FROM JAVA TO Gerard Paligot 

WHAT’S KOTLIN? 

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

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 

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

WHY USE KOTLIN? High adoption of the Android community 

WHY USE KOTLIN? - In a future, will replace Java as first language - High adoption of the Android community - Easy learning for iOS and Android developers - Null safety - Compilation speed 

WHY USE KOTLIN? Compilation speed 

WHY USE KOTLIN? - In a future, will replace Java as first language - High adoption of the Android community - Easy learning for iOS and Android developers - Null safety - Compilation speed - Less code is better! 

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 

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" // ... } 

HOW TO START // Top level build.gradle file buildscript { ext.kotlin_version = '1.2.30' // ... dependencies { // ... classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version" } } // build.gradle file of your app module // ... apply plugin: 'kotlin-android' // ... dependencies { implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version" // ... } 

HOW TO START // Top level build.gradle file buildscript { ext.kotlin_version = '1.2.30' // ... dependencies { // ... classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version" } } // build.gradle file of your app module // ... apply plugin: 'kotlin-android' // ... dependencies { implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk8:$kotlin_version" // ... } 

JAVA TO KOTLIN… JAVA STYLE 

JAVA TO KOTLIN… JAVA STYLE 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; } } 

JAVA TO KOTLIN… JAVA STYLE public class MyFragment extends Fragment { @Nullable @Override public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) { return inflater.inflate(R.layout.my_fragment, container); } } 

JAVA TO KOTLIN… JAVA STYLE public class MyFragment extends Fragment { 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()); } }); } } 

JAVA TO KOTLIN… JAVA STYLE 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; } } 

JAVA TO KOTLIN… JAVA STYLE 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 } } } 

JAVA TO KOTLIN… JAVA STYLE + private const val KEY_TITLE = "KEY_TITLE" + private const val KEY_MESSAGE = "KEY_MESSAGE" 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 } } } 

JAVA TO KOTLIN… JAVA STYLE private const val KEY_TITLE = "KEY_TITLE" private const val KEY_MESSAGE = "KEY_MESSAGE" class MyFragment : Fragment() { companion object { 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 } } } 

JAVA TO KOTLIN… JAVA STYLE private const val KEY_TITLE = "KEY_TITLE" private const val KEY_MESSAGE = "KEY_MESSAGE" + private val Bundle.title: String class MyFragment : Fragment() { companion object { 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 } } } 

JAVA TO KOTLIN… JAVA STYLE private const val KEY_TITLE = "KEY_TITLE" private const val KEY_MESSAGE = "KEY_MESSAGE" + private val Bundle.title: String + get() = getString(KEY_TITLE) class MyFragment : Fragment() { companion object { 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 } } } 

JAVA TO KOTLIN… JAVA STYLE private const val KEY_TITLE = "KEY_TITLE" private const val KEY_MESSAGE = "KEY_MESSAGE" + private var Bundle.title: String + get() = getString(KEY_TITLE) + set(value) { + putString(KEY_TITLE, value) + } class MyFragment : Fragment() { companion object { 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 } } } 

JAVA TO KOTLIN… JAVA STYLE private const val KEY_TITLE = "KEY_TITLE" private const val KEY_MESSAGE = "KEY_MESSAGE" 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) + } class MyFragment : Fragment() { companion object { 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 } } } 

JAVA TO KOTLIN… JAVA STYLE private const val KEY_TITLE = "KEY_TITLE" private const val KEY_MESSAGE = "KEY_MESSAGE" 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) } class MyFragment : Fragment() { companion object { fun newInstance(newTitle: String, newMessage: String): MyFragment { val args = Bundle() - args.putString(KEY_TITLE, newTitle) - args.putString(KEY_MESSAGE, newMessage) + args.title = newTitle + args.message = newMessage val fragment = MyFragment() fragment.arguments = args return fragment } 

JAVA TO KOTLIN… JAVA STYLE private const val KEY_TITLE = "KEY_TITLE" private const val KEY_MESSAGE = "KEY_MESSAGE" 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) } class MyFragment : Fragment() { companion object { fun newInstance(newTitle: String, newMessage: String): MyFragment { val args = Bundle() args.title = newTitle args.message = newMessage val fragment = MyFragment() fragment.arguments = args return fragment } } } 

JAVA TO KOTLIN… JAVA STYLE class MyFragment : Fragment() { companion object { fun newInstance(newTitle: String, newMessage: String): MyFragment { val args = Bundle() args.title = newTitle args.message = newMessage val fragment = MyFragment() fragment.arguments = args return fragment } } } 

JAVA TO KOTLIN… JAVA STYLE class MyFragment : Fragment() { companion object { fun newInstance(newTitle: String, newMessage: String): MyFragment { - val args = Bundle() - args.title = newTitle - args.message = newMessage + val args = Bundle().apply { + title = newTitle + message = newMessage + } val fragment = MyFragment() fragment.arguments = args return fragment } } } 

JAVA TO KOTLIN… JAVA STYLE class MyFragment : Fragment() { companion object { fun newInstance(newTitle: String, newMessage: String): MyFragment { val args = Bundle().apply { title = newTitle message = newMessage } val fragment = MyFragment() fragment.arguments = args return fragment } } } 

JAVA TO KOTLIN… JAVA STYLE class MyFragment : Fragment() { companion object { fun newInstance(newTitle: String, newMessage: String): MyFragment { - 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 + } + } } } } 

JAVA TO KOTLIN… JAVA STYLE class MyFragment : Fragment() { companion object { fun newInstance(newTitle: String, newMessage: String): MyFragment { return MyFragment().apply { arguments = Bundle().apply { title = newTitle message = newMessage } } } } } 

JAVA TO KOTLIN… JAVA STYLE private const val KEY_TITLE = "KEY_TITLE" private const val KEY_MESSAGE = "KEY_MESSAGE" 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) } class MyFragment : Fragment() { companion object { fun newInstance(newTitle: String, newMessage: String): MyFragment { return MyFragment().apply { arguments = Bundle().apply { title = newTitle message = newMessage } } } } } 

JAVA TO KOTLIN… JAVA STYLE public class MyFragment extends Fragment { @Nullable @Override public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) { return inflater.inflate(R.layout.my_fragment, container); } } 

JAVA TO KOTLIN… JAVA STYLE 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) } 

JAVA TO KOTLIN… JAVA STYLE 

JAVA TO KOTLIN… JAVA STYLE public class MyFragment extends Fragment { 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()); } }); } } 

JAVA TO KOTLIN… JAVA STYLE class MyFragment : Fragment() { 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()) } } } 

JAVA TO KOTLIN… JAVA STYLE class MyFragment : Fragment() { 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) + mTitleTextView!!.text = arguments.title mMessageTextView = view.findViewById(R.id.messageTextView) - mMessageTextView!!.text = arguments.getString(KEY_MESSAGE) + mMessageTextView!!.text = arguments.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()) } } } 

JAVA TO KOTLIN… JAVA STYLE class MyFragment : Fragment() { 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.title mMessageTextView = view.findViewById(R.id.messageTextView) mMessageTextView!!.text = arguments.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()) } } } 

JAVA TO KOTLIN… JAVA STYLE class MyFragment : Fragment() { - 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 override fun onViewCreated(view: View?, savedInstanceState: Bundle?) { super.onViewCreated(view, savedInstanceState) mTitleTextView = view!!.findViewById(R.id.titleTextView) mTitleTextView!!.text = arguments.title mMessageTextView = view.findViewById(R.id.messageTextView) mMessageTextView!!.text = arguments.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()) } } } 

JAVA TO KOTLIN… JAVA STYLE class MyFragment : Fragment() { private lateinit var mTitleTextView: TextView private lateinit var mMessageTextView: TextView private lateinit var mSubmitButton: Button override fun onViewCreated(view: View?, savedInstanceState: Bundle?) { super.onViewCreated(view, savedInstanceState) mTitleTextView = view!!.findViewById(R.id.titleTextView) mTitleTextView!!.text = arguments.title mMessageTextView = view.findViewById(R.id.messageTextView) mMessageTextView!!.text = arguments.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()) } } } 

JAVA TO KOTLIN… JAVA STYLE class MyFragment : Fragment() { private lateinit var mTitleTextView: TextView private lateinit var mMessageTextView: TextView private lateinit var mSubmitButton: Button override fun onViewCreated(view: View?, savedInstanceState: Bundle?) { super.onViewCreated(view, savedInstanceState) mTitleTextView = view!!.findViewById(R.id.titleTextView) - mTitleTextView!!.text = arguments.title + mTitleTextView.text = arguments.title mMessageTextView = view.findViewById(R.id.messageTextView) - mMessageTextView!!.text = arguments.message + mMessageTextView.text = arguments.message mSubmitButton = view.findViewById(R.id.submitButton) - mSubmitButton!!.setOnClickListener { + mSubmitButton.setOnClickListener { Log.d("MyFragment", "Somebody click on this button!") Log.d("MyFragment", "Title: " + mTitleTextView!!.toString() + ", Message: " + mMessageTextView!!.toString()) } } } 

class MyFragment : Fragment() { private lateinit var mTitleTextView: TextView private lateinit var mMessageTextView: TextView private lateinit var mSubmitButton: Button override fun onViewCreated(view: View?, savedInstanceState: Bundle?) { super.onViewCreated(view, savedInstanceState) mTitleTextView = view!!.findViewById(R.id.titleTextView) mTitleTextView.text = arguments.title mMessageTextView = view.findViewById(R.id.messageTextView) mMessageTextView.text = arguments.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()) } } } JAVA TO KOTLIN… JAVA STYLE 

class MyFragment : Fragment() { private lateinit var mTitleTextView: TextView private lateinit var mMessageTextView: TextView private lateinit var mSubmitButton: Button override fun onViewCreated(view: View?, savedInstanceState: Bundle?) { super.onViewCreated(view, savedInstanceState) mTitleTextView = view!!.findViewById(R.id.titleTextView) mTitleTextView.text = arguments.title mMessageTextView = view.findViewById(R.id.messageTextView) mMessageTextView.text = arguments.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()) + Log.d("MyFragment", "Title: $mTitleTextView, Message: $mMessageTextView") } } } JAVA TO KOTLIN… JAVA STYLE 

class MyFragment : Fragment() { private lateinit var mTitleTextView: TextView private lateinit var mMessageTextView: TextView private lateinit var mSubmitButton: Button override fun onViewCreated(view: View?, savedInstanceState: Bundle?) { super.onViewCreated(view, savedInstanceState) mTitleTextView = view!!.findViewById(R.id.titleTextView) mTitleTextView.text = arguments.title mMessageTextView = view.findViewById(R.id.messageTextView) mMessageTextView.text = arguments.message mSubmitButton = view.findViewById(R.id.submitButton) mSubmitButton.setOnClickListener { Log.d("MyFragment", "Somebody click on this button!") Log.d("MyFragment", "Title: $mTitleTextView, Message: $mMessageTextView") } } } JAVA TO KOTLIN… JAVA STYLE 

// Top level build.gradle file buildscript { ext.kotlin_version = '1.2.30' // ... dependencies { // ... classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version" } } // build.gradle file of your app module // ... apply plugin: ‘kotlin-android' + apply plugin: 'kotlin-android-extensions' // ... dependencies { implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk8:$kotlin_version" // ... } JAVA TO KOTLIN… JAVA STYLE 

class MyFragment : Fragment() { private lateinit var mTitleTextView: TextView private lateinit var mMessageTextView: TextView private lateinit var mSubmitButton: Button override fun onViewCreated(view: View?, savedInstanceState: Bundle?) { super.onViewCreated(view, savedInstanceState) mTitleTextView = view!!.findViewById(R.id.titleTextView) mTitleTextView.text = arguments.title mMessageTextView = view.findViewById(R.id.messageTextView) mMessageTextView.text = arguments.message mSubmitButton = view.findViewById(R.id.submitButton) mSubmitButton.setOnClickListener { Log.d("MyFragment", "Somebody click on this button!") Log.d("MyFragment", "Title: $mTitleTextView, Message: $mMessageTextView") } } } JAVA TO KOTLIN… JAVA STYLE 

+ import kotlinx.android.synthetic.main.my_fragment.* class MyFragment : Fragment() { - private lateinit var mTitleTextView: TextView - private lateinit var mMessageTextView: TextView - private lateinit var mSubmitButton: Button override fun onViewCreated(view: View?, savedInstanceState: Bundle?) { super.onViewCreated(view, savedInstanceState) - mTitleTextView = view!!.findViewById(R.id.titleTextView) mTitleTextView.text = arguments.title - mMessageTextView = view.findViewById(R.id.messageTextView) mMessageTextView.text = arguments.message - mSubmitButton = view.findViewById(R.id.submitButton) mSubmitButton.setOnClickListener { Log.d("MyFragment", "Somebody click on this button!") Log.d("MyFragment", "Title: $mTitleTextView, Message: $mMessageTextView") } } } JAVA TO KOTLIN… JAVA STYLE 

import kotlinx.android.synthetic.main.my_fragment.* class MyFragment : Fragment() { override fun onViewCreated(view: View?, savedInstanceState: Bundle?) { super.onViewCreated(view, savedInstanceState) titleTextView.text = arguments.title messageTextView.text = arguments.message submitButton.setOnClickListener { Log.d("MyFragment", "Somebody click on this button!") Log.d("MyFragment", "Title: $mTitleTextView, Message: $mMessageTextView") } } } JAVA TO KOTLIN… JAVA STYLE 

