Working with Strings

Strings come with many useful functions and properties.

Example

fun main() {
    val text = "  Hello, Kotlin World  "
    val trimmed = text.trim()

    println(trimmed)
    println(trimmed.uppercase())
    println(trimmed.lowercase())
    println(trimmed.length)
    println(trimmed.startsWith("Hello"))
    println(trimmed.endsWith("!"))
    println("Kotlin" in trimmed)
    println(trimmed.replace("World", "Friends"))
    println("a,b,c".split(","))
    println("Kotlin".reversed())
    println("Kotlin"[0])
    println("Kotlin".substring(1, 4))
    println("".isEmpty())
    println("   ".isBlank())
    println("ruby".replaceFirstChar { it.uppercase() })
}

Expected output

Hello, Kotlin World
HELLO, KOTLIN WORLD
hello, kotlin world
19
true
false
true
Hello, Kotlin Friends
[a, b, c]
niltoK
K
otl
true
true
Ruby

Things to notice:

• Strings are immutable: functions like uppercase() return a new string and leave the original unchanged.
• "Kotlin" in trimmed checks whether one string contains another.
• Square brackets get a character by position, starting at 0.
• substring(1, 4) takes positions 1 up to, but not including, 4.
• isEmpty() is true only for ""; isBlank() is also true for strings of only whitespace — usually what you want for user input.

Looping over characters

fun main() {
    for (c in "Hi!") {
        println(c)
    }
    println("banana".count { it == 'a' })
}

H
i
!
3

Common mistakes

• Using == vs ===. In Kotlin, == compares string content, which is what you want. (=== checks whether two references point to the same object.)
• Reading a position past the end: "Hi"[5] throws StringIndexOutOfBoundsException.

Practice

Given val email = " Ada@Example.COM ", print it trimmed and in lowercase.

fun main() {
    val email = "  Ada@Example.COM "
    println(email.trim().lowercase())
}

ada@example.com