Java vs Kotlin - Speaker Deck

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Java vs Kotlin Danny Preussler, JNation

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@PreusslerBerlin about:me • Java developer since 2003 • Kotlin developer since 2016 • Google Developer Expert for Android and Kotlin • Introduced Kotlin at Groupon and Viacom

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@PreusslerBerlin sporttotal.tv • We are streaming under- medialised sports • Amateur and semi professional football • Professional Volleyball…

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@PreusslerBerlin

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@PreusslerBerlin

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@PreusslerBerlin Kotlin adoption • Languages developers want to learn in 2019 • https://research.hackerrank.co m/developer-skills/2019

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@PreusslerBerlin Kotlin adoption Okt 18, GitHub: 'Kotlin for Android now fastest-growing programming language' https://www.zdnet.com/article/microsofts-github-kotlin-for-android- now-fastest-growing-programming-language/

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@PreusslerBerlin Kotlin adoption Stackoverflow trends of JVM languages. #kotlin is first with a strong growth during the last 2 years.

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@PreusslerBerlin Kotlin adoption • JVM Ecosystem Report 2018 (10k questionaires): What is the principal JVM language you use for your main applications? • Kotlin edges past Groovy and Scala in language usage on 2.42% https://snyk.io/blog/jvm-ecosystem-report-2018/

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@PreusslerBerlin Kotlin adoption • 1.5M+ developers edited Kotlin code in 2018 • 100M+ lines of Kotlin code • 96,000+ Kotlin GitHub repositories • Android — 1 in 4 apps (from top 1000) uses Kotlin • Server — х2 from 2017 https://www.jetbrains.com/annualreport/2018/community/

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@PreusslerBerlin 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 The year: 2010 Java6 has been around the 4th year

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@PreusslerBerlin 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 The year: 2010 Kotlin Development started internally

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@PreusslerBerlin 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 July 2010 Swift Development started

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@PreusslerBerlin 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 July 2011 Java 7

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@PreusslerBerlin 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 July 2011 JetBrains unveiled Project Kotlin

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@PreusslerBerlin 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 February 2012 JetBrains open sourced Kotlin

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@PreusslerBerlin 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 March 2014 Java 8

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@PreusslerBerlin 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 June 2014 WWDC application became the first publicly released Swift app

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@PreusslerBerlin 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 September 2014 Swift 1.0

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@PreusslerBerlin 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 Februar 2016 Kotlin 1.0

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@PreusslerBerlin 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 Google I/O 2017 Official Kotlin support for Android

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@PreusslerBerlin 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 September 2017 Java 9

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@PreusslerBerlin 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 March 2018 Java 10

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@PreusslerBerlin 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 Google I/O 2019 Kotlin becomes preferred language for Android

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@PreusslerBerlin Why Kotlin?

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@PreusslerBerlin Goals of Kotlin Modern language Faster compilation than Scala Smaller footprint than Scala Designed to work perfectly with Java (designed for 6+8)

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@PreusslerBerlin Why Kotlin? Null safe Functional aspects Strongly typed but type Inference Reduces boilerplate

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@PreusslerBerlin Kotlin is.. Strongly typed Dynamic typed Object orientated Functional kotlin java scala groovy clojure

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@PreusslerBerlin Makes developers happy Happiness

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@PreusslerBerlin Basics String someString; final String someConstantString;

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@PreusslerBerlin Basics

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@PreusslerBerlin Basics var someString: String val someConstantString: String

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@PreusslerBerlin The power of type inference Integer doSomething(String someString) { return someString.getLength(); }

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@PreusslerBerlin The power of type inference

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@PreusslerBerlin The power of type inference fun doSomething(someString: String): Int { return someString.length }

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@PreusslerBerlin The power of type inference fun doSomething(someString: String): Int = someString.length

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@PreusslerBerlin The power of type inference fun doSomething(someString: String) = someString.length

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@PreusslerBerlin Type inference in Java HashMap x = new HashMap();

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@PreusslerBerlin Type inference in Java HashMap x = new HashMap();

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@PreusslerBerlin Type inference in Java 10 var x = new HashMap();

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@PreusslerBerlin Type inference in Java 10 Difference to Kotlin: • var is not a keyword in Java 10 • it‘s a reserved type name

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@PreusslerBerlin Let’s talk about null

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@PreusslerBerlin Lets talk null I call it my billion-dollar mistake. It was the invention of the null reference in 1965 Tony Hoare ( inventor of the null reference)

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@PreusslerBerlin Lets talk null if (button != null){ button.onPressed(); }

