Write Less Code with Kotlin and Spring Boot

@ToddGinsberg
Write Less Code with Kotlin
and Spring Boot
October 7, 2019
Austin Convention Center
Todd Ginsberg
Principal Software Developer, Netspend

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@ToddGinsberg
Kotlin and Spring Boot in 30 Minutes?!

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@ToddGinsberg
No, that’s crazy!

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@ToddGinsberg
No, that’s crazy!
But stick around after!

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@ToddGinsberg
Agenda
- What Is Kotlin?

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@ToddGinsberg
Agenda
- What Is Kotlin?
- Why Kotlin with Spring Boot?

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@ToddGinsberg
Agenda
- What Is Kotlin?
- Syntax & Features

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@ToddGinsberg
Agenda
- What Is Kotlin?
- Syntax & Features
- Summary

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@ToddGinsberg
Agenda
- What Is Kotlin?
- Syntax & Features
- Summary
- Questions?

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@ToddGinsberg
Who is this Guy?
Todd Ginsberg

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@ToddGinsberg
Who is this Guy?
Todd Ginsberg
Just moved to Dallas from Chicago

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@ToddGinsberg
Who is this Guy?
Todd Ginsberg
Just moved to Dallas from Chicago
Principal Developer at Netspend
- A payments company here in Austin!
- We love Kotlin and Java!

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@ToddGinsberg
What Is Kotlin?

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@ToddGinsberg
What Is Kotlin?
Statically typed language, developed by JetBrains

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@ToddGinsberg
What Is Kotlin?
Released under Apache 2.0 license

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@ToddGinsberg
What Is Kotlin?
Designed as a general purpose language

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@ToddGinsberg
What Is Kotlin?
● Targets JVM 6 or 8 bytecode

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@ToddGinsberg
What Is Kotlin?
● Targets ECMAScript 5.1

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@ToddGinsberg
What Is Kotlin?
● Targets ECMAScript 5.1
● Targets other native platforms thanks to LLVM

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@ToddGinsberg
Major Features
100% Interoperable with Java

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@ToddGinsberg
Major Features
Designed to avoid entire classes of defects

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@ToddGinsberg
Major Features
Lots of small improvements that add up

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@ToddGinsberg
Major Features
Lots of small improvements that add up
Far less code to accomplish the same task in Java

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@ToddGinsberg
Major Features
Null-safe

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@ToddGinsberg
Major Features
Null-safe
189,000+ issues on GitHub!

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@ToddGinsberg
Major Features
Null-safe
189,000+ issues on GitHub!
14,548 duplicates on Stack Overﬂow!

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@ToddGinsberg
Major Features
Pragmatic Improvements
No shame in copying language features that
make developers more productive

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@ToddGinsberg
Major Features
Positive Community

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@ToddGinsberg
Major Features
Positive Community
Even on Stack Overﬂow!

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@ToddGinsberg
Major Features
Positive Community
Even on Stack Overﬂow!
RTFM

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@ToddGinsberg
Why Kotlin
with
Spring Boot?

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@ToddGinsberg
The Hard Work is Already Done!
- Kotlin Support since Spring Framework 5.0

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@ToddGinsberg
- Nullability support between Java and Kotlin

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@ToddGinsberg
- Excellent documentation

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@ToddGinsberg
- Thorough examples

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@ToddGinsberg
- Thorough examples
- Even better support being added for Spring Boot 2.2

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@ToddGinsberg
Similar Philosophies
“The goal of library design is to give application developers
ready-to-use tools that cover their most frequent use-cases
and solve the most popular problems. If something is needed
all the time by any kind of application, it should be simple and
straightforward to code. The correct code shall be the easiest
one to write, while advanced, rarely needed corner-cases can
and should take longer.”

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@ToddGinsberg
Similar Philosophies
“The goal of library design is to give application developers
ready-to-use tools that cover their most frequent use-cases
and solve the most popular problems. If something is needed
all the time by any kind of application, it should be simple and
straightforward to code. The correct code shall be the easiest
one to write, while advanced, rarely needed corner-cases can
and should take longer.”
-- Roman Elizarov (JetBrains)

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@ToddGinsberg
Less Code == More Room For Business Value
- Less code means less cognitive load

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@ToddGinsberg
- The most common case is the default

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@ToddGinsberg
- Fewer places for bugs to hide

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@ToddGinsberg
- Work that you once had to do is a top level concern of the
language / framework

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@ToddGinsberg
- Work that you once had to do is a top level concern of the
language / framework
- These eﬀects stack

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@ToddGinsberg
Syntax

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@ToddGinsberg
Variables and Values
var place: String = "Austin"

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@ToddGinsberg
place = "Texas" // OK!

