Understanding Kotlin Variables: From Basics to Constants

Welcome! Today, we're delving into Kotlin variables, which are critical elements in any programming language. They function as containers that store and handle data. In this lesson, we will explore what they are, learn about assigning values and assigning names to them, and discuss what constants are in Kotlin.

A variable is a slot in memory that stores data, its content can change during the execution of the program. For example, in the snippet below, the value "Alice" held by myName is replaced by "Bob":

var myName = "Alice"
println(myName) // Output: Alice

myName = "Bob"
println(myName) // Output: Bob

Kotlin has two types of variables - val, which is immutable, and var, which is mutable.

In Kotlin, you can reassign values to variables declared with var, but variables declared with val are immutable. Let's assign and reassign a value to myAge to see this in action:

var myAge = 25
println(myAge) // Outputs: 25

myAge = 30
println(myAge) // Outputs: 30

Now, let's try to alter a constant, myBirthYear:

val myBirthYear = 1991
println(myBirthYear) // Outputs: 1991

myBirthYear = 1990  // Error: Val cannot be reassigned

In Kotlin, we adhere to the camelCase naming convention, which helps deliver clear, purposeful names. Good examples include numberOfStudents, accountBalance, userDetails. Poor examples like c, xyz, temp lack clear purpose and should, therefore, be avoided.

Kotlin uses const val for constants, which are declared at the top level:

const val PI = 3.14159
println(PI) // Outputs: 3.14159

PI = 3.15   // Error: Val cannot be reassigned

Constants are used for values that are not subject to change.

In Kotlin, you can incorporate variable values into a string using a technique called string interpolation. This technique makes your code more concise and easy to read. Here's an example:

fun main() {
  val name = "Kotlin learner"
  val topicsCovered = 2
  println("Hello, $name! You've covered $topicsCovered topics today") // prints "Hello, Kotlin learner! You've covered 2 topics today"
}

In the code snippet above, $name and $topicsCovered within the quotes act as placeholders. Kotlin automatically substitutes them with the values of the name and topicsCovered variables when the code executes.