How to Declare Variable in Kotlin: Syntax and Examples

In Kotlin, you declare a variable using val for read-only variables or var for mutable variables, followed by the variable name and optional type. For example, val name: String = "John" declares a constant string variable.

Syntax

To declare a variable in Kotlin, use either val or var. val creates a read-only variable (like a constant), and var creates a variable that can change. You write the variable name, optionally specify its type with a colon, then assign a value with =.

• val: immutable variable (cannot change)
• var: mutable variable (can change)
• variableName: the name you choose
• type: optional, Kotlin can often guess it
• value: the data you assign

kotlin

val variableName: Type = value
var variableName: Type = value

Example

This example shows how to declare both a read-only and a mutable variable. It prints their values and changes the mutable one.

kotlin

fun main() {
    val name: String = "Alice"  // read-only variable
    var age: Int = 30            // mutable variable

    println("Name: $name")
    println("Age: $age")

    age = 31  // changing mutable variable
    println("New Age: $age")
}

Output

Name: Alice
Age: 30
New Age: 31

Common Pitfalls

One common mistake is trying to change a val variable, which causes an error because it is read-only. Another is forgetting to initialize a variable or giving it a wrong type.

Always use var if you plan to change the variable later, and initialize variables before use.

kotlin

fun main() {
    val constant = 10
    // constant = 20  // Error: Val cannot be reassigned

    var number: Int
    // println(number)  // Error: Variable 'number' must be initialized
    number = 5
    println(number)  // Correct usage
}

Output

5

Quick Reference

Keyword	Meaning	Can Change?	Example
val	Immutable variable (constant)	No	val pi: Double = 3.14
var	Mutable variable	Yes	var count: Int = 0

Key Takeaways

Use val for variables that should not change after assignment.

Use var for variables that need to be updated later.

Kotlin can often infer the variable type, so specifying it is optional.

Always initialize variables before using them to avoid errors.

Trying to reassign a val variable causes a compile-time error.