Kotlin is charming; The reasons Java engineers should start Kotlin.

Kotlin is charming;
The reasons Java engineers
should start Kotlin.
By Kiyotaka Soranaka:空中清高

About me
Name:空中清高(Kiyotaka Soranaka)
Work:株式会社ジャストシステム

株式会社ジャストシステム
創業38年、売上182億、平均年収900万円
Javaのプロダクト採用は1.0から。
一太郎Ark:1996年開発着手。
JUST.
SYSTEMS

いろんなwebサービスをJavaで作ってます。
などなど...
株式会社ジャストシステムJUST.
SYSTEMS

Agenda
・What is Kotlin?
・Why Kotlin?
・Kotlin VS Java
・Kotlin AND Java

Statically typed programming language
for the JVM, Android and the browser
https://kotlinlang.org/

Google is adding Kotlin as an official
programming language for Android
development
https://android-developers.googleblog.com/2017/05/android-
announces-support-for-kotlin.html

Concise
Safe
Versatile
Interoperable
Tooling
https://kotlinlang.org/

Concise
Safe
Versatile
Interoperable
Tooling

// Create a POJO in a single line
data class Person(val name: String, val age: Int)
// No need "new" keyword and Semicolons are optional.
val person = Person("Kiyotaka Soranaka", 29)
// String Templates
println("My name is ${person.name}.")
// > My name is Kiyotaka Soranaka.
Concise

// Null Safe: These codes cause error at compile time.
val str: String = null // compile error:String is non-null.
val nullable: String? = null // String? is nullable.
val length:Int = nullable.length // compile error:String? is nullable.
// Below codes don't cause error.
val length:Int = nullable!!.length // But this cause error at runtime if ‘nullable’ is null.
val length:Int = if (nullable != null) { nullable.length } else { 0 } // Smart-cast
val length:Int = nullable?.length ?: 0 // "?." is Safe Calls and "?:" is Elvis Operator.
Safe

Versatile
・Android Development.
・Write code in Kotlin and target JavaScript.
・Application Server.
・Spring Framework support Kotlin.
・Enterprise Java EE.

Interoperable
Kotlin is of 100% Java compatibility.
You can use any existing library on the JVM.

Tooling
IntelliJ IDEA is very powerful!
It is developed by JetBrains, where the team
that created Kotlin itself belongs.

Tooling
(Win)Alt + Enter, (Mac) ⌥Enter

Tooling
(Win)Ctrl + Alt + Shift + K
(Mac) ⌥⇧⌘K
Convert Java to Kotlin.

public final class Person {
public Person(String name, int age) {
this.name = name;
this.age = age;
}
private final String name;
private int age;
public String getName() { return name; }
public int getAge() { return age; }
public void setAge(int age) {
this.age = age;
}
}
Kotlin JavaKotlin VS Java

class Person(name: String, age: Int) {
val name: String = name
var age: Int = age
}
'val' and 'var' mean read-only and
mutable, respectively.
getter/setter are automatically
generated.
this.name = name;
this.age = age;
}
private int age;
this.age = age;
}
}
Kotlin: Level 1 JavaKotlin VS Java

class Person(val name: String,
var age: Int)
Properties are automatically generated.
this.name = name;
this.age = age;
}
private int age;
this.age = age;
}
}

data class Person(val name: String,
var age: Int)
Several members are automatically
generated; e.g.,
getter/setter, equals(), hashCode(),
toString(), copy(), etc..
val person = Person("Bob", 5)
println("$person")
// > Person(name=Bob, age=5)
https://kotlinlang.org/docs/reference/dat
a-classes.html
this.name = name;
this.age = age;
}
private int age;
this.age = age;
}
@Override
public boolean equals(Object obj) {
...

data class Person(val name: String = "",
var age: Int = 0)
You can use default values.
'Telescoping' is not needed.
'Telescoping' is as follows.
public Person() {
this("");
}
public Person(String name) {
this(name, 0);
}
this.name = name;
this.age = age;
}

data class Person(var height: Int = 155,
var weight: Int = 50)
By using 'Named Arguments',
you don't have to care the order of the
arguments.
val p1 = Person(height = 170, weight = 60)
val p2= Person(weight = 60, height = 170)
val p3= Person(height = 170)
val p4= Person(weight = 60)
https://kotlinlang.org/docs/reference/fun
ctions.html
public Person() {
this(155);
}
public Person(int height) {
this(height, 50);
}
public Person(int height, int weight) {
this.height = height;// cm
this.weight = weight;// kg
}
Person p1 = new Person(170, 60);
Person p2 = new Person(60, 170);

