12.4 Declaring Mutable and Immutable Variables
Mutable variables are declared using the var keyword and may be initialized with a value at creation time. For example:
var userCount = 10
If the variable is declared without an initial value, the type of the variable must also be declared (a topic which will be covered in more detail in the next section of this chapter). The following, for example, is a typical declaration where the variable is initialized after it has been declared:
var userCount: Int
userCount = 42
Immutable variables are declared using the val keyword.
val maxUserCount = 20
As with mutable variables, the type must also be specified when declaring the variable without initializing it:
val maxUserCount: Int
maxUserCount = 20
When writing Kotlin code, immutable variables should always be used in preference to mutable variables whenever possible.