Is there Kotlin after Java 8
Ivan Turčinović & Igor Buzatović
What is Kotlin?

Statically typed, JVM-targeted language

Design principles: Industrial use, tooling, safety

Compiles to JVM byte code and JavaScript

Runtime size 800K
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General
General


No release date, no spec yet
Roadmap
 M1 → M2 → … → Beta → Jetbrains project → V1.0
 Latest milestone → M7
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
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
Current status, plans, release date
General

Advantages over Java 7 are overwhelming
Java 8 introduced
 Lambdas
 Null safety (“Optional” class)
 Default methods
 Streams API
 New Date & Time API
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
Java strikes back with Java 8
But even with Java 8 Kotlin still...

Is more elegant, concise and safe

Has more cool stuff
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General
Basic syntax and rules
Functions - definition in
package or in class

Immutable/mutable
variables


No „new” keyword

Type inference

No checked exceptions

No primitive types

No static members
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More elegant?
Basic syntax and rules cont’d

Primary constructors

No fields, just properties

Bean style classes easy to declare

By default, all classes are final
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More elegant?
More elegant?
Null safety
• Null reference – Billion dollar mistake
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• Kotlin is designed in a way that aims to
eliminate NPE from our code
More elegant?
Java
Generics
 Not co-variant by default, reason?
 Wildcards, use-site variance
 Co-variance price, producers
 Contra-variance, consumers
 Bloch: "PECS: Producer extend,
Consumer super“
Kotlin
 Declaration-site variance
 out / in generics modifiers
 Supports use-site variance also
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 No wildcard syntax, out / in used
Java survey: 39% of wildcards usage can be replaced by declaration-site variance
Default arguments, named arguments
 Default argument values can
be defined
 Arguments with default
values are optional
 No more need for function
overloading (almost)
 Kotlin classes can have only
one constructor
 Arguments can be called by
name
 When passing arguments by
name ordering doesn’t
matter
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More cool stuff
Ranges
 Simpler “is it in range” check
 Can be used for any type
that implements
Comparable
 “both ends“ included
 “..” operator is translated to
“rangeTo” function
 “rangeTo” function is
implemented as extension
function on Comparable
 Numerical ranges can be
iterated over
 In both directions and in
arbitrary steps
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More cool stuff
Pattern matching
 Java’s instaceof not very practical
 No !instanceof
 Meet Kotlin’s "is“
 "is" negation "!is“
 Automatic type cast when “is”
evaluates true inside if / when
blocks
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More cool stuff
More cool stuff
Data classes
 Classes very easy to declare
 Data annotation changes
behavior of hashCode,
equals, toString functions
 Default implementation uses
all properties
javacro.User@41a7d9e7
1101519335
1926426205
false
 Off course we can override
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Output
More cool stuff
Data classes
 Classes very easy to declare
 Data annotation changes
behavior of hashCode,
equals, toString functions
 Default implementation uses
all properties
User(firstName=John,lastName=Doe)
71819599
71819599
true
 Off course we can override
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Output
Operator overloading
 Operators are translated to
corresponding functions
 Operator set is fixed
 Operator overloading is achieved by
overriding corresponding functions of
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 enable
certain operator by implementing
corresponding functions
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More cool stuff
Extension functions
 What to do if we want to add features
to existing classes
 Java: extending or utility classes
 Kotlin: add functions to existing
classes without changing their code
 Example: implement BigDecimal +
operator
 How does it work “behind the scene”?
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More cool stuff
More cool stuff
Kotlin collections

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
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
More cool stuff
Type-safe Groovy-style builders
 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
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 For generating tree-like structures (UI layouts, 3D scene graphs)
Type-safe Groovy-style builders
 Objects initialized using
function literal argument
 Extension function literal
allows referencing to object
enclosing element
 Kara web framework using it
for HTML layouts, CSS
 Android apps, XML layouts
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More cool stuff
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Thank You!