Inside Kotlin

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What’s “runs on JVM”?

JVM executes .class 00000000 ca fe ba be 00 00 00 34 00 1d 0a 00 06 00 0f 09 |.......4........| 00000010 00 10 00 11 08 00 12 0a 00 13 00 14 07 00 15 07 |................| 00000020 00 16 01 00 06 3c 69 6e 69 74 3e 01 00 03 28 29 |........()| 00000030 56 01 00 04 43 6f 64 65 01 00 0f 4c 69 6e 65 4e |V...Code...LineN| 00000040 75 6d 62 65 72 54 61 62 6c 65 01 00 04 6d 61 69 |umberTable...mai| 00000050 6e 01 00 16 28 5b 4c 6a 61 76 61 2f 6c 61 6e 67 |n...([Ljava/lang| 00000060 2f 53 74 72 69 6e 67 3b 29 56 01 00 0a 53 6f 75 |/String;)V...Sou| 00000070 72 63 65 46 69 6c 65 01 00 0a 48 65 6c 6c 6f 2e |rceFile...Hello.| 00000080 6a 61 76 61 0c 00 07 00 08 07 00 17 0c 00 18 00 |java............| 00000090 19 01 00 0b 48 65 6c 6c 6f 20 57 6f 72 6c 64 07 |....Hello World.| 000000a0 00 1a 0c 00 1b 00 1c 01 00 05 48 65 6c 6c 6f 01 |..........Hello.| 000000b0 00 10 6a 61 76 61 2f 6c 61 6e 67 2f 4f 62 6a 65 |..java/lang/Obje| 000000c0 63 74 01 00 10 6a 61 76 61 2f 6c 61 6e 67 2f 53 |ct...java/lang/S| 000000d0 79 73 74 65 6d 01 00 03 6f 75 74 01 00 15 4c 6a |ystem...out...Lj| 000000e0 61 76 61 2f 69 6f 2f 50 72 69 6e 74 53 74 72 65 |ava/io/PrintStre| 000000f0 61 6d 3b 01 00 13 6a 61 76 61 2f 69 6f 2f 50 72 |am;...java/io/Pr| 00000100 69 6e 74 53 74 72 65 61 6d 01 00 07 70 72 69 6e |intStream...prin| 00000110 74 6c 6e 01 00 15 28 4c 6a 61 76 61 2f 6c 61 6e |tln...(Ljava/lan| 00000120 67 2f 53 74 72 69 6e 67 3b 29 56 00 21 00 05 00 |g/String;)V.!...| 00000130 06 00 00 00 00 00 02 00 01 00 07 00 08 00 01 00 |................| 00000140 09 00 00 00 1d 00 01 00 01 00 00 00 05 2a b7 00 |.............*..| 00000150 01 b1 00 00 00 01 00 0a 00 00 00 06 00 01 00 00 |................| 00000160 00 01 00 09 00 0b 00 0c 00 01 00 09 00 00 00 25 |...............%| 00000170 00 02 00 01 00 00 00 09 b2 00 02 12 03 b6 00 04 |................| 00000180 b1 00 00 00 01 00 0a 00 00 00 0a 00 02 00 00 00 |................| 00000190 03 00 08 00 04 00 01 00 0d 00 00 00 02 00 0e |...............| 0000019f => “Hello World”

“runs on JVM” means generating JVM instructions

.class public Main .super java/lang/Object .method public static main([Ljava/lang/String;)V .limit stack 2 getstatic java/lang/System/out Ljava/io/PrintStream; ldc "Hello World" invokevirtual java/io/PrintStream/println(Ljava/lang/String;)V return .end method JVM assembler => “Hello World”

public class Main { public static void main(String[] args) { System.out.println("Hello World"); } } Java => “Hello World”

