kotlin-code-labs

Kotlin Language learning lab

Run the code with Kotlin Compiler:

$ kotlinc file.kt -include-runtime -d file.jar
$ java -jar file.jar

Kotlin is a cross-platform, statically typed, general purpose programming language with type inference. Kotlin is designed to interoperate fully with Java and the JVM version of Kotlin's standard library depends on the Java Class Library, but type inference allows its syntax to be more precise.

Topics

• Basics
• Concepts
• Multiplatform development
• Platforms
• Standard Library
• Kotlin Coroutines
• Serialization
• Ktor

Basic syntax

Package definition and imports

Package specification should be at the top of the source file.

package my.demo

import kotlin.text.*

// some code

It is not required to match directories and packages: Source files can be placed arbitrarily in the file system

Program entry point

An entry point of a Kotlin application is the main function

fun main(){
    println("Hello Kotlin")
}

Another form of main accepts a variable number of String arguments.

fun main(args: Array<String>){
    println(args.contentToString())
}

Print to the standard output

print prints its arguments to the standard output

print("Hello")
print("...hey hey have you seen episode")
print(43)

Functions

fun sum(a: Int, b: Int): Int {
    return a + b
}

A function body can be expression. Its return type is inferred.

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

A function that returns no meaningful value:

fun printSum(a: Int, b: Int): Unit {
    println("sum of $a and $b is ${a + b}")
}

Unit return type can be omitted

fun printSum(a: Int, b: Int){
    println("sum of $a and $b is ${a + b}")
}

Variables

Read-only local variables are defined using the keyword val.

val a: Int = 1
val b = 2
c = 3

Variables that can be reassigned use the var keyword

var x = 5
x += 3

Creating classes and instances

To define a class, use the class keyword

class shape

Properties of a class can be listed in its declaration or body

class Rectangle(var height: Double, var length: Double) {
    var perimeter = (height + length) * 2
}

The default constructor with parameters listed in the class declaration is available automatically