Kotlin

Basic Syntax

/**
 * We declare a package-level function main which returns Unit and takes
 * an Array of strings as a parameter. Note that semicolons are optional.
 */
fun main(args: Array<String>) {
    println("Hello, world!")
}

/**
 * Line 13 demonstrates string templates and array access.
 * See this pages for details:
 * http://kotlinlang.org/docs/reference/basic-types.html#strings
 * http://kotlinlang.org/docs/reference/basic-types.html#arrays
 */

fun main(args: Array<String>) {
    if (args.size == 0) {
        println("Please provide a name as a command-line argument")
        return
    }
    println("Hello, ${args[0]}!")
}

/**
 * Line 2 demonstrates the for-loop, that would have been called "enhanced"
 * if there were any other for-loop in Kotlin.
 * See http://kotlinlang.org/docs/reference/basic-syntax.html#using-a-for-loop
 */

fun main(args: Array<String>) {
    for (name in args)
        println("Hello, $name!")
}

/**
 * In this example, `val` denotes a declaration of a read-only local variable,
 * that is assigned an pattern matching expression.
 * See http://kotlinlang.org/docs/reference/control-flow.html#when-expression
 */

fun main(args: Array<String>) {
    val language = if (args.size == 0) "EN" else args[0]
    println(when (language) {
        "EN" -> "Hello!"
        "FR" -> "Salut!"
        "IT" -> "Ciao!"
        else -> "Sorry, I can't greet you in $language yet"
    })
}

/**
 * Here we have a class with a primary constructor and a member function.
 * Note that there's no `new` keyword used to create an object.
 * See http://kotlinlang.org/docs/reference/classes.html#classes
 */

class Greeter(val name: String) {
    fun greet() {
        println("Hello, ${name}");
    }
}

fun main(args: Array<String>) {
    Greeter(args[0]).greet()
}

/**
 * `if` is an expression, i.e. it returns a value.
 * Therefore there is no ternary operator (condition ? then : else),
 * because ordinary `if` works fine in this role.
 * See http://kotlinlang.org/docs/reference/control-flow.html#if-expression
 */
fun main(args: Array<String>) {
    println(max(args[0].toInt(), args[1].toInt()))
}

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

/**
 * A reference must be explicitly marked as nullable to be able hold a null.
 * See http://kotlinlang.org/docs/reference/null-safety.html#null-safety
 */
package multiplier

// Return null if str does not hold a number
fun parseInt(str: String): Int? {
    try {
        return str.toInt()
    } catch (e: NumberFormatException) {
        println("One of the arguments isn't Int")
    }
    return null
}

fun main(args: Array<String>) {
    if (args.size < 2) {
        println("No number supplied");
    } else {
        val x = parseInt(args[0])
        val y = parseInt(args[1])

        // We cannot say 'x * y' now because they may hold nulls
        if (x != null && y != null) {
            print(x * y) // Now we can
        } else {
            println("One of the arguments is null")
    }
    }
}

/**
  * The `is` operator checks if an expression is an instance of a type and more.
  * If we is-checked an immutable local variable or property, there's no need
  * to cast it explicitly to the is-checked type.
  * See this pages for details:
  * http://kotlinlang.org/docs/reference/classes.html#classes-and-inheritance
  * http://kotlinlang.org/docs/reference/typecasts.html#smart-casts
 */
fun main(args: Array<String>) {
    println(getStringLength("aaa"))
    println(getStringLength(1))
}

fun getStringLength(obj: Any): Int? {
    if (obj is String)
        return obj.length // no cast to String is needed
    return null
}

/**
 * `while` and `do..while` work as usual.
 * See http://kotlinlang.org/docs/reference/control-flow.html#while-loops
 */
fun main(args: Array<String>) {
    var i = 0
    while (i < args.size)
        println(args[i++])
}

/**
 * For loop iterates through anything that provides an iterator.
 * See http://kotlinlang.org/docs/reference/control-flow.html#for-loops
 */
fun main(args: Array<String>) {
    for (arg in args)
        println(arg)