Kotlin => “Hello World” fun main(args: Array) { println("Hello World") }

0000000 ca fe ba be 00 03 00 2d 00 18 07 00 0f 0c 00 15 0000010 00 17 01 00 16 28 5b 4c 6a 61 76 61 2f 6c 61 6e 0000020 67 2f 53 74 72 69 6e 67 3b 29 56 01 00 10 6a 61 0000030 76 61 2f 6c 61 6e 67 2f 4f 62 6a 65 63 74 07 00 0000040 04 07 00 0e 01 00 07 48 65 6c 6c 6f 2e 6a 07 00 0000050 11 01 00 04 43 6f 64 65 01 00 04 6d 61 69 6e 09 0000060 00 08 00 02 01 00 0a 53 6f 75 72 63 65 46 69 6c 0000070 65 0c 00 10 00 16 01 00 13 6a 61 76 61 2f 69 6f 0000080 2f 50 72 69 6e 74 53 74 72 65 61 6d 01 00 04 4d 0000090 61 69 6e 01 00 07 70 72 69 6e 74 6c 6e 01 00 10 00000a0 6a 61 76 61 2f 6c 61 6e 67 2f 53 79 73 74 65 6d 00000b0 01 00 0b 48 65 6c 6c 6f 20 57 6f 72 6c 64 0a 00 00000c0 06 00 0d 08 00 12 01 00 03 6f 75 74 01 00 15 28 00000d0 4c 6a 61 76 61 2f 6c 61 6e 67 2f 53 74 72 69 6e 00000e0 67 3b 29 56 01 00 15 4c 6a 61 76 61 2f 69 6f 2f 00000f0 50 72 69 6e 74 53 74 72 65 61 6d 3b 00 21 00 01 0000100 00 05 00 00 00 00 00 01 00 09 00 0a 00 03 00 01 0000110 00 09 00 00 00 15 00 02 00 01 00 00 00 09 b2 00 0000120 0b 12 14 b6 00 13 b1 00 00 00 00 00 01 00 0c 00 0000130 00 00 02 00 07 0000135 .class public Main .super java/lang/Object .method public static main([Ljava/lang/String;)V .limit stack 2 getstatic java/lang/System/out Ljava/io/PrintStream; ldc "Hello World" invokevirtual java/io/PrintStream/println(Ljava/lang/String;)V return .end method public class Main { public static void main(String[] args) { System.out.println("Hello World"); } } fun main(args: Array) { println("Hello, World") } Abstract Concrete The difference is abstraction level

.class Class Loader JVM javac kotlinc fun main(args: Array) { println("Hello, World") } public class Main { public static void main(String[] args) { System.out.println("Hello World"); } } Execution Engine JIT Compiler Garbage Collector

JetBrains/kotlin

Issue tracker

Execute now $ git clone git@github.com:JetBrains/kotlin.git $ cd kotlin $ brew install ant $ ant -f update_dependencies.xml if you want to play Kotlin

* libraries/stdlib * libraries/tools/kotlin-annotation-processing * libraries/tools/kotlin-gradle-plugin * compiler Directories that you may be going to see

What’s “stdlib"?

// Hello.kt fun main(args: Array) { println("Hello World") } $ kotlinc Hello.kt # javac Hello.java $ kotlin HelloKt # java Hello Hello World $ java HelloKt Exception in thread "main" java.lang.NoClassDefFoundError: kotlin/jvm/internal/Intrinsics at HelloKt.main(Hello.kt) Caused by: java.lang.ClassNotFoundException: kotlin.jvm.internal.Intrinsics at java.net.URLClassLoader$1.run(URLClassLoader.java:372) at java.net.URLClassLoader$1.run(URLClassLoader.java:361) at java.security.AccessController.doPrivileged(Native Method) at java.net.URLClassLoader.findClass(URLClassLoader.java:360) at java.lang.ClassLoader.loadClass(ClassLoader.java:424) at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308) at java.lang.ClassLoader.loadClass(ClassLoader.java:357) ... 1 more Try running .class

