Strings

Strings in Kotlin are represented by the type String. Generally, a string value is a sequence of characters in double quotes ("):

val str = "abcd 123"

Elements of a string are characters that you can access via the indexing operation: s[i]. You can iterate over these characters with a for loop:

fun main() {
    val str = "abcd" 
//sampleStart
    for (c in str) {
        println(c)
    }
//sampleEnd
}

Strings are immutable. Once you initialize a string, you can't change its value or assign a new value to it. All operations that transform strings return their results in a new String object, leaving the original string unchanged:

fun main() {
//sampleStart
    val str = "abcd"
   
    // Creates and prints a new String object
    println(str.uppercase())
    // ABCD
   
    // The original string remains the same
    println(str) 
    // abcd
//sampleEnd
}

To concatenate strings, use the + operator. This also works for concatenating strings with values of other types, as long as the first element in the expression is a string:

fun main() {
//sampleStart
    val s = "abc" + 1
    println(s + "def")
    // abc1def    
//sampleEnd
}

String literals

Kotlin has two types of string literals:

Escaped strings
Multiline strings

Escaped strings

Escaped strings can contain escaped characters.
Here's an example of an escaped string:

val s = "Hello, world!\n"

Escaping is done in the conventional way, with a backslash (\).
See Characters page for the list of supported escape sequences.

Multiline strings

Multiline strings can contain newlines and arbitrary text. It is delimited by a triple quote ("""), contains no escaping and can contain newlines and any other characters:

val text = """
    for (c in "foo")
        print(c)
"""

To remove leading whitespace from multiline strings, use the trimMargin() function:

val text = """
    |Tell me and I forget.
    |Teach me and I remember.
    |Involve me and I learn.
    |(Benjamin Franklin)
    """.trimMargin()

By default, a pipe symbol | is used as margin prefix, but you can choose another character and pass it as a parameter, like trimMargin(">").

String templates

String literals may contain template expressions – pieces of code that are evaluated and whose results are concatenated into the string. A template expression starts with a dollar sign ($) and consists of either a name:

fun main() {
//sampleStart
    val i = 10
    println("i = $i") 
    // i = 10
//sampleEnd
}

or an expression in curly braces:

fun main() {
//sampleStart
    val s = "abc"
    println("$s.length is ${s.length}") 
    // abc.length is 3
//sampleEnd
}

You can use templates both in multiline and escaped strings. To insert the dollar sign $ in a multiline string (which doesn't support backslash escaping) before any symbol, which is allowed as a beginning of an identifier, use the following syntax:

val price = """
${'$'}_9.99
"""

String formatting

To format a string to your specific requirements, use the String.format() function.

The String.format() function accepts a format string and one or more arguments. The format string contains one placeholder (%) for each remaining argument, followed by format specifiers. Format specifiers are formatting instructions for the respective argument. In the output, each argument fills its corresponding placeholder in the defined format:

fun main() { 
//sampleStart
    // Formats to add zeroes and make a length of seven
    val integerNumber = String.format("%07d", 31416)
    println(integerNumber)
    // 0031416

    // Formats with four decimals and sign
    val floatNumber = String.format("%+.4f", 3.141592)
    println(floatNumber)
    // +3.1416

    // Formats with uppercase for two placeholders
    val helloString = String.format("%S %S", "hello", "world")
    println(helloString)
    // HELLO WORLD
//sampleEnd    
}

The String.format() function provides similar functionality to string templates. However, the String.format() function is more versatile because there are more formatting options available.

In addition, you can assign the format string from a variable. This can be useful when the format string changes, for example, in localization cases that depend on the user locale.

Be careful when using the String.format() function because it can be easy to mismatch the number or position of the arguments with their corresponding placeholders.

Last modified: 21 November 2023