Declaring variables

Every variable must be declared. Any attempt to use a variable that hasn't been declared yet is a syntax error; thus, you are protected from accidentally assigning to a misspelled variable. The declaration also decides what kind of data you are allowed to store in the variable.

Local variables are typically declared and initialized at the same time, in which case the type of the variable is inferred to be the type of the expression you initialize it with:

var number = 42
var message = "Hello"

We now have a local variable number whose value is 42 and whose type is Int (because that's the type of the literal 42), and another local variable message whose value is "Hello" and whose type is String. Subsequent usages of the variable must use only the name, not var:

number = 10
number += 7
println(number)
println(message + " there")

However, you cannot change the type of a variable: number can only ever refer to Int values, and message can only ever refer to String values, so both number = "Test" and message = 3 are illegal and will produce syntax errors.

Read-only variables

Frequently, you'll find that during the lifetime of your variable, it only ever needs to refer to one object. Then, you can declare it with val (for "value") instead:

val message = "Hello"
val number = 42

The terminology is that var declares a mutable variable, and that val declares a read-only or assign-once variable - so both kinds are called variables.

Note that a read-only variable is not a constant per se: it can be initialized with the value of a variable (so its value doesn't need to be known at compile-time), and if it is declared inside a construct that is repeatedly invoked (such as a function or a loop), it can take on a different value on each invocation. Also, while the read-only variable may not be reassigned while it is in scope, it can still refer to an object which is in itself mutable (such as a list).

Constants

If you have a value that is truly constant, and the value is a string or a primitive type (see below) that is known at compile-time, you can declare an actual constant instead. You can only do this at the top level of a file or inside an object declaration (but not inside a class declaration):

const val x = 2

Specifying the type explicitly

If you really want to, you can both initialize and specify the type on the same line. This is mostly useful if you're dealing with a class hierarchy (more on that later) and you want the variable type to be a base type of the value's class:

val characters: CharSequence = "abc"

In this doc, we'll sometimes specify the type unnecessarily, in order to highlight what type is produced by an expression. (Also, a good IDE will be able to show you the resulting type.)

For completeness: it is also possible (but discouraged) to split the declaration and the initial assignment, and even to initialize in multiple places based on some condition. You can only read the variable at a point where the compiler can prove that every possible execution path will have initialized it. If you're creating a read-only variable in this way, you must also ensure that every possible execution path assigns to it exactly once.

val x: String
x = 3

Scopes and naming

A variable only exists inside the scope (curly-brace-enclosed block of code; more on that later) in which it has been declared - so a variable that's declared inside a loop only exists in that loop; you can't check its final value after the loop. Variables can be redeclared inside nested scopes - so if there's a parameter x to a function and you create a loop and declare an x inside that loop, the x inside the loop is a different variable than the parameter x.

Variable names should use lowerCamelCase instead of snake_case.

In general, identifiers may consist of letters, digits, and underscores, and may not begin with a digit. However, if you are writing code that e.g. autogenerates JSON based on identifiers and you want the JSON key to be a string that does not conform to these rules or that collides with a keyword, you can enclose it in backticks: `I can't believe this is not an error!` is a valid identifier.

This material was written by Aasmund Eldhuset; it is owned by Khan Academy and is licensed for use under CC BY-NC-SA 3.0 US. Please note that this is not a part of Khan Academy's official product offering.