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val name: String = "Todd"

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val name: String = "Todd"
name = "Emma" // Compile Error!

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Type Inference
val d: Int = 2
val text: String = "Todd has $d doughnuts"

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@ToddGinsberg
Type Inference
val d = 2
val text = "Todd has $d doughnuts"

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@ToddGinsberg
Equality
val name1 = "EXAMPLE"
val name2 = "example"

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Equality
// Structural Equality
name1 == name2.toUpperCase() // True!

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@ToddGinsberg
Equality
// Structural Equality
name1 == name2.toUpperCase() // True!
// Referential Equality
name1 === name2 // False!

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@ToddGinsberg
Raw Strings
val json = "{\n\"name\": \"Todd\"\n}"

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@ToddGinsberg
Raw Strings
val json = """{ "name": "Todd" }"""

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@ToddGinsberg
Raw Strings
val json =
"""
{
"name": "Todd"
}
"""

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@ToddGinsberg
Null Safety
// Guaranteed to never be null
var name: String = "Todd"

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@ToddGinsberg
Null Safety
// Guaranteed to never be null
var name: String = "Todd"
// May be null
var salary: Int? = null

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Null-Safe Traversal
var city: String? = "Austin"

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@ToddGinsberg
Null-Safe Traversal
// Not allowed, might be null!
city.toUpperCase()

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@ToddGinsberg
Null-Safe Traversal
// Not allowed, might be null!
city.toUpperCase()
// Safe traversal
city?.toUpperCase()

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@ToddGinsberg
Elvis
val lowest : Int? = listOf(1, 2, 3).min()

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@ToddGinsberg
Elvis
val lowest : Int? = listOf(1, 2, 3).min()
val lowest : Int = listOf(1, 2, 3).min() ?: 0

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@ToddGinsberg
Combine Safe-Traversal and Elvis
println(
city?.toUpperCase() ?: ”UNKNOWN”
)

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@ToddGinsberg
Manual Override
val lowest: Int? = listOf(1, 2, 3).min()

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@ToddGinsberg
Hey Kotlin, Hold My Beer!
val lowest: Int = listOf(1, 2, 3).min()!!

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@ToddGinsberg
val lowest: Int = emptyList().min()!!

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@ToddGinsberg
val lowest: Int = emptyList().min()!!
// NullPointerException!

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@ToddGinsberg
Expressions - if
val status = if (code == 42) {
"Success"
} else {
"Fail"
}

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@ToddGinsberg
Expressions - try/catch
val number = try {
code.toInt()
} catch (e: NumberFormatException) {
0
}

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@ToddGinsberg
Expressions - when
val result = when (x) {
0 -> "x is 0”
in 1..10 -> "x is between 1 and 10"
in someSet -> "x is in someSet"
is SomeType -> "x is an instance of SomeType"
parseString(s) -> "the same as parseString"
else -> "x doesn't match anything"
}

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@ToddGinsberg
Smart Casting
when (x) {
is Int -> print(x % 2 == 0)
is String -> print(x.length + 1)
is IntArray -> print(x.sum())
}

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@ToddGinsberg
Smart Casting
if(x != null) {
println(x.toString())
}

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@ToddGinsberg
Classes
class Entity : SomeInterface {
// ...
}

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@ToddGinsberg
Classes - Inheritance
open class Entity : SomeInterface {
// ...
}
class Customer : Entity() {
// ...
}

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@ToddGinsberg
Properties
public class Customer {
private String name;
public String getName() {
return name;
}
public void setName(final String name) {
this.name = name;
}
}

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@ToddGinsberg
Properties
class Customer {
var name: String? = null
}

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@ToddGinsberg
Properties
class Customer {
}
val c = Customer()

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@ToddGinsberg
Properties
class Customer {
}
val c = Customer()
c.name = "Todd"

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@ToddGinsberg
Properties
class Customer {
}
val c = Customer()
c.name = "Todd"
println("My name is ${c.name}")

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@ToddGinsberg
Properties
class Customer {
}

