String class Null safety

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 which creates a new string:

const string = 'Dart is fun';
print(string.substring(0, 4)); // 'Dart'

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

const string = 'Dart ' + 'is ' + 'fun!';
print(string); // 'Dart is fun!'

Adjacent string literals are concatenated automatically:

const string = 'Dart ' 'is ' 'fun!';
print(string); // 'Dart is fun!'

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

const string = 'dartlang';
print('$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:

const string = 'Dart';
final firstCodeUnit = string.codeUnitAt(0);
print(firstCodeUnit); // 68, aka U+0044, the code point for 'D'.
final allCodeUnits = string.codeUnits;
print(allCodeUnits); // [68, 97, 114, 116]

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

const string = 'Dart';
final charAtIndex = string[0];
print(charAtIndex); // '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, Dart uses the name 'rune' for an integer representing a Unicode code point. Use the runes property to get the runes of a string:

const string = 'Dart';
final runes = string.runes.toList();
print(runes); // [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:

const clef = '\u{1D11E}';
for (final item in clef.codeUnits) {
  print(item.toRadixString(16));
  // d834
  // dd1e
}
for (final item in clef.runes) {
  print(item.toRadixString(16)); // 1d11e
}

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

Other resources

Implemented types
Available Extensions

Constructors

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

Properties

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

Methods

allMatches(String string, [int start = 0]) Iterable<Match>
Matches this pattern against the string repeatedly.
inherited
codeUnitAt(int index) int
Returns the 16-bit UTF-16 code unit at the given index.
compareTo(String other) int
Compares this string to other.
override
contains(Pattern other, [int startIndex = 0]) bool
Whether this string contains a match of other.
endsWith(String other) bool
Whether this string ends with other.
indexOf(Pattern pattern, [int start = 0]) int
Returns the position of the first match of pattern in this string, starting at start, inclusive:
lastIndexOf(Pattern pattern, [int? start]) int
The starting position of the last match pattern in this string.
matchAsPrefix(String string, [int start = 0]) Match?
Matches this pattern against the start of string.
inherited
noSuchMethod(Invocation invocation) → dynamic
Invoked when a non-existent method or property is accessed.
inherited
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 computed string.
replaceFirst(Pattern from, String to, [int startIndex = 0]) String
Creates a new string with the first occurrence of from replaced by to.
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)?, String onNonMatch(String)?}) String
Splits the string, converts its parts, and combines them into a new string.
startsWith(Pattern pattern, [int index = 0]) bool
Whether this string starts with a match of pattern.
substring(int start, [int? end]) String
The substring of this string from start, inclusive, to end, exclusive.
toLowerCase() String
Converts all characters in this string to lower case.
toString() String
A string representation of this object.
inherited
toUpperCase() String
Converts all characters in this string to upper case.
trim() String
The string without any leading and trailing whitespace.
trimLeft() String
The string without any leading whitespace.
trimRight() String
The string without any trailing whitespace.

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
Whether other is a String with the same sequence of code units.
override
operator [](int index) String
The character (as a single-code-unit String) at the given index.