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String class

A sequence of UTF-16 code units.

Strings are mainly used to represent text. A character may be represented by multiple code points, each code point consisting of one or two code units. For example the Papua New Guinea flag character requires four code units to represent two code points, but should be treated like a single character: "🇵🇬". Platforms that do not support the flag character may show the letters "PG" instead. If the code points are swapped, it instead becomes the Guadeloupe flag "🇬🇵" ("GP").

A string can be either single or multiline. Single line strings are written using matching single or double quotes, and multiline strings are written using triple quotes. The following are all valid Dart strings:

'Single quotes';
"Double quotes";
'Double quotes in "single" quotes';
"Single quotes in 'double' quotes";

'''A
multiline
string''';

"""
Another
multiline
string""";

Strings are immutable. Although you cannot change a string, you can perform an operation on a string and assign the result to a new string:

var string = 'Dart is fun';
var newString = string.substring(0, 5);

You can use the plus (+) operator to concatenate strings:

'Dart ' + 'is ' + 'fun!'; // 'Dart is fun!'

You can also use adjacent string literals for concatenation:

'Dart ' 'is ' 'fun!';    // 'Dart is fun!'

You can use ${} to interpolate the value of Dart expressions within strings. The curly braces can be omitted when evaluating identifiers:

string = 'dartlang';
'$string has ${string.length} letters'; // 'dartlang has 8 letters'

A string is represented by a sequence of Unicode UTF-16 code units accessible through the codeUnitAt or the codeUnits members:

string = 'Dart';
string.codeUnitAt(0); // 68
string.codeUnits;     // [68, 97, 114, 116]

The string representation of code units is accessible through the index operator:

string[0];            // 'D'

The characters of a string are encoded in UTF-16. Decoding UTF-16, which combines surrogate pairs, yields Unicode code points. Following a similar terminology to Go, we use the name 'rune' for an integer representing a Unicode code point. Use the runes property to get the runes of a string:

string.runes.toList(); // [68, 97, 114, 116]

For a character outside the Basic Multilingual Plane (plane 0) that is composed of a surrogate pair, runes combines the pair and returns a single integer. For example, the Unicode character for a musical G-clef ('𝄞') with rune value 0x1D11E consists of a UTF-16 surrogate pair: 0xD834 and 0xDD1E. Using codeUnits returns the surrogate pair, and using runes returns their combined value:

var clef = '\u{1D11E}';
clef.codeUnits;         // [0xD834, 0xDD1E]
clef.runes.toList();    // [0x1D11E]

The String class can not be extended or implemented. Attempting to do so yields a compile-time error.

Other resources

See StringBuffer to efficiently build a string incrementally. See RegExp to work with regular expressions.

Also see:

Dart Cookbook for String examples and recipes.
Dart Up and Running

Implemented types

Constructors

String.fromCharCode(int charCode) factory: Allocates a new String for the specified charCode. [...]
String.fromCharCodes(Iterable<int> charCodes, [ int start = 0, int end ]) factory: Allocates a new String for the specified charCodes. [...]
String.fromEnvironment(String name, { String defaultValue }) const factory: Returns the string value of the environment declaration name. [...]

Properties

codeUnits → List<int> read-only: Returns an unmodifiable list of the UTF-16 code units of this string.
hashCode → int read-only, override: Returns a hash code derived from the code units of the string. [...]
isEmpty → bool read-only: Returns true if this string is empty.
isNotEmpty → bool read-only: Returns true if this string is not empty.
length → int read-only: The length of the string. [...]
runes → Runes read-only: Returns an Iterable of Unicode code-points of this string. [...]
runtimeType → Type read-only, inherited: A representation of the runtime type of the object.

Methods

codeUnitAt(int index) → int: Returns the 16-bit UTF-16 code unit at the given index.
compareTo(String other) → int override: Compares this string to other. [...]
contains(Pattern other, [ int startIndex = 0 ]) → bool: Returns true if this string contains a match of other: [...]
endsWith(String other) → bool: Returns true if this string ends with other. For example: [...]
indexOf(Pattern pattern, [ int start ]) → int: Returns the position of the first match of pattern in this string, starting at start, inclusive: [...]
lastIndexOf(Pattern pattern, [ int start ]) → int: Returns the position of the last match pattern in this string, searching backward starting at start, inclusive: [...]
padLeft(int width, [ String padding = ' ' ]) → String: Pads this string on the left if it is shorter than width. [...]
padRight(int width, [ String padding = ' ' ]) → String: Pads this string on the right if it is shorter than width. [...]
replaceAll(Pattern from, String replace) → String: Replaces all substrings that match from with replace. [...]
replaceAllMapped(Pattern from, String replace(Match match)) → String: Replace all substrings that match from by a string computed from the match. [...]
replaceFirst(Pattern from, String to, [ int startIndex = 0 ]) → String: Returns a new string in which the first occurrence of from in this string is replaced with to, starting from startIndex: [...]
replaceFirstMapped(Pattern from, String replace(Match match), [ int startIndex = 0 ]) → String: Replace the first occurrence of from in this string. [...]
replaceRange(int start, int end, String replacement) → String: Replaces the substring from start to end with replacement. [...]
split(Pattern pattern) → List<String>: Splits the string at matches of pattern and returns a list of substrings. [...]
splitMapJoin(Pattern pattern, { String onMatch(Match match), String onNonMatch(String nonMatch) }) → String: Splits the string, converts its parts, and combines them into a new string. [...]
startsWith(Pattern pattern, [ int index = 0 ]) → bool: Returns true if this string starts with a match of pattern. [...]
substring(int startIndex, [ int endIndex ]) → String: Returns the substring of this string that extends from startIndex, inclusive, to endIndex, exclusive. [...]
toLowerCase() → String: Converts all characters in this string to lower case. If the string is already in all lower case, this method returns this. [...]
toUpperCase() → String: Converts all characters in this string to upper case. If the string is already in all upper case, this method returns this. [...]
trim() → String: Returns the string without any leading and trailing whitespace. [...]
trimLeft() → String: Returns the string without any leading whitespace. [...]
trimRight() → String: Returns the string without any trailing whitespace. [...]
allMatches(String string, [ int start = 0 ]) → Iterable<Match> inherited: Match this pattern against the string repeatedly. [...]
matchAsPrefix(String string, [ int start = 0 ]) → Match inherited: Match this pattern against the start of string. [...]
noSuchMethod(Invocation invocation) → dynamic inherited: Invoked when a non-existent method or property is accessed. [...]
toString() → String inherited: Returns a string representation of this object.

Operators

operator *(int times) → String: Creates a new string by concatenating this string with itself a number of times. [...]
operator +(String other) → String: Creates a new string by concatenating this string with other. [...]
operator ==(Object other) → bool override: Returns true if other is a String with the same sequence of code units. [...]
operator [](int index) → String: Gets the character (as a single-code-unit String) at the given index. [...]

© 2012 the Dart project authors
Licensed under the Creative Commons Attribution-ShareAlike License v4.0.
https://api.dart.dev/stable/2.5.0/dart-core/String-class.html