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!
						

Remember The Names!

?. - Safe traversal

?: - Elvis

!! - Hold my beer!!

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;
}
						

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;
}
						

Even More Java Version

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

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);
}
						

Oh come on...

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

Kotlin Version

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

Now What?

• Learn - kotlinlang.org
• Play - try.kotlinlang.org
• Discuss - discuss.kotlinlang.org

Thanks!

 @ToddGinsberg

 #todd.ginsberg

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