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@ToddGinsberg
Properties
class Customer {
set(value) {
field = value?.toUpperCase()
}
}

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@ToddGinsberg
Properties
class Customer {
private set(value) {
field = value?.toUpperCase()
}
}

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@ToddGinsberg
Properties
class Customer {
get() {
return field?.toUpperCase()
}
}

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@ToddGinsberg
Let’s Write a POJO!
public class Person {
public String firstName;
public String lastName;
}

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@ToddGinsberg
private String firstName;
private String lastName;
public String getFirstName() {
return firstName;
}
public void setFirstName(final String firstName) {
this.firstName = firstName;
}
public String getLastName() {
return lastName;
}
public void setLastName(final String lastName) {
this.lastName = lastName;
}
}

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@ToddGinsberg
public Person() { }
public Person(final String firstName, final String lastName) {
}
return firstName;
}
}
return lastName;
}
}
}

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@ToddGinsberg
import java.util.Objects;
public Person() { }
}
return firstName;
}
}
return lastName;
}
}
@Override
public boolean equals(final Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
final Person person = (Person) o;
return Objects.equals(firstName, person.firstName) &&
Objects.equals(lastName, person.lastName);
}
@Override
public int hashCode() {
return Objects.hash(firstName, lastName);
}
}

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@ToddGinsberg
import java.util.Objects;
public Person() { }
}
return firstName;
}
}
return lastName;
}
}
@Override
public boolean equals(final Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
final Person person = (Person) o;
return Objects.equals(firstName, person.firstName) &&
Objects.equals(lastName, person.lastName);
}
@Override
public int hashCode() {
return Objects.hash(firstName, lastName);
}
@Override
public String toString() {
return "Person{" +
"firstName='" + firstName + '\'' +
", lastName='" + lastName + '\'' +
'}';
}
}

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@ToddGinsberg
Data Classes to the Rescue!
data class Person(val firstName: String,
val lastName: String)

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@ToddGinsberg
● Getters (and Setters for vars) as Properties

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@ToddGinsberg
● toString()

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@ToddGinsberg
● toString()
● hashCode() and equals()

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@ToddGinsberg
● toString()
● hashCode() and equals()
● And…

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@ToddGinsberg
Copying Data Classes
val me = Person("Todd", "Ginsberg")

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@ToddGinsberg
Copying Data Classes
val me = Person("Todd", "Ginsberg")
val emma = me.copy(firstName = "Emma")
// Person(”Emma", "Ginsberg")

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@ToddGinsberg
Data Classes as @ConfigurationProperties
@ConfigurationProperties("example.kotlin")
data class KotlinExampleProperties(
val name: String,
val description: String = "Defaults Supported!",
val myService: MyService
)
data class MyService(
val apiToken: String,
val uri: URI
)

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@ToddGinsberg
Data Classes as @ConfigurationProperties
@ConfigurationProperties("example.kotlin")
data class KotlinExampleProperties(
val name: String,
val description: String = "Defaults Supported!",
val myService: MyService
)
data class MyService(
val apiToken: String,
val uri: URI
)
Boot
2.2!

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@ToddGinsberg
FUNctions!
fun generateRandomNumber(): Int {
return 4
}

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FUNctions!
return 4
}
fun generateRandomNumber(): Int = 4

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@ToddGinsberg
FUNctions!
return 4
}
fun generateRandomNumber(): Int = 4
fun generateRandomNumber() = 4

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@ToddGinsberg
FUNctions - Default Values
fun random(offset: Int = 0): Int =
offset + 4

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offset + 4
random() // 4

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offset + 4
random() // 4
random(1) // 5

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@ToddGinsberg
FUNctions - Adding Parameters
fun combine(first: Int,
second: Int): Int =
first + second

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second: Int,
third: Int = 0): Int =
first + second + third

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second: Int,
third: Int = 0): Int =
first + second + third
combine(1, 2)
combine(1, 2, 3)

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@ToddGinsberg
FUNctions - Named Parameters
second: Int): Int =
first + second

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second: Int): Int =
first + second
combine(1, 2) // 3

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second: Int): Int =
first + second
combine(first = 1, second = 2) // 3

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second: Int): Int =
first + second
combine(second = 2, first = 1) // 3

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@ToddGinsberg
Extension FUNctions
// Java
public static boolean isEven(int i) {
return i % 2 == 0;
}

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@ToddGinsberg
Extension FUNctions
// Kotlin
fun Int.isEven(): Boolean =
this % 2 == 0

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@ToddGinsberg
Extension FUNctions
// Kotlin
fun Int.isEven(): Boolean =
this % 2 == 0
2.isEven() // True!

