Is there Kotlin after Java 8

Ivan Turčinović & Igor Buzatović

What is Kotlin?General

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� Statically typed, JVM-targeted language

� Design principles: Industrial use, tooling, safety

� Compiles to JVM byte code and JavaScript

� Runtime size 800K

Current status, plans, release date General

� No release date, no spec yet

� Roadmap� M1 → M2 → … → Beta → Jetbrains project → V1.0� Latest milestone → M7

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� Some frameworks/libraries already appeared� Kara Web Framework – Statically typed HTML/CSS � Spek – Specification/test framework� Kannotator – Infer annotations from bytecode� KotlinPrimavera - Kotlin libraries to Spring projects

Java strikes back with Java 8

� Advantages over Java 7 are overwhelming

� Java 8 introduced

� Lambdas

General

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� Lambdas

� Null safety (“Optional” class)

� Default methods

� Streams API

� New Date & Time API

But even with Java 8 Kotlin still...

� Is more elegant, concise and safe

General

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�Has more cool stuff

Basic syntax and rulesMore elegant?

� Functions - definition in

package or in class

� Immutable/mutable

variables

� No „new” keyword

� Type inference

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� Type inference

� No checked exceptions

� No primitive types

� No static members

More elegant? Basic syntax and rules cont’d

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� Primary constructors

� No fields, just properties

� Bean style classes easy to declare

� By default, all classes are final

More elegant? Null safety

• Null reference – Billion dollar mistake

• Kotlin is designed in a way that aims to eliminate NPE from our code

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GenericsMore elegant?

� Not co-variant by default, reason?

� Wildcards, use-site variance

� Co-variance price, producers

� Contra-variance, consumers

� Bloch: "PECS: Producer extend,

Consumer super“

Java

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� Declaration-site variance

� out / in generics modifiers

� Supports use-site variance also

� No wildcard syntax, out / in used

Kotlin

Java survey: 39% of wildcards usage can be replaced by declaration-site variance

Default arguments, named argumentsMore cool stuff

� Default argument values can

be defined

� Arguments with default

values are optional

� No more need for function

overloading (almost)

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overloading (almost)

� Kotlin classes can have only

one constructor

� Arguments can be called by

name

� When passing arguments by

name ordering doesn’t

matter

RangesMore cool stuff

� Simpler “is it in range” check

� Can be used for any type that implements Comparable

� “both ends“ included

� “..” operator is translated to “rangeTo” function

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“rangeTo” function

� “rangeTo” function is implemented as extension function on Comparable

� Numerical ranges can be iterated over

� In both directions and in arbitrary steps

Pattern matchingMore cool stuff

� Java’s instaceof not very practical

� No !instanceof

� Meet Kotlin’s "is“

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� "is" negation "!is“

� Automatic type cast when “is” evaluates true inside if / when blocks

Data classesMore cool stuff

� Classes very easy to declare

� Data annotation changes

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Output

javacro.User@41a7d9e711015193351926426205false

� Data annotation changes behavior of hashCode, equals, toString functions

� Default implementation uses all properties

� Off course we can override

Data classesMore cool stuff

� Classes very easy to declare

� Data annotation changes

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Output

User(firstName=John,lastName=Doe)7181959971819599true

� Data annotation changes behavior of hashCode, equals, toString functions

� Default implementation uses all properties

� Off course we can override

Operator overloadingMore cool stuff

� Operators are translated to corresponding functions

� Operator set is fixed

� Operator overloading is achieved by overriding corresponding functions of specific class

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specific class

� By adding extension functions we can enable certain operator for classes that don’t support it originally

� On our own classes we can enablecertain operator by implementingcorresponding functions

Extension functionsMore cool stuff

� What to do if we want to add features to existing classes

� Java: extending or utility classes

� Kotlin: add functions to existingclasses without changing their code

� Example: implement BigDecimal +

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� Example: implement BigDecimal + operator

� How does it work “behind the scene”?

Extension functionsMore cool stuff

� What to do if we want to add features to existing classes

� Java: extending or utility classes

� Kotlin: add functions to existingclasses without changing their code

� Example: implement BigDecimal +

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� Example: implement BigDecimal + operator

� How does it work “behind the scene”?

Kotlin collections More cool stuff

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� Read-only traits on top, co-variant

� Extended by mutable traits

� Implemented by JDK classes (compile time modifications)

� Read-only traits do not guarantee immutability

� Top level package functions for instantiating collections:

� Read-only: setOf, listOf, mapOf

� Mutable: hashSetOff, arrayListOf, hashMapOf

Type-safe Groovy-style buildersMore cool stuff

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� For generating tree-like structures (UI layouts, 3D scene graphs)

� Tree-like syntax – better insight in what is being generated

� Functions create, initialize and return builder specific objects

� Better then xml: no scheme required, type safe, interoperable with other code

Type-safe Groovy-style buildersMore cool stuff

� Objects initialized using

function literal argument

� Extension function literal

allows referencing to object

enclosing element

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enclosing element

� Kara web framework using it

for HTML layouts, CSS

� Android apps, XML layouts

Thank You!

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Thank You!