    // or
    println()
    for (i in args.indices)
        println(args[i])
}

/**
 * Check if a number lies within a range.
 * Check if a number is out of range.
 * Check if a collection contains an object.
 * See http://kotlinlang.org/docs/reference/ranges.html#ranges
 */

fun main(args: Array<String>) {
    val x = args[0].toInt()
    //Check if a number lies within a range:
    val y = 10
    if (x in 1..y - 1)
        println("OK")

    //Iterate over a range:
    for (a in 1..5)
        print("${a} ")

    //Check if a number is out of range:
    println()
    val array = arrayListOf<String>()
    array.add("aaa")
    array.add("bbb")
    array.add("ccc")

    if (x !in 0..array.size - 1)
        println("Out: array has only ${array.size} elements. x = ${x}")

    //Check if a collection contains an object:
    if ("aaa" in array) // collection.contains(obj) is called
        println("Yes: array contains aaa")

    if ("ddd" in array) // collection.contains(obj) is called
        println("Yes: array contains ddd")
    else
        println("No: array doesn't contains ddd")
}

/**
 * See http://kotlinlang.org/docs/reference/control-flow.html#when-expression
 */

fun main(args: Array<String>) {
    cases("Hello")
    cases(1)
    cases(0L)
    cases(MyClass())
    cases("hello")
}

fun cases(obj: Any) {
    when (obj) {
        1 -> println("One")
        "Hello" -> println("Greeting")
        is Long -> println("Long")
        !is String -> println("Not a string")
        else -> println("Unknown")
    }
}

class MyClass() {
}

Destructuring Declarations

/**
 * This example introduces a concept that we call destructuring declarations.
 * It creates multiple variable at once. Anything can be on the right-hand
 * side of a destructuring declaration, as long as the required number of component
 * functions can be called on it.
 * See http://kotlinlang.org/docs/reference/multi-declarations.html#multi-declarations
 */

fun main(args: Array<String>) {
    val pair = Pair(1, "one")

    val (num, name) = pair

    println("num = $num, name = $name")
}

class Pair<K, V>(val first: K, val second: V) {
    operator fun component1(): K {
        return first
    }

    operator fun component2(): V {
        return second
    }
}

/**
 *  Data class gets component functions, one for each property declared
 *  in the primary constructor, generated automatically, same for all the
 *  other goodies common for data: toString(), equals(), hashCode() and copy().
 *  See http://kotlinlang.org/docs/reference/data-classes.html#data-classes
 */

data class User(val name: String, val id: Int)

fun getUser(): User {
    return User("Alex", 1)
}

fun main(args: Array<String>) {
    val user = getUser()
    println("name = ${user.name}, id = ${user.id}")

    // or

    val (name, id) = getUser()
    println("name = $name, id = $id")

    // or

    println("name = ${getUser().component1()}, id = ${getUser().component2()}")
}

/**
 *  Kotlin Standard Library provide component functions for Map.Entry
 */

fun main(args: Array<String>) {
    val map = hashMapOf<String, Int>()
    map.put("one", 1)
    map.put("two", 2)

    for ((key, value) in map) {
        println("key = $key, value = $value")
    }
}

/**
 * Data class gets next functions, generated automatically:
 * component functions, toString(), equals(), hashCode() and copy().
 * See http://kotlinlang.org/docs/reference/data-classes.html#data-classes
 */

data class User(val name: String, val id: Int)

fun main(args: Array<String>) {
    val user = User("Alex", 1)
    println(user) // toString()

    val secondUser = User("Alex", 1)
    val thirdUser = User("Max", 2)

    println("user == secondUser: ${user == secondUser}")
    println("user == thirdUser: ${user == thirdUser}")

    // copy() function
    println(user.copy())
    println(user.copy("Max"))
    println(user.copy(id = 2))
    println(user.copy("Max", 2))
}

