# 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
  - Comparable<String>
  - Pattern

## Constructors

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

## Properties

codeUnits → List<int>
  Returns an unmodifiable list of the UTF-16 code units of this string.
  read-only
hashCode → int
  Returns a hash code derived from the code units of the string. [...]
  read-only, override
isEmpty → bool
  Returns true if this string is empty.
  read-only
isNotEmpty → bool
  Returns true if this string is not empty.
  read-only
length → int
  The length of the string. [...]
  read-only
runes → Runes
  Returns 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

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
  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>
  Match this pattern against the string repeatedly. [...]
  inherited
matchAsPrefix(String string, [ int start = 0 ]) → Match
  Match this pattern against the start of `string`. [...]
  inherited
noSuchMethod(Invocation invocation) → dynamic
  Invoked when a non-existent method or property is accessed. [...]
  inherited
toString() → String
  Returns a string representation of this object.
  inherited

## 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
  Returns true if other is a `String` with the same sequence of code units. [...]
  override
operator [](int index) → String
  Gets the character (as a single-code-unit String) at the given `index`. [...]