$ javap -c HelloKt.class Compiled from "Hello.kt" public final class HelloKt { public static final void main(java.lang.String[]); Code: 0: aload_0 1: ldc #9 // String args 3: invokestatic #15 // Method kotlin/jvm/internal/Intrinsics.checkParameterIsNotNull:(Ljava/lang/Object;Ljava/lang/String;)V 6: ldc #17 // String Hello World 8: invokestatic #23 // Method kotlin/io/ConsoleKt.println:(Ljava/lang/Object;)V 11: return } $ javap -c Hello.class Compiled from "Hello.java" public class Hello { public Hello(); Code: 0: aload_0 1: invokespecial #1 // Method java/lang/Object."":()V 4: return public static void main(java.lang.String[]); Code: 0: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream; 3: ldc #3 // String Hello World 5: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/String;)V 8: return } generated by kotlinc generated by javac

$ kotlinc Hello.kt $ kotlin HelloKt Hello World $ kotlinc Hello.kt -include-runtime -d Hello.jar $ java -jar Hello.jar Hello World -include-runtime

$ tree -L 2 . !"" META-INF # !"" MANIFEST.MF # %"" main.kotlin_module !"" HelloKt.class !"" kotlin # !"" ArrayIntrinsicsKt.class # !"" ArraysKt.class # !"" BooleanIterator.class # !"" ByteIterator.class # !"" ByteProgression$Companion.class # !"" ByteProgression.class # !"" ByteProgressionIterator.class # !"" ByteRange$Companion.class # !"" ByteRange.class # !"" CharCategory$Companion$categoryMap$1.class # !"" CharCategory$Companion$categoryMap$2.class # !"" CharCategory$Companion.class # !"" CharCategory.class # !"" CharDirectionality$Companion$directionalityMap$1.class # !"" CharDirectionality$Companion$directionalityMap$2.class # !"" CharDirectionality$Companion.class # !"" CharDirectionality.class # !"" CharIterator.class # !"" CharProgression$Companion.class # !"" CharProgression.class # !"" CharProgressionIterator.class # !"" CharRange$Companion.class # !"" CharRange.class # !"" CharsKt.class # !"" Charsets.class # !"" ClosedRange$DefaultImpls.class # !"" ClosedRange.class # !"" CollectionsKt.class # !"" ComparisonsKt$comparator$1.class # !"" ComparisonsKt$compareBy$1.class # !"" ComparisonsKt$compareBy$2.class # !"" ComparisonsKt$compareBy$3.class # !"" ComparisonsKt$compareByDescending$1.class # !"" ComparisonsKt$compareByDescending$2.class # !"" ComparisonsKt$nullsFirst$1.class # !"" ComparisonsKt$nullsLast$1.class # !"" ComparisonsKt$then$1.class # !"" ComparisonsKt$thenBy$1.class # !"" ComparisonsKt$thenBy$2.class # !"" ComparisonsKt$thenByDescending$1.class # !"" ComparisonsKt$thenByDescending$2.class # !"" ComparisonsKt$thenComparator$1.class # !"" ComparisonsKt$thenDescending$1.class # !"" ComparisonsKt.clas # !"" Deprecated.class # !"" DeprecatedBuiltinsKt.class # !"" DeprecationLevel.class # !"" DoubleIterator.class # !"" EnumsKt.class # !"" EnumsKt__EnumsKt.class # !"" ExceptionsKt.class # !"" ExceptionsKt__StandardJVMKt.class # !"" Extension.class # !"" ExtensionsKt.class # !"" ExtensionsKt__JLangJVMKt.class # !"" FloatIterator.class # !"" FloatingPointConstants.class # !"" Function.class # !"" FunctionsKt$toGenerator$1.class # !"" FunctionsKt.class # !"" HiddenDeclaration.class # !"" IndexedValue.class # !"" IndexingIterable.class # !"" IndexingIterator.class # !"" InitializedLazyImpl.class # !"" IntIterator.class # !"" IntProgression$Companion.class # !"" IntProgression.class # !"" IntProgressionIterator.class # !"" IntRange$Companion.class # !"" IntRange.class # !"" IntegerConstants.class # !"" KotlinNullPointerException.class # !"" Lazy.class # !"" LazyKt$WhenMappings.class # !"" LazyKt.class # !"" LazyThreadSafetyMode.class # !"" LongIterator.class # !"" LongProgression$Companion.class # !"" LongProgression.class # !"" LongProgressionIterator.class # !"" LongRange$Companion.class # !"" LongRange.class # !"" MapsKt.class # !"" MathKt.class # !"" MathKt__BigNumbersKt.class # !"" MathKt__NumbersKt.class # !"" NaturalOrderComparator.class # !"" NoWhenBranchMatchedException.class # !"" NotImplementedError.class # !"" Pair.class # !"" PreconditionsKt.class # !"" PreconditionsKt__AssertionsJVMKt.class # !"" PreconditionsKt__PreconditionsKt.class # !"" Progression.class # !"" Range$DefaultImpls.class # !"" Range.class # !"" RangesKt.class # !"" ReplaceWith.class # !"" ReverseOrderComparator.class # !"" ReversedComparator.class # !"" SafePublicationLazyImpl$Companion.class # !"" SafePublicationLazyImpl.class # !"" Sequence.class # !"" SequencesKt.class # !"" SetsKt.class # !"" ShortIterator.class # !"" ShortProgression$Companion.class # !"" ShortProgression.class # !"" ShortProgressionIterator.class # !"" ShortRange$Companion.class # !"" ShortRange.class # !"" StandardKt.class # !"" StandardKt__IntegersKt.class # !"" StandardKt__StandardKt.class # !"" StandardKt__SynchronizedKt.class # !"" StringsKt.class # !"" Suppress.class # !"" SynchronizedLazyImpl.class # !"" Triple.class # !"" TuplesKt.class # !"" TypeCastException.class # !"" Typography.class # !"" UNINITIALIZED_VALUE.class # !"" UninitializedPropertyAccessException.class # !"" Unit.class # !"" UnsafeLazyImpl.class # !"" UnsafeVariance.class # !"" _Assertions.class # !"" annotation # !"" collections # !"" concurrent # !"" internal # !"" io # !"" jvm # !"" math # !"" properties # !"" ranges # !"" reflect # !"" sequences # !"" support # !"" system # !"" test # !"" text # !"" throws.class # %"" util