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@ToddGinsberg
The use Extension
// Java
try (Connection conn = getConnection()) {
// ...
}

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@ToddGinsberg
The use Extension
// Kotlin
getConnection().use { conn ->
// ...
}

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@ToddGinsberg
The apply Extension
// Expression and Statements
val p = Person()
p.name = "Todd"
p.age = 21

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@ToddGinsberg
The apply Extension
// Single Expression
val p = Person().apply {
name = "Todd"
age = 21
}

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@ToddGinsberg
The apply Extension
// Single Expression
val p = Person().apply {
name = "Todd"
age = 21
address = Address().apply {
line1 = "5 Tall Cedar Rd."
…
}
}

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@ToddGinsberg
One More Thing on FUNctions
● Functions are ﬁnal by default.

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● Arguments are always ﬁnal.

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● Functions can be deﬁned in a ﬁle, outside of a class.

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● Functions can be deﬁned within another function.

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● Functions can be deﬁned within another function.
● Kotlin supports tail recursive functions.

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@ToddGinsberg
What About Checked Exceptions?
?

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@ToddGinsberg
What About Checked Exceptions?
NO

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@ToddGinsberg
Lambdas
listOf(1, 2, 3, 4)
.filter { x -> x % 2 == 0 }
.map { y -> y * 2 }
// List[4, 8]

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@ToddGinsberg
Lambdas
listOf(1, 2, 3, 4)
.filter { it % 2 == 0 }
.map { it * 2 }
// List[4, 8]

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@ToddGinsberg
So Many Things Left Out! :(
- Property delegation
- Class delegation
- Operator overloading
- Destructuring
- Higher-order functions
- Companion objects
- Import aliases
- Type aliases
- Unsigned integer types
- Coroutines
- Massive STDLIB replaces
your util classes
- Reiﬁed generics
- Inline classes
- Inﬁx functions
- Sealed classes
- Scoping functions
- DSL support

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@ToddGinsberg
Summary

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@ToddGinsberg
Why Do I Use Kotlin?
● I write far less code.

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@ToddGinsberg
● The code I write is more expressive and clear.

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@ToddGinsberg
● I avoid whole classes of defects.

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@ToddGinsberg
● Plays well with the tools I use (Spring, IDEA, Gradle)

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@ToddGinsberg
● Plays well with the tools I use (Spring, IDEA, Gradle)
● Writing Kotlin, for me, is more fun.

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@ToddGinsberg
Fun == Fun
% of developers who are developing with the
language or technology but have not
expressed interest in continuing to do so
(Stack Overflow Developer Survey 2019)

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@ToddGinsberg
Fun == Fun
% of developers who are developing with the
language or technology but have not
expressed interest in continuing to do so
(Stack Overflow Developer Survey 2019)
Kotlin

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@ToddGinsberg
Make “Bad” Choices Explicit or Impossible
Fixed in Kotlin:
-Singleton support built in
-Override keyword mandatory
-Properties over ﬁelds with getter/setter
-Mutability is minimized (val + collections)
-Inheritance prohibited by default
-Builders are easy with default values
-No checked exceptions
-Structural equality is the same everywhere
-Delegation support makes composition easier

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@ToddGinsberg
Want to Learn More?
https://play.kotlinlang.org

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@ToddGinsberg
Want to Learn More?
The State of Kotlin Support in
Spring
Go back in time for this
(or check it out on YouTube next week!)

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@ToddGinsberg
Want to Learn More?
Fully Reactive: Spring, Kotlin, and
JavaFX Playing Together
Today from 4:20pm–5:30pm
In this room!

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@ToddGinsberg
Want to Learn More?
Klingon Kotlin (in Anger, er,
Passion) with Project Reactor and
Spring Boot: Ha' HIja'!
Tomorrow from 11:30am–12:40pm
In this room!

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#springone
@s1p
@ToddGinsberg
[email protected]
https://todd.ginsberg.com
http://bit.ly/S1P2019-Kotlin
Thank You!

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Write Less Code with Kotlin and Spring Boot

Write Less Code with Kotlin and Spring Boot

Todd Ginsberg

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