Kotlin - Better Java

KOTLIN
BETTER JAVA
Dariusz Lorenc

JAVA IS EVOLVING SLOWLY
▸ Java 5 - 2005
▸ for each, autoboxing, generics, better concurrency
▸ Java 6 - 2006
▸ Java 7 - 2011
▸ Java 8 - 2014
▸ lambdas, streams
▸ Java 9 - 2017?

” MOST PEOPLE TALK ABOUT JAVA THE LANGUAGE,
AND THIS MAY SOUND ODD COMING FROM ME, BUT
I COULD HARDLY CARE LESS. AT THE CORE OF THE
JAVA ECOSYSTEM IS THE JVM. “
James Gosling, 
Creator of the Java Programming Language

OTHER JVM LANGUAGES
▸ Groovy
▸ dynamic language
▸ Scala
▸ too many features / steeper learning curve
▸ slow compilation time
▸ too much emphasis on FP?
▸ Clojure
▸ dynamic language, dynamically typed
▸ different syntex, too LISPy?

KOTLIN - KEY FEATURES
▸ Statically typed, statically compiled
▸ Built on proven solutions, but simpliﬁed
▸ Great Java inter-op
▸ OO at core
▸ FP-friendly
▸ function types
▸ immutability
▸ lets you program in functional style but does not force you

ANY BETTER?
▸ Fixes some basic java problems
▸ Draws inspiration from Effective Java
▸ Provides more modern syntax & concepts
▸ Avoids (java’s) boilerplate
▸ Focuses on pragmatism

VARIABLES / PROPERTIES
var greeting: String
private val hello: String = “hello”
*no static ﬁelds/properties

FUNCTIONS
fun greet(name: String): String {
return “hello $name”
}
private fun print(name: String) {
println(greet(name))
}
*no static functions

CLASSES
class Hello : Greeting {
private val greeting: String = “hello”
override fun greet() {
print(greeting)
}
}

WHY DO I CARE
Comparing to Java, Kotlin allows me to write
▸ safer
▸ concise and readable
code, so I can be
‣ more productive
but can still leverage
‣ java’s rich ecosystem
‣ JVM platform

JAVA - WHAT COULD POSSIBLY GO WRONG
public class JavaGreeting {
private String greeting;
public JavaGreeting(String greeting) {
this.greeting = greeting;
}
public String getGreeting() {
return this.greeting;
}
}

KOTLIN - FINAL PROPERTIES
class KotlinGreeting(val greeting: String)

private final String greeting;
}
public int length() {
return this.greeting.length();
}
}

KOTLIN - NULL SAFETY
class KotlinGreeting(val greeting: String) {
fun length() = greeting.length
}
class KotlinGreeting(val greeting: String?) {
…
greeting?.length //enforced by compiler
greeting!!.length //enforced by compiler
}

public class JavaGreetings {
private final List<String> greetings;
public JavaGreetings(List<String> greetings) {
if (greetings == null)
throw new NullPointerException();
this.greetings = greetings;
}
public void addGreeting(String greeting) {
this.greetings.add(greeting);
}
}

KOTLIN - IMMUTABLE COLLECTIONS
class KotlinGreetings(
private val greetings: List<String>) {
fun addGreeting(greeting: String) {
greetings.add(“hola”) //doesn’t compile
}
}
class KotlinGreetings(
private val greetings: MutableList<String>)

interface Greeting { }
public class Hello extends Greeting {
public String say() {
return “hello”;
}
}
if (greeting instanceOf Greeting) {
Hello hello = (Hello) greeting
hello.say()
}

KOTLIN - SMART CAST
if (greeting is Hello) {
greeting.say()
}

public String say() { return “hello”; }
say() == “hello”

KOTLIN - REFERENTIAL EQUALITY
fun say() = “hello”
say() == “hello” //true
say() === “hello” //false
say() !== “hello” //true

}
public boolean equals(Object aGreeting) {
//some ide-generated code
}
}

KOTLIN - DATA CLASSES
data class KotlinGreeting(val greeting: String)