.class + stdlib JVM kotlinc fun main(args: Array) { println("Hello, World") } Class Loader Execution Engine JIT Compiler Garbage Collector From the viewpoint of architecture

Kotlin $ tree libraries/stdlib/src -d kotlin/libraries/stdlib/src/ !"" generated %"" kotlin !"" collections !"" comparisons !"" concurrent !"" internal !"" io # %"" files !"" jvm # %"" internal # %"" unsafe !"" properties !"" ranges !"" system !"" text # %"" regex %"" util 17 directories, 70 files Scala $ tree scala/src/library -d scala/src/library/ %"" scala !"" annotation # !"" meta # %"" unchecked !"" beans !"" collection # !"" concurrent # !"" convert # !"" generic # !"" immutable # !"" mutable # !"" parallel # # !"" immutable # # %"" mutable # %"" script !"" compat !"" concurrent # !"" duration # !"" forkjoin # %"" impl !"" io !"" math !"" ref !"" reflect # %"" macros # %"" internal !"" runtime # %"" java8 !"" sys # %"" process !"" text %"" util !"" control !"" hashing %"" matching 35 directories, 751 files

Kotlin: 70 files Scala: 751 files Kotlin provides convenient syntax and extension methods (== better Java) Scala has a lot of classes for full support of functional programming (== realizing different paradigm)

“But we have proguard, right?” “Ah, yeah…”

Modifying stdlib is easy // libraries/stdlib/src/kotlin/text/Strings.kt public inline fun String.hello(): String = "Hello World" // Hello.kt “”.hello() $ ant runtime $ dist/kotlinc/bin/kotlinc Hello.kt && dist/kotlinc/bin/kotlin HelloKt `ant runtime` is finished around a minute!

