Andrey Breslav

http://kotlinlang.org

JetBrains

• Pragmatic language for industry– Started in 2010– Open Source– 1.0 Released in Feb, 2016

• For JVM, Android, and JavaScript– Interoperability: 100%

• Try online: https://try.kotl.in

bytecode

Kotlin

Technicalities

• Few compiler optimizations – inlning as a language feature

• Compiler intergated into the IDE

• Incrementality

Requirements

• Statically typed

• “Toolable” (is this a word at all?)

• Interoperable (yes, C++ has been there!)

• Learnable (“democratic”)

Comparing to others

Over 500K LOC of Kotlin:

IntelliJ IDEARider (C# IDE)

YouTrackJetBrains Account

(and more)

In Open-Source Projects, Kotlin itself excluded

In Open-Source Projects, Kotlin itself excluded

IDE

Compiler

(Incremental)Build System

Java*.kt *.java

compiler

Javacompiler

“Billion dollar mistake”— Sir C. A. R. Hoare

Java String s = null;

s.length();

Errors At Runtime

Kotlin val s: Strings.length()

val s: String? = nulls.length()

Errors At Compile Time

= nullNullable

type

Check and use val s: String? = …if (s != null) { s.length()}

Check and exit if (s == null) return

s.length()

Rock’n’Roll s?.length()s!!.length()(s ?: “…”).length()

Kotlin is good with nulls

What about Java?

Java (as seen from Kotlin)public class JavaClass { public String foo(List<String> l) {…}}

String

String?

List<String>

List<String?>

List<String>?

List<String?>?

Safest!

Java Interop: All Nullable

javaValue.toString().length() + 1

javaValue?.toString()?.length()!! + 1

val l: List<String> = javaValue?.getList()!!

ArrayList<String?>

Your safest option doesn’t

work!

Annotationspublic class JavaClass { @NotNull public String foo(@NotNull List<String> l) {…}}

String

String?

List<String>

List<String?>

List<String>?

List<String?>?

Annotations are cumbersome

ANDdon’t really help!

Pick Two

• Null-safety• Convenience• Java Interop

Platform Types!

Thanks toDr. Ross Tate of

Java: Platform Typespublic class JavaClass { public String foo(Bar<String> l) {…}}

String! Bar<String!>!

Platform Type

Dereferencing Platform Values

s: String s: String? s: String!

s.length() s.length() s.length()

s?.length() s?.length() s?.length()

s!!.length() s!!.length() s!!.length()

NPE

Assignability

String String?

String!

Platform Type

Not Transitive!NPE

Some Notes

• Platform Types are Not Denotable!– String! is notation, not syntax

• Pure Kotlin is Null-Safe• Kotlin+Java is as safe as Java

• Annotations Still Applicable– @NotNull String in Java becomes String in Kotlin

• Dynamic Type for JavaScript Interop– JS is even unsafer than Java

Nullable types: Summary

Cost• More Complexity• Less Null-Safety

Benefit• Smooth Interop

http://kotlinlang.org

Images Used

• http://blogs.msdn.com/cfs-filesystemfile.ashx/__key/communityserver-blogs-components-weblogfiles/00-00-01-32-02-metablogapi/8054.image_5F00_thumb_5F00_35C6E986.png

• http://www.complete-coherence.com/wp-content/uploads/2015/06/Is-business-rational-or-emotional.jpg

• http://www.mightyheaton.com/wp-content/uploads/2012/05/33-1024x908.jpg

• https://openclipart.org/image/2400px/svg_to_png/97543/text-file-icon.png

• http://fileinfo.com/img/icons/files/128/class-737.png

• http://1.bp.blogspot.com/-UK5uEhDELUA/T9duhHrDeII/AAAAAAAAB20/nKpYXYhqub4/s1600/application-x-jar%5B1%5D.png