String.length / String.isEmpty()

// String length
val len: Int = string.length

// Empty checks
val empty: Boolean = string.isEmpty()      // true when length is 0
val blank: Boolean = string.isBlank()      // true when empty or whitespace only
val notEmpty: Boolean = string.isNotEmpty()
val notBlank: Boolean = string.isNotBlank()

fun main() {
    val s1 = "Hello, Kotlin!"
    val s2 = ""
    val s3 = "   "

    // Get the character count using length.
    println(s1.length)        // 14
    println(s2.length)        // 0
    println(s3.length)        // 3

    // isEmpty / isNotEmpty
    println(s1.isEmpty())     // false
    println(s2.isEmpty())     // true
    println(s1.isNotEmpty())  // true

    // isBlank / isNotBlank (whitespace-only strings are also considered blank)
    println(s2.isBlank())     // true
    println(s3.isBlank())     // true (spaces only)
    println(s1.isBlank())     // false

    // Count characters matching a condition using count.
    val vowelCount = s1.count { it in "aeiouAEIOU" }
    println(vowelCount)       // 3 (e, o, i)

    // Example: checking user input for blank
    val input = "  "
    if (input.isBlank()) {
        println("Input is empty")  // Input is empty
    }
}

length is a property, so you do not need parentheses to access it. Note that this differs from Java's length() method.

isEmpty() and isBlank() behave differently: for a string containing only spaces or tabs, isEmpty() returns false while isBlank() returns true. For form input validation, it is generally recommended to use isBlank().

For extracting substrings, see string.substring() / drop() / take(). For splitting and joining strings, see string.split() / joinToString().