Kotlin Variables: `val` for Constants, `var` for Variables

Why it exists

Programs need to store and manage data. Kotlin provides two ways to declare variables to make code safer and more predictable. By distinguishing between data that can change (mutable) and data that cannot (immutable), the language helps developers prevent common bugs caused by unintended modifications.

The mental model

Think of val as a value written in permanent ink. Once you assign it, it's fixed and cannot be changed. Think of var as a value written on a whiteboard; you can erase it and write a new value at any time. This distinction forces you to be intentional about which data in your program is allowed to change.

How it works

You declare a variable using either the val keyword for read-only variables or var for mutable ones. The syntax is the keyword, followed by the variable name, an optional type, and the assigned value. For example: val pi = 3.14 or var counter = 0. Kotlin's type inference often allows you to omit the type (like :Double or :Int) if the value is assigned at declaration. If you declare a variable without initializing it, you must specify its type, like val name: String. You can then assign a value to name later, but only once since it's a val.

When to use it

Use val as your default for any value that will not change after its initial assignment. This is common for configuration settings, API response data, or properties of an object that are set at creation. Use var only when you have a clear need for a value to be reassigned, such as a loop counter, an accumulator that totals up values, or a property that tracks a changing state like a user's current score.

When not to use it

Avoid using var when val would suffice. This is a common beginner mistake. Using var everywhere makes the code harder to reason about, as any variable could potentially change anywhere in the code. This increases cognitive load and the risk of bugs. Always start with val and only change it to var if the compiler tells you a reassignment is needed and it makes logical sense.

One canonical example

Imagine tracking a user's score in an app. The app's name is a constant, but the score changes. You would define the name with val and the score with var. For example: val appName = "BrainBites" and var currentScore = 0. Later, you can update the score with currentScore = 100. If you tried to change appName, the compiler would give you an error because a val cannot be reassigned.