Strings and string templates

String immutability and characters

The operations trim, replace, and uppercase return a new string. Calling them without assigning or using the result does not change the original variable. You can iterate over a String with for, read a character by index, and compare contents with ==. For case-insensitive comparison, use equals(other, ignoreCase = true).

On the JVM, length counts UTF-16 code units. Some characters outside the Basic Multilingual Plane are represented by a pair of Char values; a combined letter may consist of several code points. Thus, rearranging Char values is not always a correct way to rearrange visible characters. Explicitly define the alphabet for teaching algorithms.

Char.isDigit() recognizes more digits than just ASCII 0..9. If a protocol format allows only decimal ASCII digits, check character in '0'..'9'. isLetter is useful for a broad notion of a letter but does not prove membership in a particular alphabet. uppercaseChar returns one Char and does not cover all transformations that may require several characters.

Templates and multiline literals

$name inserts a simple variable, and ${expression} inserts an expression's value. To place a name next to adjacent letters, braces are required; otherwise, the compiler may read a longer name. A nullable value becomes the text null; if you need another label, use the Elvis operator in the template.

An ordinary string supports the escape sequences \n, \t, \", and \\. A multiline literal in triple quotes preserves line breaks and does not process these escape sequences. trimIndent removes common indentation, while trimMargin removes the part of each line up to a specified marker, which defaults to a vertical bar.

Kotlin 2.4.20 supports multi-dollar interpolation. A $$ prefix before a literal means that two consecutive dollar signs start interpolation; a single dollar sign remains text. This is useful for JSON schemas, command templates, and text with prices.

fun main() {
    val name = "Ada"
    val text = $$"""Cost $10; user=$$name"""
    println(text)
    val missing: String? = null
    println("User: ${missing ?: "anonymous"}")
}

Cost $10; user=Ada
User: anonymous

Searching, slicing, and formatting

substring(start, end) uses a half-open interval: the start is included, and the end is excluded. The difference between the indices equals the result's length in code units. Invalid bounds cause an exception. After indexOf, check for -1 instead of immediately passing the result as a slice index.

split returns parts based on a delimiter; this does not yet prove that the format is valid. For a name;score record, check both the number of parts and each field. trim removes leading and trailing whitespace but does not normalize all internal whitespace. replace can mean literal replacement or Regex replacement, depending on the argument type.

padStart and padEnd pad to a minimum length but do not truncate a long string. repeat requires a nonnegative repetition count. For a table, separately define what happens to an excessively long value: reject, wrap, or shorten it.

JVM formatting with String.format(Locale.ROOT, "%.2f", value) specifies a reproducible decimal point. Without an explicit locale, the format may depend on the environment. Human-readable formats and machine data exchange formats may have different requirements.

StringBuilder and incremental text construction

Repeated result += piece in a loop may create many intermediate strings. StringBuilder accumulates fragments in a mutable buffer, and toString() produces the result. It also provides insertion, replacement, and deletion operations by index.

buildString { append(...); appendLine(...) } is a concise way to use a local builder. The braces define a block that works with it; lambdas with receivers will be studied in detail in Topic 11. For these examples, it is enough to understand that append adds a fragment to the resulting string.