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@PreusslerBerlin Lets talk null if (button.isPresent()){ button.get().onPressed(); }

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@PreusslerBerlin Lets talk null

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@PreusslerBerlin Lets talk null button?.onPressed()

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@PreusslerBerlin Lets talk null someString?.length

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@PreusslerBerlin Lets talk null someString?.length ?: 0

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@PreusslerBerlin Lets talk null someString!!.length

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@PreusslerBerlin Lets talk null val someString : String? …

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@PreusslerBerlin Lets talk null • Equivalent in Java: @NonNull but runtime crash vs compiler only • Nullability is Part of Typesystem

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@PreusslerBerlin Lets talk null fun some(thing: String?) {…} public final void some( @Nullable String thing) {…}

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@PreusslerBerlin Lets talk null fun some(thing: String) {…} public final void some( @NotNull String thing) { checkParameterIsNotNull(thing, "thing"); }

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@PreusslerBerlin Smart casts Object someString … if (someString instanceOf String) { ((String)someString).getLength(); }

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@PreusslerBerlin Smart casts

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@PreusslerBerlin Smart casts val someString : Any if (someString is String) { someString.length }

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@PreusslerBerlin Getter, setter view.setVisibility(View.GONE);

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@PreusslerBerlin Getter, setter

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@PreusslerBerlin Getter, setter view.visibility = View.GONE

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@PreusslerBerlin Lambdas • Came in Java-8 • Android devs had to rely on Retrolambda very long

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@PreusslerBerlin Lambdas list.forEach( element -> System.out.println(element) );

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@PreusslerBerlin Lambdas

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@PreusslerBerlin Lambdas list.forEach{ element -> System.out.println(element) }

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@PreusslerBerlin Lambdas list.forEach{ System.out.println(it) }

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@PreusslerBerlin Collections List myList = Arrays.asList("1", "2", "3")

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@PreusslerBerlin Collections

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@PreusslerBerlin Collections val myList = listOf("1", "2", "3") val myList = mutablelistOf("1", "2", "3")

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@PreusslerBerlin Streams tags.stream() .map(tag -> tag.replace("#", "")) .collect(joining(",");

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@PreusslerBerlin Streams

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@PreusslerBerlin Streams tags.map{ it.replace("#", "") } .join(",")

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@PreusslerBerlin Streams tags.asSequence(). .map{ it.replace("#", "") } .join(",")

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@PreusslerBerlin default parameters vs overloading class Api { Api(String baseURl) { this(baseURl, CacheStrategy.NO_CACHE) } Api(String baseURl, CacheStrategy cache) { } }

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@PreusslerBerlin default parameters vs overloading

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@PreusslerBerlin default parameters vs overloading class Api( baseUrl: String, cache: CacheStrategy = CacheStrategy.NO_CACHE )

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@PreusslerBerlin default parameters vs overloading class Api( private val baseUrl: String, val cache: CacheStrategy = CacheStrategy.NO_CACHE )

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@PreusslerBerlin Data classes data class Team( val name: String? = null, val logo: String? = null )

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@PreusslerBerlin Sealed classes sealed class LoadingState { class Loading : LoadingState() class Loaded : LoadingState() class Empty : LoadingState() }

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@PreusslerBerlin Lazy initialization public class LazyField { private String value; public String getValue() { if (value == null) { value = computeValue(); } return value } private String computeValue() { .... } }

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@PreusslerBerlin Lazy initialization

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@PreusslerBerlin Lazy initialization class LazyField { val: String by lazy { computeValue() } private fun computeValue()= … }

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@PreusslerBerlin Lazy initialization • Framework class like Activities: no control over constructor • How use the compiler to check for null?

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@PreusslerBerlin Late initialization • Framework class like Activities: no control over constructor • How use the compiler to check for null?

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@PreusslerBerlin Late initialization View someView; ... someView = findViewbyId(R.id.myView);

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@PreusslerBerlin Late initialization

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@PreusslerBerlin Late initialization lateinit someView: View ... someView = findViewbyId(R.id.myView);

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@PreusslerBerlin Primitives in Kotlin

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@PreusslerBerlin Primitives in Kotlin • No primitives

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@PreusslerBerlin Primitives in Kotlin • No primitives int a; Integer b; a: Integer

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@PreusslerBerlin Primitives in Kotlin • No primitives • Boxing? Therefore no boxing issues (mostly) int a; Integer b; a: Integer

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@PreusslerBerlin Primitives in Kotlin: Costs? • The numeric type usages in Kotlin are compiled into JVM primitives where possible. • Compiler decides

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@PreusslerBerlin Primitives in Kotlin: Costs? • The numeric type usages in Kotlin are compiled into JVM primitives where possible. • Compiler decides • Works because of missing widening int a = 1; double b = a; val a:Int = 1 val b:Double = a

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@PreusslerBerlin Primitives in Kotlin • Nullable type cannot be primitive • Primitives cannot be used as a generic type argument. • List is equivalent of Java List • Therefore IntArray as type!

