String.toInt() / toDouble()

// Converts a string to an integer. Throws NumberFormatException if conversion fails.
string.toInt()
string.toInt(radix: Int)  // Converts using the specified radix

// Safe version that returns null if conversion fails.
string.toIntOrNull()

// Converts a number to a string.
number.toString()
number.toString(radix: Int)  // Converts to a string using the specified radix

fun main() {
    // Convert a string to an integer.
    val numStr = "42"
    val num = numStr.toInt()
    println(num + 8) // 50

    // Returns null if conversion fails.
    val invalid = "abc"
    val result = invalid.toIntOrNull()
    println(result) // null

    // Use the Elvis operator to provide a default value.
    val value = "xyz".toIntOrNull() ?: 0
    println(value) // 0

    // Convert to a floating-point number.
    val pi = "3.14159"
    println(pi.toDouble()) // 3.14159

    // Convert a hexadecimal string to an integer.
    val hex = "ff"
    println(hex.toInt(radix = 16)) // 255

    // Convert a number to a string.
    val n = 255
    println(n.toString())          // 255
    println(n.toString(radix = 2)) // 11111111 (binary)
    println(n.toString(radix = 16)) // ff (hexadecimal)

    // Numbers are also converted automatically inside string templates.
    val score = 98
    println("Your score is ${score} points.")
}

Conversion functions such as toInt() throw a NumberFormatException if the string is not in the expected format. When handling user input or external data, always use the safe variants such as toIntOrNull() and check for null.

toString() is a method available on every Kotlin object. When called on a number, you can specify a radix (2–36) to convert it to binary, octal, hexadecimal, or any other base. Inside a string template (${}), toString() is called automatically.