}
public boolean equals(JavaGreeting aGreeting) {
}
public int hashCode() {
}
}

KOTLIN - OVERRIDE
class KotlinGreeting(val greeting: String) {
//computer says NO!
override fun equals(aGreeting: KotlinGreeting):
Boolean {
…
}
override fun hashCode(): Int {
…
}
}

protected void sayHi() { … }
protected void sayHello() {
}
}
public class HelloGreeting extends JavaGreeting {
protected void sayHi() {
sayHello();
}
}
sayHi();

KOTLIN - FINAL CLASSES & FUNCTIONS
class KotlinGreeting {
fun sayHi() { … }
fun sayHello() { … }
}
//computer says NO!
class HelloGreeting : KotlinGreeting {
}

OTHER SAFETY FEATURES
▸ Serialisable & inner classes
▸ No statics
▸ Functional code
▸ Safer multithreading
▸ More testable code

CONCISE & NOT READABLE
String one, two, three = two = one = "";
boolean a = false, b = one==two ? two==three : a

EXPRESSION BODY
fun greeting(): String {
return “Hello Kotlin”
}
fun greeting(): String = “Hello Kotlin”

TYPE INFERENCE
val greeting: String = “Hello Kotlin”
val greeting = “Hello Kotlin”
fun greeting(): String = “Hello Kotlin”
fun greeting() = “Hello Kotlin”

CLASS DECLARATION + CONSTRUCTOR + PROPERTIES
class KotlinGreeting(val greeting: String)
}
public String getGreeting() {
return this.greeting;
}
}

NAMED ARGUMENTS - SAY NO TO BUILDERS
class LangGreeting(
val greeting: String,
val lang: String
)
LangGreeting(greeting = “Hello”, lang = “Kotlin”)
LangGreeting(lang = “Java”, greeting = “bye, bye!”)

DEFAULT VALUES
class LangGreeting(
val greeting = “Hello”,
val lang: String
)
LangGreeting(lang = “Kotlin”)
LangGreeting(lang = “Java”, greeting = “bye, bye!”)

DATA CLASS
data class KotlinGreeting(val greeting: String)
‣ equals()
‣ hashCode()
‣ toString()
‣ copy()
‣ componentN()

DATA CLASSES & IMMUTABILITY
data class LangGreeting(
val lang: String
)
val kotlinGreeting = LangGreeting(
greeting = “Hello”,
lang = “Kotlin”
)
val javaGreeting = kotlinGreeting.copy(
lang = “Java”
)

DATA CLASSES & DESTRUCTURING
data class LangGreeting(
val lang: String
)
val (greeting, lang) = kotlinGreeting

LAMBDAS
val ints = 1..10
val doubled = ints.map { value -> value * 2 }
val doubled = ints.map { it * 2 }

DELEGATION
interface Greeting {
fun print()
}
class KotlinGreeting(val greeting: String) :
Greeting {
override fun print() { print(greeting) }
}
class DecoratedGreeting(g: Greeting) : Greeting by g

WHEN EXPRESSION
when (x) {
0, 1 -> print("x == 0 or x == 1")
else -> print("otherwise")
}
val y = when (x) {
0, 1 -> true
else -> false
}

CONCISE & READABLE - OTHER FEATURES
▸ Better defaults: ﬁnal, public?, nested classes are not inner
▸ Extension functions
▸ Default imports
▸ Sealed classes

AND THERE IS MORE …
▸ Coroutines - experimental in Kotlin 1.1
▸ Better Kotlin support in Spring 5
▸ Spek
▸ Kotlin/Native
▸ Kotlin to JavaScript
▸ Gradle Kotlin Script

JAVA INTEROP & GOTCHAS
▸ Using Java libs in Kotlin is straightforward
▸ Using Kotlin libs in Java
▸ Understand how Kotlin compiles to Java
▸ Final classes - spring, mockito …
▸ mockito-kotlin wrapper
▸ kotlin-allopen & spring plugin
▸ `when` is a keyword
▸ “$” is used for string interpolation

REFERENCES
▸ http://kotlinlang.org/docs/reference/
▸ Kotlin in Action [Book]
▸ https://github.com/Kotlin/kotlin-koans
▸ https://try.kotlinlang.org/
▸ https://github.com/KotlinBy/awesome-kotlin

Kotlin - Better Java

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