Kotlin fundamentals Part I: variables & constants

Variables are the building blocks to know when you learn a new programming language. In today's article we are going to talk about variables in Kotlin.

Declaring variables: val and var

To declare a variable in Kotlin we use the keyword val or var. If declared with val, a variable is immutable. It means that once set the value of that variable can't be changed throughout the execution of the program. On the other hand, if a variable is declared with the keyword var, its value can change during the execution of the program.

val age = 18 // value can't change
var name = "Lionel Messi"

When using the val keyword to declare a variable, you must set the initial value for that variable.

Kotlin is a statically typed language which means that every variable should have a specific type. To specify a type when declaring a variable, you put a semicolon and the type after the variable name

val age: Int = 18
val salary: Double = 3500.5
var name: String = "David"
var isAdmin: Boolean = false

Kotlin has built-in types like other programming language:

• Int for integers
• Double for floating numbers
• Boolean for boolean value
• String for text

Although, you are not forced to specify the variable type thanks to a mechanism called "type inference". It means that Kotlin can infer (or simply guess ) the type of the variable

val country = "Madagascar"

In the example, Kotlin will assume the the variable country is a String due to the presence of the double quotes

Define a constant

To define a constant in Kotlin you need to define a dedicated class and a companion object inside the class

class Constants {
    companion object {
        const val NUMBER_OF_DAYS_IN_YEAR = 365
    }
}

To use that constant in your code, you need to import it like shown in the example below

import Constants.Companion.NUMBER_OF_DAYS_IN_YEAR
fun main() {
    println("One year has $NUMBER_OF_DAYS_IN_YEAR")
}

Working with String

To create a string you can call the String constructor and assign it to a variable

val myString = String()

Or by assigning a value when creating the variable

val myString = "Hello Kotlin"

To get the length of a string, use the length property

val myString = "Kotlin"
myString.length

Many methods are available for string manipulation

val myString = "Hello World"
myString.uppercase() // "HELLO WORLD"
val position = myString.indexOf("W") // 7
myString.substring(position) // "World"

Like in Java, you can build a string using the StringBuilder class

var builder = StringBuilder("Hello EveryOne ")
    builder.append("Kotlin is cool")

It will create the string "Hello EveryOne. Kotlin is cool"

That's all for today. See you in my next article