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KOTLIN TO BINARY TO JAVA 

KOTLIN TO BINARY TO JAVA 

class MyClass abstract class MyAbsClass open class MyOpenClass data class MyDataClass(val field: String) KOTLIN TO BINARY TO JAVA 

KOTLIN TO BINARY TO JAVA 

public final class MyClass { } public abstract class MyAbsClass { } public class MyOpenClass { } KOTLIN TO BINARY TO JAVA 

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) { // ... } } KOTLIN TO BINARY TO JAVA 

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

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(); } } KOTLIN TO BINARY TO JAVA 

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

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; } } KOTLIN TO BINARY TO JAVA 

/** A function that takes 0 arguments. */ public interface Function0 : Function { /** Invokes the function. */ public operator fun invoke(): R } /** A function that takes 1 argument. */ public interface Function1 : Function { /** Invokes the function with the specified argument. */ public operator fun invoke(p1: P1): R } /** A function that takes 2 arguments. */ public interface Function2 : Function { /** Invokes the function with the specified arguments. */ public operator fun invoke(p1: P1, p2: P2): R } // ... /** A function that takes 22 arguments. */ public interface Function22 : 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 } KOTLIN TO BINARY TO JAVA 

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

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)); } } KOTLIN TO BINARY TO JAVA 

object MyObject { var test: String = "" } fun main(args: Array) { MyObject.test = "new value" } KOTLIN TO BINARY TO JAVA 

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 = ""; } } KOTLIN TO BINARY TO JAVA 

TRAPS 

fun m(typeClass: AbsType) { } open class AbsType class SubType1 : AbsType() class SubType2 : AbsType() TRAPS 

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

fun m(typeClass: AbsType) { 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 SubType1 : AbsType() - class SubType2 : AbsType() + class SubType2(val children: List) : AbsType() TRAPS 

fun m(typeClass: AbsType) { 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") else -> println("No type identified") } } } else -> println("No type identified") } } open class AbsType class SubType1 : AbsType() class SubType2(val children: List) : AbsType() TRAPS 

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

WTF (˽°□°҂˽Ɨ ˍʓˍ 

someObject?.takeIf { status }.apply { doThis() } WTF (˽°□°҂˽Ɨ ˍʓˍ 

MyNullSafetyClass var7 = status ? someObject : null; doThis(); WTF (˽°□°҂˽Ɨ ˍʓˍ 

- someObject?.takeIf { status }.apply { doThis() } + someObject?.takeIf { status }?.apply { doThis() } WTF (˽°□°҂˽Ɨ ˍʓˍ 

MyNullSafetyClass var7 = status ? someObject : null; if((status ? someObject : null) != null) { doThis(); } WTF (˽°□°҂˽Ɨ ˍʓˍ 

null.apply { ... } WTF (˽°□°҂˽Ɨ ˍʓˍ 

class MyClass class MyClass2(val test: String, val test2: String) class MyClass3(val clazz: MyClass2?) fun main(args: Array) { val m: MyClass? = MyClass() // No compilation error. if (m != null) { m.toString() } // No compilation error. "${m!!.toString()}${m.toString()}" // No compilation error. val myClass3 = MyClass3(MyClass2("", "")) val test = myClass3.clazz!!.test val test2 = myClass3.clazz.test2 } WTF (˽°□°҂˽Ɨ ˍʓˍ 

class MyClass class MyClass2(val test: String, val test2: String) class MyClass3(val clazz: MyClass2?) class MyClass4 { private var myClass2: MyClass2? = null fun m() { val test = myClass2!!.test // error, wtf this shit?! val test2 = myClass2.test2 } } WTF (˽°□°҂˽Ɨ ˍʓˍ 

class MyClass class MyClass2(val test: String, val test2: String) class MyClass3(val clazz: MyClass2?) class MyClass4 { private var myClass2: MyClass2? = null fun m() { if (myClass2 != null) { val test = myClass2!!.test val test2 = myClass2!!.test2 } } } WTF (˽°□°҂˽Ɨ ˍʓˍ 

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 

FROM JAVA TO Gerard Paligot