Delegated properties

/**
 * There's some new syntax: you can say `val 'property name': 'Type' by 'expression'`.
 * The expression after by is the delegate, because get() and set() methods
 * corresponding to the property will be delegated to it.
 * Property delegates don't have to implement any interface, but they have
 * to provide methods named getValue() and setValue() to be called.</p>
 */

import kotlin.reflect.KProperty

class Example {
    var p: String by Delegate()

    override fun toString() = "Example Class"
}

class Delegate() {
    operator fun getValue(thisRef: Any?, prop: KProperty<*>): String {
        return "$thisRef, thank you for delegating '${prop.name}' to me!"
    }

    operator fun setValue(thisRef: Any?, prop: KProperty<*>, value: String) {
        println("$value has been assigned to ${prop.name} in $thisRef")
    }
}

fun main(args: Array<String>) {
    val e = Example()
    println(e.p)
    e.p = "NEW"
}

/**
 * Delegates.lazy() is a function that returns a delegate that implements a lazy property:
 * the first call to get() executes the lambda expression passed to lazy() as an argument
 * and remembers the result, subsequent calls to get() simply return the remembered result.
 * If you want thread safety, use blockingLazy() instead: it guarantees that the values will
 * be computed only in one thread, and that all threads will see the same value.
 */

class LazySample {
    val lazy: String by lazy {
        println("computed!")
        "my lazy"
    }
}

fun main(args: Array<String>) {
    val sample = LazySample()
    println("lazy = ${sample.lazy}")
    println("lazy = ${sample.lazy}")
}

/**
 * The observable() function takes two arguments: initial value and a handler for modifications.
 * The handler gets called every time we assign to `name`, it has three parameters:
 * a property being assigned to, the old value and the new one. If you want to be able to veto
 * the assignment, use vetoable() instead of observable().
 */
import kotlin.properties.Delegates

class User {
    var name: String by Delegates.observable("no name") {
        d, old, new ->
        println("$old - $new")
    }
}

fun main(args: Array<String>) {
    val user = User()
    user.name = "Carl"
}

/**
 * Users frequently ask what to do when you have a non-null var, but you don't have an
 * appropriate value to assign to it in constructor (i.e. it must be assigned later)?
 * You can't have an uninitialized non-abstract property in Kotlin. You could initialize it
 * with null, but then you'd have to check every time you access it. Now you have a delegate
 * to handle this. If you read from this property before writing to it, it throws an exception,
 * after the first assignment it works as expected.
 */

import kotlin.properties.Delegates

class User {
    var name: String by Delegates.notNull()

    fun init(name: String) {
        this.name = name
    }
}

fun main(args: Array<String>) {
    val user = User()
    // user.name -> IllegalStateException
    user.init("Carl")
    println(user.name)
}

/**
 * Properties stored in a map. This comes up a lot in applications like parsing JSON
 * or doing other "dynamic" stuff. Delegates take values from this map (by the string keys -
 * names of properties). Of course, you can have var's as well,
 * that will modify the map upon assignment (note that you'd need MutableMap instead of read-only Map).
 */

class User(val map: Map<String, Any?>) {
    val name: String by map
    val age: Int     by map
}

fun main(args: Array<String>) {
    val user = User(mapOf(
            "name" to "John Doe",
            "age"  to 25
    ))

    println("name = ${user.name}, age = ${user.age}")
}

Callable References

/**
 * "Callable References" or "Feature Literals", i.e. an ability to pass
 * named functions or properties as values. Users often ask
 * "I have a foo() function, how do I pass it as an argument?".
 * The answer is: "you prefix it with a `::`".
 */

fun main(args: Array<String>) {
    val numbers = listOf(1, 2, 3)
    println(numbers.filter(::isOdd))
}

fun isOdd(x: Int) = x % 2 != 0

/**
 * The composition function return a composition of two functions passed to it:
 * compose(f, g) = f(g(*)).
 * Now, you can apply it to callable references.
 */

fun main(args: Array<String>) {
    val oddLength = compose(::isOdd, ::length)
    val strings = listOf("a", "ab", "abc")
    println(strings.filter(oddLength))
}

fun isOdd(x: Int) = x % 2 != 0
fun length(s: String) = s.length

fun <A, B, C> compose(f: (B) -> C, g: (A) -> B): (A) -> C {
    return { x -> f(g(x)) }
}