What’s “kotlinc"?

Text Tokens AST JVM bytecode Machine code Lexer Parser Code generator Frontend Backend Compiler

.kt Kt Tokens AST Node .class Machine code Compiler JFlex PsiParser ASM compiler/frontend compiler/backend

Lexical analysis Source program Lexical analyzer Parser Read characters Push back extra characters Token Ask for next token

Lexical analysis with JFlex * JFlex - fast lexical analyzer generator * Input: scanner specification file * Output: a lexical analyzer _JetLexer.java JFlex Kotlin.flex Lexical specification Read .flex file Generate lexer

switch (zzAction < 0 ? zzAction : ZZ_ACTION[zzAction]) { case 22: { return KtTokens.LBRACKET ; } case 106: break; case 78: { return KtTokens.FUN_KEYWORD ; } case 107: break; case 48: { return KtTokens.IN_KEYWORD ; } case 108: break; case 58: { return KtTokens.GTEQ ; } case 109: break; case 87: { return KtTokens.TRUE_KEYWORD ; } ... "return" { return KtTokens.RETURN_KEYWORD ;} "while" { return KtTokens.WHILE_KEYWORD ;} "break" { return KtTokens.BREAK_KEYWORD ;} "class" { return KtTokens.CLASS_KEYWORD ;} "throw" { return KtTokens.THROW_KEYWORD ;} ... "++" { return KtTokens.PLUSPLUS ; } "--" { return KtTokens.MINUSMINUS; } "<=" { return KtTokens.LTEQ ; } ">=" { return KtTokens.GTEQ ; } "==" { return KtTokens.EQEQ ; } "!=" { return KtTokens.EXCLEQ ; } ... HEX_DIGIT=[0-9A-Fa-f] ESCAPE_SEQUENCE=\\(u{HEX_DIGIT}{HEX_DIGIT}{HEX_DIGIT}{HEX_DIGIT}|[^\n]) CHARACTER_LITERAL="'"([^\\\'\n]|{ESCAPE_SEQUENCE})*("'"|\\)? {CHARACTER_LITERAL} { return KtTokens.CHARACTER_LITERAL; } _JetLexer.java JFlex Kotlin.flex Lexical specification

Parser Source program Lexical analyzer Parser Read characters Push back extra characters Token Ask for next token

::= "+" | ::= "*" | ::= "(" ")" | ::= integer Inside parser: formal grammar and automata e.g. yacc generates push down automata from context-free grammar

IntelliJ Platform SDK http:/ /www.jetbrains.org/intellij/sdk/docs/reference_guide/custom_language_support/implementing_parser_and_psi.html

Optimizer * Remove redundant boxing * Remove redundant null check * Remove redundant GOTO * Remove dead code * …

