From Java to Kotlin

FROM JAVA TO
Gerard Paligot

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WHAT’S KOTLIN?

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WHAT’S KOTLIN?
- Started in mid 2010
- Avoid huge Java codebase
- Want a modern, expressive language
- Easy to introduce in the existing environment
Presentation

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WHAT’S KOTLIN?
- Full Java interoperability
- Compile as fast as Java
- More concise than Java
- Prevent more kinds of errors than Java
- Was simpler than Scala
Design Goals

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WHY USE KOTLIN?
- In a future, will replace Java as ﬁrst language
- High adoption of the Android community

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WHY USE KOTLIN?
High adoption of the Android community

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WHY USE KOTLIN?
- Easy learning for iOS and Android developers
- Null safety
- Compilation speed

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WHY USE KOTLIN?
Compilation speed

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WHY USE KOTLIN?
- Easy learning for iOS and Android developers
- Null safety
- Compilation speed
- Less code is better!

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HOW TO START?
- Experimenting with a legacy Java/Android project
- Run automatic conversion to Kotlin and learn what changed
- Practice more and inform what’s new at each new Kotlin version

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HOW TO START
// Top level build.gradle file
buildscript {
ext.kotlin_version = '1.2.30'
// ...
dependencies {
// ...
classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
}
}
// build.gradle file of your app module
// ...
apply plugin: 'kotlin-android'
// ...
dependencies {
implementation "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
// ...
}

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HOW TO START
buildscript {
// ...
dependencies {
// ...
}
}
// ...
// ...
dependencies {
implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
// ...
}

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HOW TO START
buildscript {
// ...
dependencies {
// ...
}
}
// ...
// ...
dependencies {
// ...
}

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JAVA TO KOTLIN… JAVA STYLE

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public class MyFragment extends Fragment {
private static final String KEY_TITLE = "KEY_TITLE";
private static final String KEY_MESSAGE = "KEY_MESSAGE";
public static MyFragment newInstance(String newTitle, String newMessage) {
Bundle args = new Bundle();
args.putString(KEY_TITLE, newTitle);
args.putString(KEY_MESSAGE, newMessage);
MyFragment fragment = new MyFragment();
fragment.setArguments(args);
return fragment;
}
}

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@Nullable
@Override
public View onCreateView(LayoutInflater inflater,
@Nullable ViewGroup container,
@Nullable Bundle savedInstanceState) {
return inflater.inflate(R.layout.my_fragment, container);
}
}

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private TextView mTitleTextView;
private TextView mMessageTextView;
private Button mSubmitButton;
@Override
public void onViewCreated(View view, @Nullable Bundle savedInstanceState) {
super.onViewCreated(view, savedInstanceState);
mTitleTextView = view.findViewById(R.id.titleTextView);
mTitleTextView.setText(getArguments().getString(KEY_TITLE));
mMessageTextView = view.findViewById(R.id.messageTextView);
mMessageTextView.setText(getArguments().getString(KEY_MESSAGE));
mSubmitButton = view.findViewById(R.id. submitButton);
mSubmitButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Log.d("MyFragment", "Somebody click on this button!");
Log.d("MyFragment", "Title: " + mTitleTextView.toString()
+ ", Message: " + mMessageTextView.toString());
}
});
}
}

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private static final String KEY_TITLE = "KEY_TITLE";
private static final String KEY_MESSAGE = "KEY_MESSAGE";
public static MyFragment newInstance(String newTitle, String newMessage) {
Bundle args = new Bundle();
args.putString(KEY_TITLE, newTitle);
args.putString(KEY_MESSAGE, newMessage);
MyFragment fragment = new MyFragment();
fragment.setArguments(args);
return fragment;
}
}

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class MyFragment : Fragment() {
companion object {
private val KEY_TITLE = "KEY_TITLE"
private val KEY_MESSAGE = "KEY_MESSAGE"
fun newInstance(newTitle: String, newMessage: String): MyFragment {
val args = Bundle()
args.putString(KEY_TITLE, newTitle)
args.putString(KEY_MESSAGE, newMessage)
val fragment = MyFragment()
fragment.arguments = args
return fragment
}
}
}

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+ private const val KEY_TITLE = "KEY_TITLE"
+ private const val KEY_MESSAGE = "KEY_MESSAGE"
companion object {
- private val KEY_TITLE = "KEY_TITLE"
- private val KEY_MESSAGE = "KEY_MESSAGE"
val args = Bundle()
return fragment
}
}
}

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private const val KEY_TITLE = "KEY_TITLE"
private const val KEY_MESSAGE = "KEY_MESSAGE"
companion object {
val args = Bundle()
return fragment
}
}
}

