
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onActivityResult
override fun onActivityResult(requestCode: Int,
resultCode: Int, data: Intent) {
super.onActivityResult(requestCode, resultCode,
data)
ActivityResult.onResult(requestCode, resultCode,
data).into(this)
}
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onActivityResult
override fun onActivityResult(requestCode: Int,
resultCode: Int, data: Intent) {
super.onActivityResult(requestCode, resultCode,
data)
ActivityResult.onResult(requestCode, resultCode,
data).into(this)
}
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onActivityResult
override fun onActivityResult(requestCode: Int,
resultCode: Int, data: Intent?) {
super.onActivityResult(requestCode, resultCode,
data)
ActivityResult.onResult(requestCode, resultCode,
data).into(this)
}
☺
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e.g. support.v4.app.Fragment
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Support Library 27.0.0
‣support.v4.app.Fragment API
annotated with @Nullable ✨
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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();
}
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Option #1: ‘!!’
fragment.arguments!!
fragment.context!!
fragment.activity!!
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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
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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?
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2. Code Diversion
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Data Binding
private val binding by lazy {
DataBindingUtil.setContentView
(this,
R.layout.activity_home)
}
binding.button.setOnClickListener {
someAction.doAction()
}
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Data Binding
private val binding by lazy {
DataBindingUtil.setContentView
(this,
R.layout.activity_home)
}
binding.button.setOnClickListener {
someAction.doAction()
}
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Data Binding
…
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…
Data Binding
activity_home.xml
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Data Binding
private val binding by lazy {
DataBindingUtil.setContentView
(this,
R.layout.activity_home)
}
binding.button.setOnClickListener {
someAction.doAction()
}
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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;
…
}
}
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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
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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?
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3. Method Counts
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Build/
‘Analyze APK…’
Check what’s inside classes.dex
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kotlin package
‣ProGuarded
‣Defined methods: 259
‣Referenced methods: 293
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‣ProGuard Enabled for Release build
‣MultiDex Enabled only for Debug
build
‣Json Parser: Moshi
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moshi + kotlin
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moshi + kotlin
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moshi + kotlin
If you never annotate @Json,
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moshi + kotlin
✅ ‘com.squareup.moshi:moshi:1.5.0'
If you never annotate @Json,
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moshi + kotlin
If you annotate @Json,
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moshi + kotlin
If you annotate @Json,
✅ ‘com.squareup.moshi:moshi-kotlin:1.5.0’
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‣ProGuard Enabled for Release build
‣MultiDex Enabled only for Debug
build
‣Json Parser: Moshi
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‣ProGuard Enabled for Release build
‣MultiDex Enabled only for Debug
build
‣Json Parser: Moshi
→ Add moshi-kotlin extension
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Build/
‘Analyze APK…’
Check what’s inside classes.dex
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kotlin package
‣ProGuarded
‣Defined methods: 9632
‣Referenced methods: 10396
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Dex Limit : 65536 Methods
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Unable to execute dex:method
ID not in [0, 0xfffff]: 65536
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‘Compare with previous APK…’
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Things I Care
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@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 $constructor_marker) {
this();
}
}
}
Decompiled Bytecode
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e.g. Companion Object
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class MainActivity : AppCompatActivity() {
companion object {
val SOME_VALUE = 1
}
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
println("$SOME_VALUE")
}
}
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class MainActivity : AppCompatActivity() {
companion object {
val SOME_VALUE = 1
}
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
println("$SOME_VALUE")
}
}
Tools/Kotlin/
‘Show Kotlin Bytecode’
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class MainActivity : AppCompatActivity() {
companion object {
val SOME_VALUE = 1
}
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
println("$SOME_VALUE")
}
}
Tools/Kotlin/
‘Show Kotlin Bytecode’
(Configure own shortcut!)
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companion object {
val SOME_VALUE = 1
}
println("$SOME_VALUE")
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@Metadata(
mv = {1, 1, 7},
bv = {1, 0, 2},
k = 1,
d1 = {"\u0000\u001a\n\u0002\u0018\u0002\n\u0002\u0018\u0002\n\u0002\b\u0002\n\u0002\u0010\u0002\n\u0000\n\u0002\u0018\u0002\n\u0002\b\u0002\u0018\u0000
\u00072\u00020\u0001:\u0001\u0007B\u0005¢\u0006\u0002\u0010\u0002J\u0012\u0010\u0003\u001a\u00020\u00042\b\u0010\u0005\u001a\u0004\u0018\u00010\u0006H\u0014¨\u0006\b"},
d2 = {"Lcom/shaunkawano/androidkotlinsimpleoverheadsample/MainActivity;", "Landroid/support/v7/app/AppCompatActivity;", "()V", "onCreate", "", "savedInstanceState", "Landroid/os/
Bundle;", "Companion", "production sources for module app"}
)
public final class MainActivity extends AppCompatActivity {
public static final int SOME_VALUE = 1;
public static final MainActivity.Companion Companion = new MainActivity.Companion((DefaultConstructorMarker)null);
private HashMap _$_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 $constructor_marker) {
this();
}
}
}
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public final class MainActivity extends AppCompatActivity {
public static final int SOME_VALUE = 1;
public static final MainActivity.Companion Companion = new
MainActivity.Companion((DefaultConstructorMarker)null);
private HashMap _$_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 $constructor_marker) {
this();
}
}
}
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public final class MainActivity extends AppCompatActivity {
public static final int SOME_VALUE = 1;
public static final MainActivity.Companion Companion = new
MainActivity.Companion((DefaultConstructorMarker)null);
private HashMap _$_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 $constructor_marker) {
this();
}
}
}
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companion object {
val SOME_VALUE = 1
}
println("$SOME_VALUE")
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companion object {
val SOME_VALUE = 1
}
println("$SOME_VALUE")
public static final int SOME_VALUE = 1;
public static final class Companion {
public final int getSOME_VALUE() {
return MainActivity.SOME_VALUE;
}
}
…
String var2 = "" + Companion.getSOME_VALUE();
System.out.println(var2);
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companion object {
val SOME_VALUE = 1
}
println("$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);
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companion object {
val SOME_VALUE = 1
}
println("$SOME_VALUE")
public static final int SOME_VALUE = 1;
public static final class Companion {
public final int getSOME_VALUE() {
return MainActivity.SOME_VALUE;
}
}
…
String var2 = "" + Companion.getSOME_VALUE();
System.out.println(var2);
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companion object {
val SOME_VALUE = 1
}
println("$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
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‘const’ keyword
‣Compile-Time-Constants
‣String or Primitive of top-level or
members of object
‣No ‘getter’ to be generated
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companion object {
const val SOME_VALUE = 1
}
println(“$SOME_VALUE")
public static final int SOME_VALUE = 1;
public static final class Companion {
…
}
String var2 = "1";
System.out.println(var2);
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companion object {
const val SOME_VALUE = 1
}
println(“$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?
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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
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@JvmField
‣Tell Kotlin compiler not to
generate setter/getter
‣Expose property as a field in Java
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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
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Conclusion
‣No rush when converting
‣Review your decompiled byte
code
‣Always doubt; never blind yourself
with the phrase ‘Kotlin is cute’
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shaunkawano
Switching from Java
to Kotlin