kotlin-const-var-and-val-keywords

In this tutorial, we’ll be outlining the key differences between the const, var, and val keywords in the Kotlin language.

To put these keywords into context, we’ll be comparing them to their Java equivalents.

To understand these keywords, we have to understand two of the major categories of type systems a language can follow – manifest typing and inferred typing.

Firstly, we’ll start with var, Kotlin’s keyword representing mutable, non-final variables. Once initialized, we’re free to mutate the data held by the variable.

Let’s take a look at how this works:

Behind the scenes, myVariable initializes with the Int data type.

Although Kotlin uses type inference, we also have the option to specify the data type when we initialize the variable:

Variables declared as one data type and then initialized with a value of the wrong type will result in an error:

Kotlin’s val keyword works much in the same as the var keyword, but with one key difference – the variable is read-only. The use of val is much like declaring a new variable in Java with the final keyword.

For example, in Kotlin, we’d write:

Whereas in Java, we’d write:

Much like final variables in Java, val variables must be assigned at declaration, or in a Class constructor:

Like val, variables defined with the const keyword are immutable. The difference here is that const is used for variables that are known at compile-time.

Declaring a variable const is much like using the static keyword in Java.

Let’s see how to declare a const variable in Kotlin:

And the analogous code written in Java would be:

In this article, we’ve taken a quick look at the difference between manifest and inferred typing.

Then, we looked at the difference between Kotlin’s var, val, and const keywords.