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+ private val Bundle.title: String
companion object {
val args = Bundle()
return fragment
}
}
}

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+ private val Bundle.title: String
+ get() = getString(KEY_TITLE)
companion object {
val args = Bundle()
return fragment
}
}
}

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+ private var Bundle.title: String
+ get() = getString(KEY_TITLE)
+ set(value) {
+ putString(KEY_TITLE, value)
+ }
companion object {
val args = Bundle()
return fragment
}
}
}

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private var Bundle.title: String
get() = getString(KEY_TITLE)
set(value) {
putString(KEY_TITLE, value)
}
+ private var Bundle.message: String
+ get() = getString(KEY_MESSAGE)
+ set(value) {
+ putString(KEY_MESSAGE, value)
+ }
companion object {
val args = Bundle()
return fragment
}
}
}

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set(value) {
}
private var Bundle.message: String
get() = getString(KEY_MESSAGE)
set(value) {
putString(KEY_MESSAGE, value)
}
companion object {
val args = Bundle()
- args.putString(KEY_TITLE, newTitle)
- args.putString(KEY_MESSAGE, newMessage)
+ args.title = newTitle
+ args.message = newMessage
return fragment
}

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set(value) {
}
set(value) {
}
companion object {
val args = Bundle()
args.title = newTitle
args.message = newMessage
return fragment
}
}
}

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companion object {
val args = Bundle()
args.title = newTitle
args.message = newMessage
return fragment
}
}
}

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companion object {
- val args = Bundle()
- args.title = newTitle
- args.message = newMessage
+ val args = Bundle().apply {
+ title = newTitle
+ message = newMessage
+ }
return fragment
}
}
}

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companion object {
val args = Bundle().apply {
title = newTitle
message = newMessage
}
return fragment
}
}
}

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companion object {
- val args = Bundle().apply {
- title = newTitle
- message = newMessage
- }
- val fragment = MyFragment()
- fragment.arguments = args
- return fragment
+ return MyFragment().apply {
+ arguments = Bundle().apply {
+ title = newTitle
+ message = newMessage
+ }
+ }
}
}
}

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companion object {
return MyFragment().apply {
arguments = Bundle().apply {
title = newTitle
}
}
}
}
}

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set(value) {
}
set(value) {
}
companion object {
return MyFragment().apply {
arguments = Bundle().apply {
title = newTitle
}
}
}
}
}

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@Nullable
@Override
public View onCreateView(LayoutInflater inflater,
@Nullable ViewGroup container,
@Nullable Bundle savedInstanceState) {
return inflater.inflate(R.layout.my_fragment, container);
}
}

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override fun onCreateView(inflater: LayoutInflater?,
container: ViewGroup?,
savedInstanceState: Bundle?): View? {
return inflater?.inflate(R.layout.my_fragment, container)
?: LayoutInflater.from(context).inflate(R.layout.my_fragment, container)
}

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private TextView mTitleTextView;
private TextView mMessageTextView;
private Button mSubmitButton;
@Override
public void onViewCreated(View view, @Nullable Bundle savedInstanceState) {
super.onViewCreated(view, savedInstanceState);
mTitleTextView = view.findViewById(R.id.titleTextView);
mTitleTextView.setText(getArguments().getString(KEY_TITLE));
mMessageTextView = view.findViewById(R.id.messageTextView);
mMessageTextView.setText(getArguments().getString(KEY_MESSAGE));
mSubmitButton = view.findViewById(R.id. submitButton);
mSubmitButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Log.d("MyFragment", "Somebody click on this button!");
+ ", Message: " + mMessageTextView.toString());
}
});
}
}

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private var mTitleTextView: TextView? = null
private var mMessageTextView: TextView? = null
private var mSubmitButton: Button? = null
override fun onViewCreated(view: View?, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
mTitleTextView = view!!.findViewById(R.id.titleTextView)
mTitleTextView!!.text = arguments.getString(KEY_TITLE)
mMessageTextView = view.findViewById(R.id.messageTextView)
mMessageTextView!!.text = arguments.getString(KEY_MESSAGE)
mSubmitButton = view.findViewById(R.id.submitButton)
mSubmitButton!!.setOnClickListener {
Log.d("MyFragment", "Somebody click on this button!")
Log.d("MyFragment", "Title: " + mTitleTextView!!.toString()
+ ", Message: " + mMessageTextView!!.toString())
}
}
}

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- mTitleTextView!!.text = arguments.getString(KEY_TITLE)
+ mTitleTextView!!.text = arguments.title
- mMessageTextView!!.text = arguments.getString(KEY_MESSAGE)
+ mMessageTextView!!.text = arguments.message
}
}
}