Code generator AST to 0000000 ca fe ba be 00 03 00 2d 00 18 07 00 0f 0c 00 15 0000010 00 17 01 00 16 28 5b 4c 6a 61 76 61 2f 6c 61 6e 0000020 67 2f 53 74 72 69 6e 67 3b 29 56 01 00 10 6a 61 0000030 76 61 2f 6c 61 6e 67 2f 4f 62 6a 65 63 74 07 00 0000040 04 07 00 0e 01 00 07 48 65 6c 6c 6f 2e 6a 07 00 0000050 11 01 00 04 43 6f 64 65 01 00 04 6d 61 69 6e 09 0000060 00 08 00 02 01 00 0a 53 6f 75 72 63 65 46 69 6c 0000070 65 0c 00 10 00 16 01 00 13 6a 61 76 61 2f 69 6f 0000080 2f 50 72 69 6e 74 53 74 72 65 61 6d 01 00 04 4d 0000090 61 69 6e 01 00 07 70 72 69 6e 74 6c 6e 01 00 10 00000a0 6a 61 76 61 2f 6c 61 6e 67 2f 53 79 73 74 65 6d 00000b0 01 00 0b 48 65 6c 6c 6f 20 57 6f 72 6c 64 0a 00 00000c0 06 00 0d 08 00 12 01 00 03 6f 75 74 01 00 15 28 00000d0 4c 6a 61 76 61 2f 6c 61 6e 67 2f 53 74 72 69 6e 00000e0 67 3b 29 56 01 00 15 4c 6a 61 76 61 2f 69 6f 2f 00000f0 50 72 69 6e 74 53 74 72 65 61 6d 3b 00 21 00 01 0000100 00 05 00 00 00 00 00 01 00 09 00 0a 00 03 00 01 0000110 00 09 00 00 00 15 00 02 00 01 00 00 00 09 b2 00 0000120 0b 12 14 b6 00 13 b1 00 00 00 00 00 01 00 0c 00 0000130 00 00 02 00 07 0000135

Kotlin, Groovy, Clojure, JRuby, IntelliJ, Eclipse, Lombok, Jamaica, AspectJ, … and a lot of products are using ASM

MethodVisitor methodVisitor = classWriter.visitMethod(Opcodes.ACC_PUBLIC + Opcodes.ACC_STATIC, "main", "([Ljava/lang/String;)V", null, null); methodVisitor.visitCode(); methodVisitor.visitFieldInsn(Opcodes.GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;"); methodVisitor.visitLdcInsn("Hello World"); methodVisitor.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V"); methodVisitor.visitInsn(Opcodes.RETURN); methodVisitor.visitEnd(); Hello World

.class public Main .super java/lang/Object .method public static main([Ljava/lang/String;)V .limit stack 2 getstatic java/lang/System/out Ljava/io/PrintStream; ldc "Hello World" invokevirtual java/io/PrintStream/println(Ljava/lang/String;)V return .end method JVM assembler

