Are Kotlin strings mutable?

No. Kotlin strings are immutable — once created, their content cannot change. Functions like uppercase() and replace() return new strings rather than modifying the original.

How do I convert a String to a number in Kotlin?

Use .toInt(), .toDouble(), .toLong(), etc. For safe conversion that returns null on failure, use .toIntOrNull(), .toDoubleOrNull().

Beginner 8 min readKotlin 2.0

Kotlin Strings — Declaration, Access, and Common Operations

A String in Kotlin is an immutable sequence of characters. This tutorial covers everything from declaration to common string functions.

What You Will Learn

Declare and access Kotlin strings
Get length and individual characters
Concatenate strings
Common string functions: uppercase, lowercase, trim, replace
Check if a string is empty or blank

Declaring Strings

Strings are declared with double quotes. Kotlin strings are immutable — once created, their content cannot be changed.

Basic String Declaration

kotlin
fun main() {
    val language = "Kotlin"
    val greeting = "Hello, World!"
    println(language)
    println(greeting)
    println(language.length)
}

Output

Kotlin Hello, World! 6

.length gives the number of characters in the string. "Kotlin" has 6 characters.

Beginner Tip: Strings use double quotes ". Single quotes ' are for Char (a single character). "A" is a String; 'A' is a Char.

Accessing Characters

Access individual characters by index (0-based). The first character is at index 0.

Character Access

kotlin
fun main() {
    val word = "Kotlin"
    println(word[0])   // first character
    println(word[5])   // last character
    println(word.first())
    println(word.last())
}

Output

K n K n

word[0] accesses the character at index 0. .first() and .last() are convenience functions for the first and last characters.

Common Mistake: Accessing an index that is out of bounds (e.g., word[10] on a 6-character string) throws StringIndexOutOfBoundsException.

Common String Functions

Kotlin's String class has many useful built-in functions.

Common String Operations

kotlin
fun main() {
    val text = "  Hello, Kotlin!  "
    println(text.trim())          // remove whitespace
    println(text.trim().uppercase())
    println(text.trim().lowercase())
    println(text.trim().replace("Kotlin", "World"))
    println(text.trim().contains("Hello"))
    println(text.trim().startsWith("Hello"))
    println(text.trim().endsWith("!"))
}

Output

Hello, Kotlin! HELLO, KOTLIN! hello, kotlin! Hello, World! true true true

.trim() removes leading and trailing whitespace. .uppercase() / .lowercase() change case. .replace() substitutes substrings. .contains(), .startsWith(), .endsWith() return booleans.

Checking Empty and Blank Strings

Kotlin provides helpful functions to check whether a string has content.

isEmpty vs isBlank

kotlin
fun main() {
    val empty = ""
    val blank = "   "
    val filled = "Kotlin"

    println(empty.isEmpty())   // true
    println(blank.isEmpty())   // false
    println(blank.isBlank())   // true
    println(filled.isBlank())  // false
}

Output

true false true false

isEmpty() returns true only for a string with zero characters. isBlank() returns true for an empty string or one containing only whitespace.

Best Practice: Use isBlank() instead of isEmpty() for user input validation. A string with only spaces is effectively empty for most purposes.

String Concatenation

Combine strings with + or use string templates (preferred).

Concatenation vs Templates

kotlin
fun main() {
    val first = "Kotlin"
    val second = "Guide"

    // Concatenation with +
    println(first + second)

    // String template (preferred)
    println("$first$second")
    println("Welcome to $first$second.com")
}

Output

KotlinGuide KotlinGuide Welcome to KotlinGuide.com

+ concatenates strings. String templates are more readable and preferred in Kotlin style.

Practice Exercise

Exercisepredict output

What does this print? val s = " Kotlin " println(s.trim().length)

Quick Quiz

What is the difference between isEmpty() and isBlank()?

Kotlin Strings — Declaration, Access, and Common Operations

What You Will Learn

Declaring Strings

Basic String Declaration

Accessing Characters

Character Access

Common String Functions

Common String Operations

Checking Empty and Blank Strings

isEmpty vs isBlank

String Concatenation

Concatenation vs Templates

Practice Exercise

Quick Quiz

Frequently Asked Questions

Related Tutorials