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mTitleTextView!!.text = arguments.title
mMessageTextView!!.text = arguments.message
}
}
}

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- private var mTitleTextView: TextView? = null
- private var mMessageTextView: TextView? = null
- private var mSubmitButton: Button? = null
+ private lateinit var mTitleTextView: TextView
+ private lateinit var mMessageTextView: TextView
+ private lateinit var mSubmitButton: Button
}
}
}

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private lateinit var mTitleTextView: TextView
private lateinit var mMessageTextView: TextView
private lateinit var mSubmitButton: Button
}
}
}

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- mTitleTextView!!.text = arguments.title
+ mTitleTextView.text = arguments.title
- mMessageTextView!!.text = arguments.message
+ mMessageTextView.text = arguments.message
- mSubmitButton!!.setOnClickListener {
+ mSubmitButton.setOnClickListener {
}
}
}

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mTitleTextView.text = arguments.title
mMessageTextView.text = arguments.message
mSubmitButton.setOnClickListener {
+ ", Message: " + mMessageTextView.toString())
}
}
}

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- Log.d("MyFragment", "Title: " + mTitleTextView.toString()
- + ", Message: " + mMessageTextView.toString())
+ Log.d("MyFragment", "Title: $mTitleTextView, Message: $mMessageTextView")
}
}
}

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Log.d("MyFragment", "Title: $mTitleTextView, Message: $mMessageTextView")
}
}
}

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buildscript {
// ...
dependencies {
// ...
}
}
// ...
apply plugin: ‘kotlin-android'
+ apply plugin: 'kotlin-android-extensions'
// ...
dependencies {
// ...
}

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}
}
}

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+ import kotlinx.android.synthetic.main.my_fragment.*
- private lateinit var mTitleTextView: TextView
- private lateinit var mMessageTextView: TextView
- private lateinit var mSubmitButton: Button
- mTitleTextView = view!!.findViewById(R.id.titleTextView)
- mMessageTextView = view.findViewById(R.id.messageTextView)
- mSubmitButton = view.findViewById(R.id.submitButton)
}
}
}

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import kotlinx.android.synthetic.main.my_fragment.*
titleTextView.text = arguments.title
messageTextView.text = arguments.message
submitButton.setOnClickListener {
}
}
}

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KOTLIN TO BINARY TO JAVA

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class MyClass
abstract class MyAbsClass
open class MyOpenClass
data class MyDataClass(val field: String)

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public final class MyClass {
}
public abstract class MyAbsClass {
}
public class MyOpenClass {
}

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public final class MyDataClass {
@NotNull
private final String field;
@NotNull
public final String getField() { return this.field; }
public MyDataClass(@NotNull String field) { // ... }
@NotNull
public final MyDataClass copy(@NotNull String field) { // ... }
public String toString() { // ... }
public int hashCode() { // ... }
public boolean equals(Object var1) { // ... }
}

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open class MethodExamples {
fun m() {}
open fun mOpen() {}
}
interface Method2Examples {
fun mOpen() {}
}
class Usage : MethodExamples(), Method2Examples{
override fun mOpen() {
super.mOpen()
super.mOpen()
}
}

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public class MethodExamples {
public final void m() {
}
public void mOpen() {
}
}
public interface Method2Examples {
void mOpen();
public static final class DefaultImpls {
public static void mOpen(Method2Examples $this) {
}
}
}
public final class Usage extends MethodExamples implements Method2Examples {
public void mOpen() {
Method2Examples.DefaultImpls.mOpen(this);
super.mOpen();
}
}

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class MyClass(val listener: (test: String) -> Unit) {
fun m() {
listener("Result")
}
}
fun main(args: Array) {
MyClass { it ->
// Do something really cool!
}
}

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public final class MyClass {
@NotNull
private final Function1 listener;
public final void m() {
this.listener.invoke("Result");
}
@NotNull
public final Function1 getListener() {
return this.listener;
}
public MyClass(@NotNull Function1 listener) {
Intrinsics.checkParameterIsNotNull(listener, "listener");
super();
this.listener = listener;
}
}

