A Brief Tour of Kotlin

Basics - Classes

class Customer(name: String) {
    init {
        println("Customer initialized with value ${name}")
    }
    ...
}

Classes define their constructor in the class header
Init blocks are executed in order and act as constructor body
Classes are final by default
Look! No semicolons!
Look! String interpolation!

Basics - Inheritance

open class Entity(name: String) { 
	...
}
class Customer(name: String) : Entity(name) {
    init {
        println("Customer initialized with value ${name}")
    }
    ...
}

Basics - var/val

var name: String = "Todd" // Initial
name = "Emma"             // OK

val name: String = "Todd" // Initial
name = "Emma"             // Compiler Error

Basics - Equality

val s1 = "CJUG"
val s2 = "cjug"
s1 == s2.toUpperCase()  // (Structural) True!
s1 === s2               // (Referential) False

Basics - No New Keyword

val me: Person = Person("Todd")

Basics - No Ternary

val status: String = if(x == 42) "Success" else "Fail"

Use if expression instead!

Basics - FUNctions!

fun generateRandomNumber(): Int {
    return 4
}

fun generateRandomNumber(): Int = 4

Basics - Named and Optional Parameters

fun generateRandomNumber(start: Int = 0): Int = start + 4

generateRandomNumber()    // 4
generateRandomNumber(1)   // 5
generateRandomNumber(start = 1)   // 5

Type Inference

Can't Someone Else Do It?

var result : Int = add(1, 1)
fun negate(number: Int): Int = 0-number

var result = add(1, 1) 
fun negate(number: Int) = 0-number

Smart Casting

No need to cast

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

Null Safety

Null checking is built in
Must declare when you want to allow nulls

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

Traverse safely

var homeTown : String? = null
println(homeTown.toUpperCase())  // Not allowed

val homeTown : String? = ...
println(homeTown?.toUpperCase()) // OK!
                ^^

Elvis

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

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

Combine Them!

val homeTown : String? = ...
println(homeTown?.toUpperCase() ?: "Unknown")

Manual Override

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

val lowest : Int = listOf(1, 2, 3).min()!!

val lowest : Int = listOf<Int>().min()!!
// KotlinNullPointerException!

Extension Methods

Extend existing types with your own functions!

fun String.initialCaps(): String =
    when(this.length) {
        0 ->    this
        1 ->    this.toUpperCase()
        else -> this[0].toUpperCase() + this.substring(1).toLowerCase()
    }

Pssst! Hey, 'when' is an expression returning a value!

Lambdas

listOf(2,3,4).map { a -> a * 2 } // [4,6,8]

listOf(2,3,4).map { it * 2 } // [4,6,8]

Lambdas are Closures

var sum = 0
ints.filter { it > 0 }.forEach {
    sum += it
}
print(sum)

Can access variables in their outer scope

Properties

class Person() {
    var name: String? = null
}

val e = Person()
e.name = "emma"
println("Person's name is ${e.name}")
// -> "Person's name is emma"

Properties (continued)

class Person() {
    var name: String? = null
        set(value) {
            field = value?.initialCaps() ?: value
        }
}

val e = Person()
e.name = "emma"
println("Person's name is ${e.name}")
// -> "Person's name is Emma"

Propeties - Constructor

class Customer(val name: String) {
    ...
}

Data Classes

Java version...

public class PersonDto {
    public String firstName;
    public String lastName;
}

Only 4 lines

Still Java Version

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;
}

Up to 20 lines

Even More Java Version

@Override
public String toString() {
    return "PersonDto{" +
            "firstName='" + firstName + '\'' +
            ", lastName='" + lastName + '\'' +
            '}';
}

OK, 29 lines

Even Yet Still More Java Version

@Override
public boolean equals(Object o) {
    if (this == o) {
        return true;
    }
    if (o == null || getClass() != o.getClass()) {
        return false;
    }

    PersonDto personDto = (PersonDto) o;

    if (!firstName.equals(personDto.firstName)) {
        return false;
    }
    return lastName.equals(personDto.lastName);
}

46 lines already?

Oh come on...

@Override
public int hashCode() {
    int result = firstName.hashCode();
    result = 31 * result + lastName.hashCode();
    return result;
}

A lean mean 51 lines

Kotlin Version

data class PersonDto(var firstName: String, var lastName: String)

Only 1 line!