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@PreusslerBerlin Operator overloading

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@PreusslerBerlin Operator overloading operator fun plusAssign(callback: Callback) { callbacks.add(callback) } operator fun minusAssign(callback: Callback) { callbacks.remove(callback) } adapterCallbacks += {…}

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@PreusslerBerlin Expression Translated to a++ a.inc() + see below a-- a.dec() + see below Expression Translated to +a a.unaryPlus() -a a.unaryMinus() !a a.not() Expression Translated to a + b a.plus(b) a - b a.minus(b) a * b a.times(b) a / b a.div(b) a % b a.rem(b), a.mod(b) (deprecated) a..b a.rangeTo(b)

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@PreusslerBerlin Operator overloading Expression Translated to a > b a.compareTo(b) > 0 a < b a.compareTo(b) < 0 a >= b a.compareTo(b) >= 0 a <= b a.compareTo(b) <= 0 Expression Translated to a == b a?.equals(b) ?: (b === null) a != b !(a?.equals(b) ?: (b === null))

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@PreusslerBerlin Operator overloading Expression Translated to a in b b.contains(a) a !in b !b.contains(a) Expression Translated to a[i] a.get(i) a[i, j] a.get(i, j) a[i_1, ..., i_n] a.get(i_1, ..., i_n) a[i] = b a.set(i, b) a[i, j] = b a.set(i, j, b) a[i_1, ..., i_n] = b a.set(i_1, ..., i_n, b)

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@PreusslerBerlin Operator overloading Expression Translated to a() a.invoke() a(i) a.invoke(i) a(i, j) a.invoke(i, j) a(i_1, ..., i_n) a.invoke(i_1, ..., i_n) Expression Translated to a += b a.plusAssign(b) a -= b a.minusAssign(b) a *= b a.timesAssign(b) a /= b a.divAssign(b) a %= b a.remAssign(b), a.modAssign(b) (deprecated)

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@PreusslerBerlin Operator overloading • Range expression for (i in 1..4) print(i) operator fun rangeTo(other: Int) = IntRange(this, other)

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@PreusslerBerlin Operator overloading in RxKotlin disposables += clips.subscribeBy( onError = { logE(it) }, onNext = { onNewClip(it) } )

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@PreusslerBerlin Getter / Setter var live = true private set

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@PreusslerBerlin Getter / Setter val live get() = category.live val live = category.live

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@PreusslerBerlin Getter / Setter val live get() = category.live

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@PreusslerBerlin Getter / Setter val live get() = category.live public final boolean getLive() { return category.getLive(); }

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@PreusslerBerlin Delegated properties var something: Something by lazy{} var something: Something by inject{}

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@PreusslerBerlin Delegation • inheritance Effective Java: Item 16: Favor composition over inheritance class FirebaseFeatureFlags ( private val defaults: DefaultFeatureFlags ) : FeatureFlags by defaults {

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@PreusslerBerlin Singleton class Singleton { final static Singleton instance = new Singleton() private Singleton() {} }

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@PreusslerBerlin Singleton

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@PreusslerBerlin Singleton object Singleton

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@PreusslerBerlin Extension functions class Utils { public static boolean isNonNullOrEmpty(String string){ … } } isNonNullOrEmpty(“hallo“)

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@PreusslerBerlin Extension functions

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@PreusslerBerlin Extension functions fun String?.isNonNullOrEmpty() = {…} “hallo“.isNonNullOrEmpty()

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@PreusslerBerlin Extension functions fun String?.isNonNullOrEmpty() = {…} public static final boolean isNonNullOrEmpty( @Nullable String $receiver) {… }

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@PreusslerBerlin Standard functions val calendar = Calendar.getInstance(Locale.US).apply { set(Calendar.DAY_OF_MONTH, 3) set(Calendar.MONTH, Calendar.MARCH) set(Calendar.YEAR, 1990) }

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@PreusslerBerlin Standard functions

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@PreusslerBerlin inlining

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@PreusslerBerlin inline •Avoid runtime penalties of wrappers by inline”ing