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/** A function that takes 0 arguments. */
public interface Function0 : Function {
/** Invokes the function. */
public operator fun invoke(): R
}
/** A function that takes 1 argument. */
/** Invokes the function with the specified argument. */
public operator fun invoke(p1: P1): R
}
/** Invokes the function with the specified arguments. */
public operator fun invoke(p1: P1, p2: P2): R
}
// ...
public interface Function22P11, in P12, in P13, in P14, in P15, in P16, in P17, in P18, in P19, in P20, in P21, in P22, out R> :
Function {
/** Invokes the function with the specified arguments. */
public operator fun invoke(p1: P1, p2: P2, p3: P3, p4: P4, p5: P5, p6: P6, p7: P7, p8: P8, p9: P9
p10: P10, p11: P11, p12: P12, p13: P13, p14: P14, p15: P15, p16: P16, p17: P17, p18: P18, p19: P19, p
P20, p21: P21, p22: P22): R
}

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fun MutableList.swap(index1: Int, index2: Int) {
val tmp = this[index1]
this[index1] = this[index2]
this[index2] = tmp
}

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public final class ExtensionsKt {
public static final void swap(@NotNull List $receiver, int index1, int index2) {
Intrinsics.checkParameterIsNotNull($receiver, "$receiver");
int tmp = ((Number)$receiver.get(index1)).intValue();
$receiver.set(index1, $receiver.get(index2));
$receiver.set(index2, Integer.valueOf(tmp));
}
}

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object MyObject {
var test: String = ""
}
MyObject.test = "new value"
}

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public final class MyObject {
@NotNull
private static String test;
public static final MyObject INSTANCE;
@NotNull
public final String getTest() {
return test;
}
public final void setTest(@NotNull String var1) {
Intrinsics.checkParameterIsNotNull(var1, "");
test = var1;
}
static {
MyObject var0 = new MyObject();
INSTANCE = var0;
test = "";
}
}

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TRAPS

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fun m(typeClass: AbsType) {
}
open class AbsType
class SubType1 : AbsType()
TRAPS

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+ when (typeClass) {
+ is SubType1 -> println("Typed by subtype 1")
+ is SubType2 -> println("Typed by subtype 2")
+ else -> println("No type identified")
+ }
}
open class AbsType
TRAPS

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when (typeClass) {
is SubType1 -> println("Typed by subtype 1")
- is SubType2 -> println("Typed by subtype 2")
+ is SubType2 -> {
+ println("Typed by subtype 2")
+ typeClass.children.forEach {
+ when (it) {
+ is SubType1 -> println("Child typed by subtype 1")
+ is SubType2 -> println("Child typed by subtype 2")
+ else -> println("No type identified")
+ }
+ }
+ }
else -> println("No type identified")
}
}
open class AbsType
- class SubType2 : AbsType()
+ class SubType2(val children: List) : AbsType()
TRAPS

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when (typeClass) {
is SubType1 -> println("Typed by subtype 1")
is SubType2 -> {
println("Typed by subtype 2")
typeClass.children.forEach {
when (it) {
is SubType1 -> println("Child typed by subtype 1")
is SubType2 -> println("Child typed by subtype 2")
}
}
}
}
}
open class AbsType
class SubType2(val children: List) : AbsType()
TRAPS

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variable.toList()
variable.toInt()
variable.joinToString { ... }
variable.copy()
variable.plus(...)
variable.minus(...)
...
variable.doEverything()
TRAPS

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WTF (˽°□°҂˽Ɨ ˍʓˍ

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someObject?.takeIf { status }.apply { doThis() }

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MyNullSafetyClass var7 = status ? someObject : null;
doThis();

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- someObject?.takeIf { status }.apply { doThis() }
+ someObject?.takeIf { status }?.apply { doThis() }

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MyNullSafetyClass var7 = status ? someObject : null;
if((status ? someObject : null) != null) {
doThis();
}

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null.apply { ... }

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class MyClass
class MyClass2(val test: String, val test2: String)
class MyClass3(val clazz: MyClass2?)
val m: MyClass? = MyClass()
// No compilation error.
if (m != null) {
m.toString()
}
"${m!!.toString()}${m.toString()}"
val myClass3 = MyClass3(MyClass2("", ""))
val test = myClass3.clazz!!.test
val test2 = myClass3.clazz.test2
}

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class MyClass
class MyClass4 {
private var myClass2: MyClass2? = null
fun m() {
val test = myClass2!!.test
// error, wtf this shit?!
val test2 = myClass2.test2
}
}

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class MyClass
class MyClass4 {
private var myClass2: MyClass2? = null
fun m() {
if (myClass2 != null) {
val test = myClass2!!.test
val test2 = myClass2!!.test2
}
}
}

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SUMMARY
- Kotlin is mature and ready for production
- Designed to be compatible and better than Java
- Easy to learn and high adoption by the Android community
- Be curious

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FROM JAVA TO
Gerard Paligot

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From Java to Kotlin

From Java to Kotlin

Gerard

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