Kotlin Notes

September 4, 2019 1733 words 9 minutes

Contents

Table of Contents
Basics
Collections
OOP
Nulls and Exceptions
Lambdas and Higher-Order Functions
References

Basics

Kotlin requires one main per app:

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fun main(args: Array<String>) { // the args part can be omitted
    println("Hello Kotlin!")
}

Shorter if

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println(if (x > y) "x is greater" else "x is not greater")
return if (x > y) x else y

var vs val:
- When using var, we can assign another value to the variable.
- When using val, the reference to the object stays forever. However, if the variable is an array, the array itself can be updated. (Similar to let in Swift)
Primitive types are also objects controlled by references.

Similar to Swift, we can specify the type:

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var z: Int = 6
var x: Long = z.toLong() // similarly, toFloat(), toByte(), etc.

Arrays:

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var myArray = arrayOf(1, 2, 3)
var myLength = myArray.size
var explicitArray: Array<Int> = arrayOf(1, 2, 3)

Strings:

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var x = 42
var myString = "x is $x"
// When accessing a property or function of a object, use ${}
var myArray = arrayOf(1, 2, 3)
var arraySize = "The size is ${myArray.size}"
var firstItem = "The first item is ${myArray[0]}"
"12.345-6.A".split(".", "-")  // splits at both . and -
// We don't need to escape for triple quote strings for regular expressions.
val regex = """(.+)/(.+)\.(.+)""".toRegex()
// We can also use triple quotes for multiline strings like in Python.

Functions;

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// We can have default values like in Python.
fun foo(bar: Int = 1): Int { // Unit means no return value, or just omit it.
    // And Nothing means the function never returns.
    // stuff
    return 1
}

var result = foo(1)

fun max(a: Int, b: Int): Int = if (a > b) a else b // also works

fun listOf<T> (vararg values: T): List<T> {...}  // vararg makes it variadic.
// And we need to explicitly unpack the array when passing:
listOf("args:", *args)

Loops:

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for (x in 1..100) println(x) // end inclusive
for (x in 1 until 100) println(x) // not end inclusive
for (x in 15 downTo 1) println(x) // end inclusive
for (x in 1..100 step 2) println(x)
for (item in items) println(item)

Input:
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val userInput = readLine()

when:

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when (x) {
    0 -> println("It's 0")
    1, 2 -> println("It's 1 or 2")
    else -> {
        println("It's not 0.")
        println("It's not 1 nor 2.")
    }
}
// We can also use when without the argument, then each case needs to be a Boolean.

We can use the qualified this to access the this from an outer scope: this@MainActivity.

How to compile and run in the command line:

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kotlinc main.kt -include-runtime -d main.jar && java -jar main.jar "optional args"

Collections

List, Set, Map, MutableList, MutableSet, MutableMap.
listOf(), mutableListOf(). mList.set(1, "foo"), .shuffle(), .last(), .max().
mapOf(0 to 'a', 1 to 'b', 2 to 'c') for ((key, value) in mMap). (to actually creates a Pair<K, V>).

Add out (<out T>) to make the generics covariant (use a subtype when a supertype is expected) - achieving polymorphism (like <? extends E> in Java). Add in to make it contravariant - the opposite of covariance (use a supertype when a subtype is expected) (like <? super E> in Java). Producer (read-only) out, consumer in (write-only).

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fun <T> copy(src: List<out T>, dst: MutableList<in T>) {
    for (x in src) {
        dst.add(x)
    }
}
val src = listOf<Int>(1, 2, 3)
val dst = mutableListOf<Any>()
copy<Number>(src, dst)

We can use in to check existence just like in Python.

Like enumerate() in Python, we have .withIndex():

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for ((index, element) in collection.withIndex()) {...}

OOP

Example class:

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class Dog(val name: String, var weight: int, val breed: String = "default") {
    var temperament: String = ""
    // All properties must be initialized.
    // Or:
    // lateinit var temperament: String
    fun bark() {
        println("Woof!")
    }
}

Custom getters and setters:

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...
val weightInKgs: Double
    get() = weight / 2.2
var weight = weightParam
    set(value) {
        if (value > 0) field = value // field is a keyword
    }
...

In fact, the complier adds getters and setters automatically:

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var myProperty: String
    get() = field
    set(value) {
        field = value
    }

Classes, variables, and methods are final by default. To enable inheritance and overriding, add open before them. We also need to add override to variables.
The init {} blocks are called during initialization.
If a property is defined using val in the superclass, we must override it in the subclass if we want to assign a new value to it. However, if it’s defined using var, we just need to reassign it in the init {} block without using override.
abstract for classes, variables, and methods. These properties must all be overridden later.
An interface lets us define common behavior OUTSIDE a superclass hierarchy. (Not IS-A, but share a property.) A class can have multiple interfaces, but can only inherit from a single direct superclass. class X: A, B {} (No parentheses. class X: A() means we are inheriting.)
Use is to check the type: if (animal is Wolf). And use as for explicit casting.
Any is the superclass of everything.
Add data to the start of the class to make it behave like a struct in Swift. Then we can use == or .equals to test the equivalence. (Equal objects have the same .hashCode() value.) (And .toString() returns the value of each property.) (BTW, === is used for referential check (identity).)

