KOTLIN–A modern programming language that makes developers happier.

fun main() {

    val name = "stranger"        // Declare your first variable

    println("Hi, $name!")        // ...and use it!

    print("Current count:")

    for (i in 0..10) {           // Loop over a range from 0 to 10

        print(" $i")

    }

}

import kotlinx.coroutines.*

​

suspend fun main() {                                // A function that can be suspended and resumed later

    val start = System.currentTimeMillis()

    coroutineScope {                                // Create a scope for starting coroutines

        for (i in 1..10) {

            launch {                                // Start 10 concurrent tasks

                delay(3000L - i * 300)              // Pause their execution

                log(start, "Countdown: $i")

            }

        }

    }

    // Execution continues when all coroutines in the scope have finished

    log(start, "Liftoff!")

}

​

fun log(start: Long, msg: String) {

    println("$msg " +

            "(on ${Thread.currentThread().name}) " +

            "after ${(System.currentTimeMillis() - start)/1000F}s")

}

fun main() {

    // Who sent the most messages?

    val frequentSender = messages

        .groupBy(Message::sender)

        .maxByOrNull { (_, messages) -> messages.size }

        ?.key                                                 // Get their names

    println(frequentSender) // [Ma]

​

    // Who are the senders?

    val senders = messages

        .asSequence()                                         // Make operations lazy (for a long call chain)

        .filter { it.body.isNotBlank() && !it.isRead }        // Use lambdas...

        .map(Message::sender)                                 // ...or member references

        .distinct()

        .sorted()

        .toList()                                             // Convert sequence back to a list to get a result

    println(senders) // [Adam, Ma]

}

​

data class Message(                                          // Create a data class

    val sender: String,

    val body: String,

    val isRead: Boolean = false,                              // Provide a default value for the argument

)

​

val messages = listOf(                                       // Create a list

    Message("Ma", "Hey! Where are you?"),

    Message("Adam", "Everything going according to plan today?"),

    Message("Ma", "Please reply. I've lost you!"),

)

// Tests

// The following example works for JVM only

import org.junit.Test

import kotlin.test.*

​

class SampleTest {

    @Test

    fun `test sum`() {                  // Write test names with whitespaces in backticks

        val a = 1

        val b = 41

        assertEquals(42, sum(a, b), "Wrong result for sum($a, $b)")

    }

​

    @Test

    fun `test computation`() {

        assertTrue("Computation failed") {

            setup()                     // Use lambda returning the test subject

            compute()

        }

    }

}

​

// Sources

fun sum(a: Int, b: Int) = a + b

fun setup() {}

fun compute() = true