Variables

Use val when the variable cannot be changed after it is initialized, that is, when the assigned value will be immutable:

val name: String = "Kotlin"

In this example, the variable name was initialized with the value “Kotlin” and cannot be reassigned to another value later. This guarantees that the value of a variable remains constant.

Use var when the variable can be changed after initialization, that is, when you need to assign new values over time:

var name: String = "Kotlin"
name = "Kotlin 123" // Value reassignment

Here, name is mutable and its value can be modified as many times as needed.

Variables in Kotlin can be declared in different scopes, such as local or global. This means that access to a variable can depend on where it was defined.

In this example, name is a class-level variable (global within the class), while template is a local variable, visible only inside the register method:

class Language {
    val name = "Kotlin"  // Global-level variable in the class

    fun register() {
        val template = "Language: <%s>."
        println(template.format(name))  // Using the local and global variable
    }
}

The variable name can be accessed anywhere in the class, and outside it if the class is instantiated. The variable template, on the other hand, can only be used inside the register function:

val language = Language()

println(language.name)  // Accessing the global variable 'name'
language.register()     // Accessing the local variable 'template' inside the function

Complete example:

class Language {
    val name = "Kotlin"

    fun register() {
        val template = "Language: <%s>."
        println(template.format(name))
    }
}

fun main() {
    val language = Language()
    
    println(language.name)  // Kotlin
    language.register()     // Language: <Kotlin>.
}

Whenever possible, prefer val over var, since this promotes immutability in the code, which can prevent bugs and make maintenance easier. Define variables with the narrowest possible scope. This helps limit access and avoid accidental interference with the value.