JVM instruction set

$ javap -v Hello.class Classfile /Users/kentaro-takiguchi/workspace/kotlin/Hello.class Last modified Mar 21, 2016; size 415 bytes MD5 checksum bd70c66b565a03a1fc2b96589b11a4cc Compiled from "Hello.java" public class Hello minor version: 0 major version: 52 flags: ACC_PUBLIC, ACC_SUPER Constant pool: #1 = Methodref #6.#15 // java/lang/Object."":()V #2 = Fieldref #16.#17 // java/lang/System.out:Ljava/io/PrintStream; #3 = String #18 // Hello World #4 = Methodref #19.#20 // java/io/PrintStream.println:(Ljava/lang/String;)V #5 = Class #21 // Hello #6 = Class #22 // java/lang/Object #7 = Utf8 #8 = Utf8 ()V #9 = Utf8 Code #10 = Utf8 LineNumberTable #11 = Utf8 main #12 = Utf8 ([Ljava/lang/String;)V #13 = Utf8 SourceFile #14 = Utf8 Hello.java #15 = NameAndType #7:#8 // "":()V #16 = Class #23 // java/lang/System #17 = NameAndType #24:#25 // out:Ljava/io/PrintStream; #18 = Utf8 Hello World #19 = Class #26 // java/io/PrintStream #20 = NameAndType #27:#28 // println:(Ljava/lang/String;)V #21 = Utf8 Hello #22 = Utf8 java/lang/Object #23 = Utf8 java/lang/System #24 = Utf8 out #25 = Utf8 Ljava/io/PrintStream; #26 = Utf8 java/io/PrintStream #27 = Utf8 println #28 = Utf8 (Ljava/lang/String;)V { public Hello(); descriptor: ()V flags: ACC_PUBLIC Code: stack=1, locals=1, args_size=1 0: aload_0 1: invokespecial #1 // Method java/lang/Object."":()V 4: return LineNumberTable: line 1: 0 public static void main(java.lang.String[]); descriptor: ([Ljava/lang/String;)V flags: ACC_PUBLIC, ACC_STATIC Code: stack=2, locals=1, args_size=1 0: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream; 3: ldc #3 // String Hello World 5: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/String;)V 8: return LineNumberTable: line 3: 0 line 4: 8 } SourceFile: "Hello.java" $ hexdump -C Hello.class 00000000 ca fe ba be 00 00 00 34 00 1d 0a 00 06 00 0f 09 |.......4........| 00000010 00 10 00 11 08 00 12 0a 00 13 00 14 07 00 15 07 |................| 00000020 00 16 01 00 06 3c 69 6e 69 74 3e 01 00 03 28 29 |........()| 00000030 56 01 00 04 43 6f 64 65 01 00 0f 4c 69 6e 65 4e |V...Code...LineN| 00000040 75 6d 62 65 72 54 61 62 6c 65 01 00 04 6d 61 69 |umberTable...mai| 00000050 6e 01 00 16 28 5b 4c 6a 61 76 61 2f 6c 61 6e 67 |n...([Ljava/lang| 00000060 2f 53 74 72 69 6e 67 3b 29 56 01 00 0a 53 6f 75 |/String;)V...Sou| 00000070 72 63 65 46 69 6c 65 01 00 0a 48 65 6c 6c 6f 2e |rceFile...Hello.| 00000080 6a 61 76 61 0c 00 07 00 08 07 00 17 0c 00 18 00 |java............| 00000090 19 01 00 0b 48 65 6c 6c 6f 20 57 6f 72 6c 64 07 |....Hello World.| 000000a0 00 1a 0c 00 1b 00 1c 01 00 05 48 65 6c 6c 6f 01 |..........Hello.| 000000b0 00 10 6a 61 76 61 2f 6c 61 6e 67 2f 4f 62 6a 65 |..java/lang/Obje| 000000c0 63 74 01 00 10 6a 61 76 61 2f 6c 61 6e 67 2f 53 |ct...java/lang/S| 000000d0 79 73 74 65 6d 01 00 03 6f 75 74 01 00 15 4c 6a |ystem...out...Lj| 000000e0 61 76 61 2f 69 6f 2f 50 72 69 6e 74 53 74 72 65 |ava/io/PrintStre| 000000f0 61 6d 3b 01 00 13 6a 61 76 61 2f 69 6f 2f 50 72 |am;...java/io/Pr| 00000100 69 6e 74 53 74 72 65 61 6d 01 00 07 70 72 69 6e |intStream...prin| 00000110 74 6c 6e 01 00 15 28 4c 6a 61 76 61 2f 6c 61 6e |tln...(Ljava/lan| 00000120 67 2f 53 74 72 69 6e 67 3b 29 56 00 21 00 05 00 |g/String;)V.!...| 00000130 06 00 00 00 00 00 02 00 01 00 07 00 08 00 01 00 |................| 00000140 09 00 00 00 1d 00 01 00 01 00 00 00 05 2a b7 00 |.............*..| 00000150 01 b1 00 00 00 01 00 0a 00 00 00 06 00 01 00 00 |................| 00000160 00 01 00 09 00 0b 00 0c 00 01 00 09 00 00 00 25 |...............%| 00000170 00 02 00 01 00 00 00 09 b2 00 02 12 03 b6 00 04 |................| 00000180 b1 00 00 00 01 00 0a 00 00 00 0a 00 02 00 00 00 |................| 00000190 03 00 08 00 04 00 01 00 0d 00 00 00 02 00 0e |...............| 0000019f

Can we use Kotlin?

Before Jack & Jill

After Jack & Jill We may be able to keep using Kotlin

Finally, Android will become NOT JAVA. We won’t be able to obtain Java updates until Google will do (By any chance, it may be the same as now ). Why should we keep using Java??? My concern is…