val cache = HashMap<String, Calendar>()
fun getJJUGDate(): Date {
var jjugCal = cache["JJUG"]
if (jjugCal == null) {
jjugCal = Calendar.getInstance()
jjugCal.set(2017, Calendar.MAY, 20)
cache.put("JJUG", jjugCal)
}
return jjugCal!!.time
}
Map<String, Calendar> cache = new
HashMap<>();
public Date getJJUGDate() {
Calendar jjugCal = cache.get("JJUG");
jjugCal = Calendar.getInstance();
jjugCal.set(2017, Calendar.MAY, 20);
cache.put("JJUG", jjugCal);
}
return jjugCal.getTime();
}

}
}
HashMap<>();
}
}
This code almost the same as Java.
But you can modify more concise.

}
}
HashMap<>();
}
}
To be more concise.
・Simplify the code to initialize the calendar.
・Change the type of "jjugCal" to read-only Calendar.

val jjugCal = cache["JJUG"] ?:
Calendar.getInstance().apply {
set(2017, Calendar.MAY, 20)
cache.put("JJUG", this)
}
return jjugCal.time
}
HashMap<>();
}
}

}
return jjugCal.time
}
HashMap<>();
}
}
Use "?:"(Elvis Operator).
Use "apply"(scoping function).
Type of "jjugCal" is read-only Calendar.

}
return jjugCal.time
}
HashMap<>();
}
}
To be more concise.
・Delete local variable "jjugCal".
・Use kotlin stdlib function.

return cache.getOrPut("JJUG", {
}
}).time
}
HashMap<>();
}
}

}
}).time
}
HashMap<>();
}
}
Use kotlin.stdlib function.
Delete the local variable "jjugCal".

}
}).time
}
HashMap<>();
}
}
To be more concise.
・"return" and return type is optional.

fun getJJUGDate() =
cache .getOrPut("JJUG", {
}
}).time
HashMap<>();
}
}

fun getJJUGDate() =
}
}).time
HashMap<>();
}
}
"return" is omitted.

Kotlin JavaKotlin AND Java
package example.kotlin
Import example.java.Person
fun example() {
val p = Person("Bob", 5)
p.age = p.age + 1
}
package example.java;
public class Person {
this.name = name;
this.age = age;
}
private int age;
this.age = age;
}
}

You can use any existing library on JVM.
Jackson, for example,
data class Data(val id:Int = 0,
val name:String = "")
val mapper = ObjectMapper()
val data = Data(10, "Soranaka")
val json = mapper
.writeValueAsString(data)
println(json)
// > {"id":10,"name":"Soranaka"}
val data2 = mapper
.readValue(json, Data::class.java)
println(data2)
// > Data(id=10, name=Soranaka)

data class Data(val id:Int,var name:String) Data data = new Data(10, "J");
data.setName(data.getName() + "JUG");
System.out.println(data);
// > Data(id=10, name=JJUG)

class KotlinUtil {
val cache =
HashMap<String, Calendar>()
fun getJJUGDate() =
}
}).time
}
KotlinUtil kotlinUtil = new KotlinUtil();
Date jjugDate = kotlinUtil.getJJUGDate();

class KotlinUtil {
val cache =
fun getJJUGDate() =
}
}).time
}
KotlinUtil kotlinUtil = new KotlinUtil();
Date jjugDate = kotlinUtil.getJJUGDate();
Do you want it to be singleton?

object KotlinUtil {
val cache =
fun getJJUGDate() =
}
}).time
}
Date jjugDate =
KotlinUtil.INSTANCE.getJJUGDate();

object KotlinUtil {
val cache =
fun getJJUGDate() =
}
}).time
}
Date jjugDate =
KotlinUtil.INSTANCE.getJJUGDate();
Do you want it to be static?

class KotlinUtil {
companion object {
private val cache =
@JvmStatic
fun getJJUGDate() =
cache.getOrPut("JJUG", {
}
}).time
}
}
Date jjugDate = KotlinUtil.getJJUGDate();

株式会社ジャストシステム
Kotlinを使える仕事に興味を持った方
ぜひ、一緒に働きましょう！
JUST.
SYSTEMS

Thank you
Kiyotaka Soranaka:空中清高
@soranakk

Kotlin is charming; The reasons Java engineers should start Kotlin.

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