For data objects, we have destructuring declaration:

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val (title, number) = r
// is equivalent to
val title = r.component1
val number = r.component2
// And this can be used to return multiple values from a function, or we can use Pair.

Named arguments (var r = Recipe(title = "title", foo = "bar")) are also available like in Swift.

Secondary constructors:

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constructor(foo: Boolean) : this(0, foo) {} // Calls the primary constructor.

enum class:

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enum class Color {
    RED, YELLOW, BLUE
}

fun foo(color: Color) =  // Returns the value directly.
    when (color) {
        Color.RED -> 1
        Color.YELLOW -> 2
        Color.BLUE -> 3
    }

Everything is public by default. We also have private, protected, and internal (for a module).

Operator overloading:

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data class Point(val x: Int, val y: Int) {
    operator fun plus(other: Point): Point {
        return Point(x + other.x, y + other.y)
    }
}

Nulls and Exceptions

Use ? for nullable objects just like in Swift.
w?.eat() is a safe call. It’s only called when w is not null.

Like optional binding in Swift, use let:

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w?.let {
    // executed when w is not null
}

The Elvis operator ?:

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w?.hunger ?: -1 // if null, return -1

!! is like ! in Swift. It throws a NullPointerException if the value is null: w!!.hunger.

Like Java, the same try catch finally block for exception handling. And the same throw. But we can use try as an expression, which returns the value to the variable:

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val number = try {
    Integer.parseInt(reader.readLine())
} catch (e: NumberFormatException) {
    return  // Or null if we want the flow to continue.
}
println(number)

as? is the conditional cast, which returns null is the casting is not possible:
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val foo = bar as? Person ?: return false
.filterNotNull() returns a list with null filtered out.

Lambdas and Higher-Order Functions

We can assign a lambda to a variable:

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val addFive: (Int) -> Int = {x: Int -> x + 5}
val result = addFive.invoke(1) // 6
// or
val result = addFive(1)

{it + 5} is equivalent to {x -> x + 5}.

Lambdas can also be passed to functions.

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fun convert(x: Double, converter: (Double) -> Double) : Double {
    return converter(x)
}
println(convert(1, {c: Double -> c * 1.8 + 32}))

.min() and .max() work with basic types: val mMax = mList.max()
minBy {} and maxBy {} work with all types: val mMaxQuantity = groceries.maxBy {it.quantity}. Also sumBy {} and sumByDouble {} : mMap.values.sumBy {it}. We can also use :: reference like in Java 8 (::foo calls the top-level function foo).
filter {}: val pricesOver3 = groceries.filter {it.price > 3.0}. Also filterNot {}. We can also use .count() instead of .filter().size.
map {}: val doubleInts = ints.map {it * 2}
forEach {}: groceries.forEach {println(it.name)}
Closure means that a lambda can access any local variable that it captures (the variables declared in the outer scope). Closures are functions that are aware of the surroundings.
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var sum = 0 ints.filter { it > 0 }.forEach { // sum is captured sum += it }
groupBy {} returns a Map<Key, List> (Key depends on the key’s type).
fold {}: val sumOfInts = ints.fold(0) {mSum, item -> mSum + item}

all and any:

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listOf(Person("Alice", 27), Person("Bob", 31)).all {it.age < 28}

flatMap {} and flatten():

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val books = listOf(Book("Thursday Next", listOf("Jasper Fforde")),
                    Book("Mort", listOf("Terry Pratchett")),
                    Book("Good Omens", listOf("Terry Pratchett","Neil Gaiman")))

println(books.flatMap { it.authors }.toSet())
// [Jasper Fforde, Terry Pratchett, Neil Gaiman]
// Or use .flatten() when we don't do any transformation:
listOfLists.flatten()

Like a Python generator or Java stream, Kotlin also has the lazy sequence:

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people.asSequence()
    .map(Person::name)
    .filter {it.startsWith("A")}
    .toList()  // Converts the sequence back.

For SAM (Single Abstract Method) interface, we can pass a lambda instead of implementing a class:

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fun interface IntPredicate {
    fun accept(i: Int): Boolean
}

val isEven = IntPredicate {it % 2 == 0}
// Instead of
val isEven = object : IntPredicate {
    override fun accpet(i: Int): Boolean {
        return i % 2 == 0
    }
}

println(isEven.accept(8))
// Another example:
button.setOnClickListener { view ->
    ...
}

References

Head First Kotlin: A Brain-Friendly Guide