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@PreusslerBerlin inline •Powerful with reified: Allows generics with type check: T::class inline fun Activity.start() { startActivity(Intent(this, T::class.java)) }

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inline inline fun Activity.start() { startActivity(Intent(this, T::class.java)) } start() $receiver$iv.startActivity( new Intent( (Context)$receiver$iv, MainActivity.class) );

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@PreusslerBerlin inline • Inline classes • Experimental inline class Time(private val long timeMs)

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@PreusslerBerlin inline • Inline classes • Experimental • Inline classes are final • Can lead to weird boxing issues • Not compatible with @Parcelize inline class Time(private val long timeMs)

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@PreusslerBerlin Some changes

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@PreusslerBerlin In/out https://twitter.com/fmuntenescu/status/982268265853476867

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@PreusslerBerlin Static? class Job { static String ERROR = null }

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@PreusslerBerlin Static? class Job { companion object { var ERROR: String = null } }

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@PreusslerBerlin Other changes • Classes/methods public by default

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@PreusslerBerlin Other changes • Classes/methods public by default • Classes/methods final by default (vs open)

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@PreusslerBerlin Other changes • Classes/methods public by default • Classes/methods final by default (vs open) • Inner class: static by default (vs inner)

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@PreusslerBerlin Other changes • Classes/methods public by default • Classes/methods final by default (vs open) • Inner class: static by default (vs inner) • Stricter usage of protected

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@PreusslerBerlin Other changes • Classes/methods public by default • Classes/methods final by default (vs open) • Inner class: static by default (vs inner) • Stricter usage of protected • New visibility: internal • One file can have multiple public classes

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@PreusslerBerlin Other changes • void -> Unit • Object -> Any

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@PreusslerBerlin Other changes • void -> Unit • Object -> Any • Plus Nothing

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@PreusslerBerlin Bad news something ? 1 : 0 something != null ? something : "" L

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@PreusslerBerlin Bad news if (something ) 1 else 0 L something ?: ""

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@PreusslerBerlin Java Interop

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@PreusslerBerlin Java Interop public final class FileKt { public static final void foobar() {..} } //File.kt fun foobar(){..}

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@PreusslerBerlin Java Interop • @JvmOverloads • @JvmStatic • @JvmField • @JvmName • @Throws

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@PreusslerBerlin Java Interop • Careful with „sneaky“ Kotlin exceptions • @Throws

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@PreusslerBerlin Java Interop @Metadata( mv = {1, 1, 13}, bv = {1, 0, 3}, k = 1, d1 = {"\u0000\u0014\n\u0002\u0018\u0002\n\u0002\u0010 \u0000\n\u0002\b\u0002\n\u0002\u0010\u0002\n\u00 02\b\u0003\u0018\u00002\u00020\u0001:\u0001\u000 6B\u0005¢\u0006\u0002\u0010\u0002J\b\u0010\u0003 \u001a\u00020\u0004H\u0007J\u0006\u0010\u0005\u0 01a\u00020\u0004¨\u0006\u0007"}, d2 = {"Lde/jodamob/android/kotlin/T_TestingTest;", "", "()V", "test", "", "throwSomething", "Something", "app"} )

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@PreusslerBerlin Java Interop • Performance in most case same as Java https://de.slideshare.net/intelliyole/kotlin-bytecode-generation-and-runtime-performance

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@PreusslerBerlin Compile time

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@PreusslerBerlin

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@PreusslerBerlin Sum up

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@PreusslerBerlin Kotlin Less code

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@PreusslerBerlin Less Code Kotlin .. helps to increase a cut in the lines of code by approximately 40 % (JetBrains) … tests show … the number of code lines reduces by 30… https://belitsoft.com/apps-development-services/java-vs-kotlin

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@PreusslerBerlin Kotlin Less code No more null

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@PreusslerBerlin Kotlin Less code Easy to learn No more null

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@PreusslerBerlin Kotlin No revolution

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@PreusslerBerlin Kotlin No revolution Just evolution

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@PreusslerBerlin Kotlin No revolution (pragmatic) evolution

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@PreusslerBerlin Kotlin ‘Kotlin isn't revolutionary (with the possible exception of its null-handling) but feels like a very careful amalgamation of some of the best features of other languages. Rob Fletcher, Netflix https://belitsoft.com/apps-development-services/java-vs-kotlin

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@PreusslerBerlin Kotlin is … „Scala for Dummies” https://towardsdatascience.com/introduction-to-kotlin-statistics-cdad3be88b5