What is the difference between var, val, and const in Kotlin?

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In Kotlin, which is Google’s most preferred language for Android Development, there are various keywords used to declare variables. In this shot, we’ll go over the difference among each of these keywords and the best instances to use each of them.

var

var name = "peter"
var name: String = "peter"

var is used to declare a mutable variable. A mutable variable is a variable that can be changed or that can be reassigned in the course of the program.

In the example above, both lines of code are basically the same, except that the data type is indicated in the second line.

Unlike Java, indicating the data type is optional in Kotlin, as Kotlin can infer the data type of the variable according to the value assigned to it. In var, values are assigned during run time.

val

val is used to declare immutable variables. Once a value is assigned to a variable with the val keyword, it cannot be altered or reassigned throughout the program. Val is similar to the final keyword in Java.

val age = 35

Throughout the program, it is expected that the value of age will remain 35. If you try to assign another value to age, an error will be thrown. val is also a run-time constant, meaning its value has to be assigned during run time.

const

const is also used to declare variables that do not change in the course of the program, but const is a compile-time constant, meaning its value has to be assigned at compile-time, unlike var and val. Since const needs to know its value at compile-time, you cannot assign functions or constructors to it because the values of functions are determined at run-time.

const is mostly declared at the top of the file as follows:

const val language = "en"

You can also declare const in an object as follows:

object Website {
     const val URL = "google"  
     }

The object keyword is just like the static keyword in Java. object makes it possible to access that variable anywhere in your program.

In summary: