Queue<E> abstract interface

abstract interface class Queue<E> implements Iterable<E>

A Queue is a collection that can be manipulated at both ends. One can iterate over the elements of a queue through forEach or with an Iterator.

It is generally not allowed to modify the queue (add or remove entries) while an operation in the queue is being performed, for example during a call to forEach. Modifying the queue while it is being iterated will most likely break the iteration. This goes both for using the iterator directly, or for iterating an Iterable returned by a method like map or where.

Example:

dart

final queue = Queue<int>(); // ListQueue() by default
print(queue.runtimeType); // ListQueue

// Adding items to queue
queue.addAll([1, 2, 3]);
queue.addFirst(0);
queue.addLast(10);
print(queue); // {0, 1, 2, 3, 10}

// Removing items from queue
queue.removeFirst();
queue.removeLast();
print(queue); // {1, 2, 3}

Implemented types

Iterable<E>

Implementers

Available Extensions

Constructors

Queue() factory

factory Queue()

Creates a queue.

Implementation

dart

factory Queue() = ListQueue<E>;

Queue.from() factory

factory Queue.from(Iterable<dynamic> elements)

Creates a queue containing all elements.

The element order in the queue is as if the elements were added using addLast in the order provided by elements.iterator.

All the elements should be instances of E. The elements iterable itself may have any element type, so this constructor can be used to down-cast a Queue, for example as:

dart

Queue<SuperType> superQueue = ...;
Queue<SubType> subQueue =
    Queue<SubType>.from(superQueue.whereType<SubType>());

Implementation

dart

factory Queue.from(Iterable elements) = ListQueue<E>.from;

Queue.of() factory

factory Queue.of(Iterable<E> elements)

Creates a queue from elements.

The element order in the queue is as if the elements were added using addLast in the order provided by elements.iterator.

Implementation

dart

factory Queue.of(Iterable<E> elements) = ListQueue<E>.of;

Properties

first no setter inherited

E get first

The first element.

Throws a StateError if this is empty. Otherwise returns the first element in the iteration order, equivalent to this.elementAt(0).

Inherited from Iterable.

Implementation

dart

E get first {
  Iterator<E> it = iterator;
  if (!it.moveNext()) {
    throw IterableElementError.noElement();
  }
  return it.current;
}

firstOrNull extension no setter

T? get firstOrNull

The first element of this iterator, or null if the iterable is empty.

Available on Iterable<E>, provided by the IterableExtensions<T> extension

Implementation

dart

T? get firstOrNull {
  var iterator = this.iterator;
  if (iterator.moveNext()) return iterator.current;
  return null;
}

hashCode no setter inherited

int get hashCode

The hash code for this object.

A hash code is a single integer which represents the state of the object that affects operator == comparisons.

All objects have hash codes. The default hash code implemented by Object represents only the identity of the object, the same way as the default operator == implementation only considers objects equal if they are identical (see identityHashCode).

If operator == is overridden to use the object state instead, the hash code must also be changed to represent that state, otherwise the object cannot be used in hash based data structures like the default Set and Map implementations.

Hash codes must be the same for objects that are equal to each other according to operator ==. The hash code of an object should only change if the object changes in a way that affects equality. There are no further requirements for the hash codes. They need not be consistent between executions of the same program and there are no distribution guarantees.

Objects that are not equal are allowed to have the same hash code. It is even technically allowed that all instances have the same hash code, but if clashes happen too often, it may reduce the efficiency of hash-based data structures like HashSet or HashMap.

If a subclass overrides hashCode, it should override the operator == operator as well to maintain consistency.

Inherited from Object.

Implementation

dart

external int get hashCode;

indexed extension no setter

Iterable<Record> get indexed

Pairs of elements of the indices and elements of this iterable.

The elements are (0, this.first) through (this.length - 1, this.last), in index/iteration order.

Available on Iterable<E>, provided by the IterableExtensions<T> extension

Implementation

dart

@pragma('vm:prefer-inline')
Iterable<(int, T)> get indexed => IndexedIterable<T>(this, 0);

isEmpty no setter inherited

bool get isEmpty

Whether this collection has no elements.

May be computed by checking if iterator.moveNext() returns false.

Example:

dart

final emptyList = <int>[];
print(emptyList.isEmpty); // true;
print(emptyList.iterator.moveNext()); // false

Inherited from Iterable.

Implementation

dart

bool get isEmpty => !iterator.moveNext();

isNotEmpty no setter inherited

bool get isNotEmpty

Whether this collection has at least one element.

May be computed by checking if iterator.moveNext() returns true.

Example:

dart

final numbers = <int>{1, 2, 3};
print(numbers.isNotEmpty); // true;
print(numbers.iterator.moveNext()); // true

Inherited from Iterable.

Implementation

dart

bool get isNotEmpty => !isEmpty;

iterator no setter inherited

Iterator<E> get iterator

A new Iterator that allows iterating the elements of this Iterable.

Iterable classes may specify the iteration order of their elements (for example List always iterate in index order), or they may leave it unspecified (for example a hash-based Set may iterate in any order).

Each time iterator is read, it returns a new iterator, which can be used to iterate through all the elements again. The iterators of the same iterable can be stepped through independently, but should return the same elements in the same order, as long as the underlying collection isn't changed.

Modifying the collection may cause new iterators to produce different elements, and may change the order of existing elements. A List specifies its iteration order precisely, so modifying the list changes the iteration order predictably. A hash-based Set may change its iteration order completely when adding a new element to the set.

Modifying the underlying collection after creating the new iterator may cause an error the next time Iterator.moveNext is called on that iterator. Any modifiable iterable class should specify which operations will break iteration.

Inherited from Iterable.

Implementation

dart

Iterator<E> get iterator;

last no setter inherited

E get last

The last element.

Throws a StateError if this is empty. Otherwise may iterate through the elements and returns the last one seen. Some iterables may have more efficient ways to find the last element (for example a list can directly access the last element, without iterating through the previous ones).

Inherited from Iterable.

Implementation

dart

E get last {
  Iterator<E> it = iterator;
  if (!it.moveNext()) {
    throw IterableElementError.noElement();
  }
  E result;
  do {
    result = it.current;
  } while (it.moveNext());
  return result;
}

lastOrNull extension no setter

T? get lastOrNull

The last element of this iterable, or null if the iterable is empty.

This computation may not be efficient. The last value is potentially found by iterating the entire iterable and temporarily storing every value. The process only iterates the iterable once. If iterating more than once is not a problem, it may be more efficient for some iterables to do:

dart

var lastOrNull = iterable.isEmpty ? null : iterable.last;

Available on Iterable<E>, provided by the IterableExtensions<T> extension

Implementation

dart

T? get lastOrNull {
  if (this is EfficientLengthIterable) {
    if (isEmpty) return null;
    return last;
  }
  var iterator = this.iterator;
  if (!iterator.moveNext()) return null;
  T result;
  do {
    result = iterator.current;
  } while (iterator.moveNext());
  return result;
}

length no setter inherited

int get length

The number of elements in this Iterable.

Counting all elements may involve iterating through all elements and can therefore be slow. Some iterables have a more efficient way to find the number of elements. These must override the default implementation of length.

Inherited from Iterable.

Implementation

dart

int get length {
  assert(this is! EfficientLengthIterable);
  int count = 0;
  Iterator<Object?> it = iterator;
  while (it.moveNext()) {
    count++;
  }
  return count;
}

nonNulls extension no setter

Iterable<T> get nonNulls

The non-null elements of this iterable.

The same elements as this iterable, except that null values are omitted.

Available on Iterable<E>, provided by the NullableIterableExtensions<T extends Object> extension

Implementation

dart

Iterable<T> get nonNulls => NonNullsIterable<T>(this);

runtimeType no setter inherited

Type get runtimeType

A representation of the runtime type of the object.

Inherited from Object.

Implementation

dart

external Type get runtimeType;

single no setter inherited

E get single

Checks that this iterable has only one element, and returns that element.

Throws a StateError if this is empty or has more than one element. This operation will not iterate past the second element.

Inherited from Iterable.

Implementation

dart

E get single {
  Iterator<E> it = iterator;
  if (!it.moveNext()) throw IterableElementError.noElement();
  E result = it.current;
  if (it.moveNext()) throw IterableElementError.tooMany();
  return result;
}

singleOrNull extension no setter

T? get singleOrNull

The single element of this iterator, or null.

If the iterator has precisely one element, this is that element. Otherwise, if the iterator has zero elements, or it has two or more, the value is null.

Available on Iterable<E>, provided by the IterableExtensions<T> extension

Implementation

dart

T? get singleOrNull {
  var iterator = this.iterator;
  if (iterator.moveNext()) {
    var result = iterator.current;
    if (!iterator.moveNext()) return result;
  }
  return null;
}

wait extension no setter

Future<List<T>> get wait

Waits for futures in parallel.

Waits for all the futures in this iterable. Returns a list of the resulting values, in the same order as the futures which created them, if all futures are successful.

Similar to Future.wait, but reports errors using a ParallelWaitError, which allows the caller to handle errors and dispose successful results if necessary.

The returned future is completed when all the futures have completed. If any of the futures do not complete, nor does the returned future.

If any future completes with an error, the returned future completes with a ParallelWaitError. The ParallelWaitError.values is a list of the values for successful futures and null for futures with errors. The ParallelWaitError.errors is a list of the same length, with null values for the successful futures and an AsyncError with the error for futures which completed with an error.

Available on Iterable<E>, provided by the FutureIterable<T> extension

Implementation

dart

Future<List<T>> get wait {
  var results = [for (var f in this) _FutureResult<T>(f)];
  if (results.isEmpty) return Future<List<T>>.value(<T>[]);

  @pragma('vm:awaiter-link')
  final c = Completer<List<T>>.sync();

  _FutureResult._waitAll(results, (errors) {
    if (errors == 0) {
      c.complete([for (var r in results) r.value]);
    } else {
      var errorList = [for (var r in results) r.errorOrNull];
      c.completeError(
        ParallelWaitError<List<T?>, List<AsyncError?>>(
          [for (var r in results) r.valueOrNull],
          errorList,
          errorCount: errors,
          defaultError: errorList.firstWhere(_notNull),
        ),
      );
    }
  });
  return c.future;
}

Methods

add()

void add(E value)

Adds value at the end of the queue.

Implementation

dart

void add(E value);

addAll()

void addAll(Iterable<E> iterable)

Adds all elements of iterable at the end of the queue. The length of the queue is extended by the length of iterable.

Implementation

dart

void addAll(Iterable<E> iterable);

addFirst()

void addFirst(E value)

Adds value at the beginning of the queue.

Implementation

dart

void addFirst(E value);

addLast()

void addLast(E value)

Adds value at the end of the queue.

Implementation

dart

void addLast(E value);

any() inherited

bool any(bool Function(E element) test)

Checks whether any element of this iterable satisfies test.

Checks every element in iteration order, and returns true if any of them make test return true, otherwise returns false. Returns false if the iterable is empty.

Example:

dart

final numbers = <int>[1, 2, 3, 5, 6, 7];
var result = numbers.any((element) => element >= 5); // true;
result = numbers.any((element) => element >= 10); // false;

Inherited from Iterable.

Implementation

dart

bool any(bool test(E element)) {
  for (E element in this) {
    if (test(element)) return true;
  }
  return false;
}

asNameMap() extension

Map<String, T> asNameMap()

Creates a map from the names of enum values to the values.

The collection that this method is called on is expected to have enums with distinct names, like the values list of an enum class. Only one value for each name can occur in the created map, so if two or more enum values have the same name (either being the same value, or being values of different enum type), at most one of them will be represented in the returned map.

Available on Iterable<E>, provided by the EnumByName<T extends Enum> extension

Implementation

dart

Map<String, T> asNameMap() => <String, T>{
  for (var value in this) value._name: value,
};

byName() extension

T byName(String name)

Finds the enum value in this list with name name.

Goes through this collection looking for an enum with name name, as reported by EnumName.name. Returns the first value with the given name. Such a value must be found.

Available on Iterable<E>, provided by the EnumByName<T extends Enum> extension

Implementation

dart

T byName(String name) {
  for (var value in this) {
    if (value._name == name) return value;
  }
  throw ArgumentError.value(name, "name", "No enum value with that name");
}

cast() override

Queue<R> cast<R>()

Provides a view of this queue as a queue of R instances, if necessary.

If this queue contains only instances of R, all read operations will work correctly. If any operation tries to access an element that is not an instance of R, the access will throw instead.

Elements added to the queue (e.g., by using addFirst or addAll) must be instances of R to be valid arguments to the adding function, and they must also be instances of E to be accepted by this queue as well.

Methods which accept Object? as argument, like contains and remove, will pass the argument directly to the this queue's method without any checks. That means that you can do queueOfStrings.cast<int>().remove("a") successfully, even if it looks like it shouldn't have any effect.

Implementation

dart

Queue<R> cast<R>();

clear()

void clear()

Removes all elements in the queue. The size of the queue becomes zero.

Implementation

dart

void clear();

contains() inherited

bool contains(Object? element)

Whether the collection contains an element equal to element.

This operation will check each element in order for being equal to element, unless it has a more efficient way to find an element equal to element. Stops iterating on the first equal element.

The equality used to determine whether element is equal to an element of the iterable defaults to the Object.== of the element.

Some types of iterable may have a different equality used for its elements. For example, a Set may have a custom equality (see Set.identity) that its contains uses. Likewise the Iterable returned by a Map.keys call should use the same equality that the Map uses for keys.

Example:

dart

final gasPlanets = <int, String>{1: 'Jupiter', 2: 'Saturn'};
final containsOne = gasPlanets.keys.contains(1); // true
final containsFive = gasPlanets.keys.contains(5); // false
final containsJupiter = gasPlanets.values.contains('Jupiter'); // true
final containsMercury = gasPlanets.values.contains('Mercury'); // false

Inherited from Iterable.

Implementation

dart

bool contains(Object? element) {
  for (E e in this) {
    if (e == element) return true;
  }
  return false;
}

elementAt() inherited

E elementAt(int index)

Returns the indexth element.

The index must be non-negative and less than length. Index zero represents the first element (so iterable.elementAt(0) is equivalent to iterable.first).

May iterate through the elements in iteration order, ignoring the first index elements and then returning the next. Some iterables may have a more efficient way to find the element.

Example:

dart

final numbers = <int>[1, 2, 3, 5, 6, 7];
final elementAt = numbers.elementAt(4); // 6

Inherited from Iterable.

Implementation

dart

E elementAt(int index) {
  RangeError.checkNotNegative(index, "index");
  var iterator = this.iterator;
  var skipCount = index;
  while (iterator.moveNext()) {
    if (skipCount == 0) return iterator.current;
    skipCount--;
  }
  throw IndexError.withLength(
    index,
    index - skipCount,
    indexable: this,
    name: "index",
  );
}

elementAtOrNull() extension

T? elementAtOrNull(int index)

The element at position index of this iterable, or null.

The index is zero based, and must be non-negative.

Returns the result of elementAt(index) if the iterable has at least index + 1 elements, and null otherwise.

Available on Iterable<E>, provided by the IterableExtensions<T> extension

Implementation

dart

T? elementAtOrNull(int index) {
  RangeError.checkNotNegative(index, "index");
  if (this is EfficientLengthIterable) {
    if (index >= length) return null;
    return elementAt(index);
  }
  var iterator = this.iterator;
  do {
    if (!iterator.moveNext()) return null;
  } while (--index >= 0);
  return iterator.current;
}

every() inherited

bool every(bool Function(E element) test)

Checks whether every element of this iterable satisfies test.

Checks every element in iteration order, and returns false if any of them make test return false, otherwise returns true. Returns true if the iterable is empty.

Example:

dart

final planetsByMass = <double, String>{0.06: 'Mercury', 0.81: 'Venus',
  0.11: 'Mars'};
// Checks whether all keys are smaller than 1.
final every = planetsByMass.keys.every((key) => key < 1.0); // true

Inherited from Iterable.

Implementation

dart

bool every(bool test(E element)) {
  for (E element in this) {
    if (!test(element)) return false;
  }
  return true;
}

expand() inherited

Iterable<T> expand<T>(Iterable<T> Function(E element) toElements)

Expands each element of this Iterable into zero or more elements.

The resulting Iterable runs through the elements returned by toElements for each element of this, in iteration order.

The returned Iterable is lazy, and calls toElements for each element of this iterable every time the returned iterable is iterated.

Example:

dart

Iterable<int> count(int n) sync* {
  for (var i = 1; i <= n; i++) {
    yield i;
   }
 }

var numbers = [1, 3, 0, 2];
print(numbers.expand(count)); // (1, 1, 2, 3, 1, 2)

Equivalent to:

dart

Iterable<T> expand<T>(Iterable<T> toElements(E e)) sync* {
  for (var value in this) {
    yield* toElements(value);
  }
}

Inherited from Iterable.

Implementation

dart

Iterable<T> expand<T>(Iterable<T> toElements(E element)) =>
    ExpandIterable<E, T>(this, toElements);

firstWhere() inherited

E firstWhere(bool Function(E element) test, {(E Function())? orElse})

The first element that satisfies the given predicate test.

Iterates through elements and returns the first to satisfy test.

Example:

dart

final numbers = <int>[1, 2, 3, 5, 6, 7];
var result = numbers.firstWhere((element) => element < 5); // 1
result = numbers.firstWhere((element) => element > 5); // 6
result =
    numbers.firstWhere((element) => element > 10, orElse: () => -1); // -1

If no element satisfies test, the result of invoking the orElse function is returned. If orElse is omitted, it defaults to throwing a StateError. Stops iterating on the first matching element.

Inherited from Iterable.

Implementation

dart

E firstWhere(bool test(E element), {E orElse()?}) {
  for (E element in this) {
    if (test(element)) return element;
  }
  if (orElse != null) return orElse();
  throw IterableElementError.noElement();
}

fold() inherited

T fold<T>(T initialValue, T Function(T previousValue, E element) combine)

Reduces a collection to a single value by iteratively combining each element of the collection with an existing value

Uses initialValue as the initial value, then iterates through the elements and updates the value with each element using the combine function, as if by:

dart

var value = initialValue;
for (E element in this) {
  value = combine(value, element);
}
return value;

Example of calculating the sum of an iterable:

dart

final numbers = <double>[10, 2, 5, 0.5];
const initialValue = 100.0;
final result = numbers.fold<double>(
    initialValue, (previousValue, element) => previousValue + element);
print(result); // 117.5

Inherited from Iterable.

Implementation

dart

T fold<T>(T initialValue, T combine(T previousValue, E element)) {
  var value = initialValue;
  for (E element in this) value = combine(value, element);
  return value;
}

followedBy() inherited

Iterable<E> followedBy(Iterable<E> other)

Creates the lazy concatenation of this iterable and other.

The returned iterable will provide the same elements as this iterable, and, after that, the elements of other, in the same order as in the original iterables.

Example:

dart

var planets = <String>['Earth', 'Jupiter'];
var updated = planets.followedBy(['Mars', 'Venus']);
print(updated); // (Earth, Jupiter, Mars, Venus)

Inherited from Iterable.

Implementation

dart

Iterable<E> followedBy(Iterable<E> other) {
  var self = this; &#47;&#47; TODO(lrn): Remove when we can promote `this`.
  if (self is EfficientLengthIterable<E>) {
    return FollowedByIterable<E>.firstEfficient(self, other);
  }
  return FollowedByIterable<E>(this, other);
}

forEach() inherited

void forEach(void Function(E value) action)

Invokes action on each element of this iterable in iteration order.

Example:

dart

final numbers = <int>[1, 2, 6, 7];
numbers.forEach(print);
// 1
// 2
// 6
// 7

Inherited from Iterable.

Implementation

dart

void forEach(void action(E element)) {
  for (E element in this) action(element);
}

join() inherited

String join([String separator = ""])

Converts each element to a String and concatenates the strings.

Iterates through elements of this iterable, converts each one to a String by calling Object.toString, and then concatenates the strings, with the separator string interleaved between the elements.

Example:

dart

final planetsByMass = <double, String>{0.06: 'Mercury', 0.81: 'Venus',
  0.11: 'Mars'};
final joinedNames = planetsByMass.values.join('-'); // Mercury-Venus-Mars

Inherited from Iterable.

Implementation

dart

String join([String separator = ""]) {
  Iterator<E> iterator = this.iterator;
  if (!iterator.moveNext()) return "";
  var first = iterator.current.toString();
  if (!iterator.moveNext()) return first;
  var buffer = StringBuffer(first);
  &#47;&#47; TODO(51681): Drop null check when de-supporting pre-2.12 code.
  if (separator == null || separator.isEmpty) {
    do {
      buffer.write(iterator.current.toString());
    } while (iterator.moveNext());
  } else {
    do {
      buffer
        ..write(separator)
        ..write(iterator.current.toString());
    } while (iterator.moveNext());
  }
  return buffer.toString();
}

lastWhere() inherited

E lastWhere(bool Function(E element) test, {(E Function())? orElse})

The last element that satisfies the given predicate test.

An iterable that can access its elements directly may check its elements in any order (for example a list starts by checking the last element and then moves towards the start of the list). The default implementation iterates elements in iteration order, checks test(element) for each, and finally returns that last one that matched.

Example:

dart

final numbers = <int>[1, 2, 3, 5, 6, 7];
var result = numbers.lastWhere((element) => element < 5); // 3
result = numbers.lastWhere((element) => element > 5); // 7
result = numbers.lastWhere((element) => element > 10,
    orElse: () => -1); // -1

If no element satisfies test, the result of invoking the orElse function is returned. If orElse is omitted, it defaults to throwing a StateError.

Inherited from Iterable.

Implementation

dart

E lastWhere(bool test(E element), {E orElse()?}) {
  var iterator = this.iterator;
  &#47;&#47; Potential result during first loop.
  E result;
  do {
    if (!iterator.moveNext()) {
      if (orElse != null) return orElse();
      throw IterableElementError.noElement();
    }
    result = iterator.current;
  } while (!test(result));
  &#47;&#47; Now `result` is actual result, unless a later one is found.
  while (iterator.moveNext()) {
    var current = iterator.current;
    if (test(current)) result = current;
  }
  return result;
}

map() inherited

Iterable<T> map<T>(T Function(E e) toElement)

The current elements of this iterable modified by toElement.

Returns a new lazy Iterable with elements that are created by calling toElement on each element of this Iterable in iteration order.

The returned iterable is lazy, so it won't iterate the elements of this iterable until it is itself iterated, and then it will apply toElement to create one element at a time. The converted elements are not cached. Iterating multiple times over the returned Iterable will invoke the supplied toElement function once per element for on each iteration.

Methods on the returned iterable are allowed to omit calling toElement on any element where the result isn't needed. For example, elementAt may call toElement only once.

Equivalent to:

dart

Iterable<T> map<T>(T toElement(E e)) sync* {
  for (var value in this) {
    yield toElement(value);
  }
}

Example:

dart

var products = jsonDecode('''
[
  {"name": "Screwdriver", "price": 42.00},
  {"name": "Wingnut", "price": 0.50}
]
''');
var values = products.map((product) => product['price'] as double);
var totalPrice = values.fold(0.0, (a, b) => a + b); // 42.5.

Inherited from Iterable.

Implementation

dart

Iterable<T> map<T>(T toElement(E e)) => MappedIterable<E, T>(this, toElement);

noSuchMethod() inherited

dynamic noSuchMethod(Invocation invocation)

Invoked when a nonexistent method or property is accessed.

A dynamic member invocation can attempt to call a member which doesn't exist on the receiving object. Example:

dart

dynamic object = 1;
object.add(42); // Statically allowed, run-time error

This invalid code will invoke the noSuchMethod method of the integer 1 with an Invocation representing the .add(42) call and arguments (which then throws).

Classes can override noSuchMethod to provide custom behavior for such invalid dynamic invocations.

A class with a non-default noSuchMethod invocation can also omit implementations for members of its interface. Example:

dart

class MockList<T> implements List<T> {
  noSuchMethod(Invocation invocation) {
    log(invocation);
    super.noSuchMethod(invocation); // Will throw.
  }
}
void main() {
  MockList().add(42);
}

This code has no compile-time warnings or errors even though the MockList class has no concrete implementation of any of the List interface methods. Calls to List methods are forwarded to noSuchMethod, so this code will log an invocation similar to Invocation.method(#add, [42]) and then throw.

If a value is returned from noSuchMethod, it becomes the result of the original invocation. If the value is not of a type that can be returned by the original invocation, a type error occurs at the invocation.

The default behavior is to throw a NoSuchMethodError.

Inherited from Object.

Implementation

dart

@pragma("vm:entry-point")
@pragma("wasm:entry-point")
external dynamic noSuchMethod(Invocation invocation);

reduce() inherited

E reduce(E Function(E value, E element) combine)

Reduces a collection to a single value by iteratively combining elements of the collection using the provided function.

The iterable must have at least one element. If it has only one element, that element is returned.

Otherwise this method starts with the first element from the iterator, and then combines it with the remaining elements in iteration order, as if by:

dart

E value = iterable.first;
iterable.skip(1).forEach((element) {
  value = combine(value, element);
});
return value;

Example of calculating the sum of an iterable:

dart

final numbers = <double>[10, 2, 5, 0.5];
final result = numbers.reduce((value, element) => value + element);
print(result); // 17.5

Consider using fold if the iterable can be empty.

Inherited from Iterable.

Implementation

dart

E reduce(E combine(E value, E element)) {
  Iterator<E> iterator = this.iterator;
  if (!iterator.moveNext()) {
    throw IterableElementError.noElement();
  }
  E value = iterator.current;
  while (iterator.moveNext()) {
    value = combine(value, iterator.current);
  }
  return value;
}

remove()

bool remove(Object? value)

Removes a single instance of value from the queue.

Returns true if a value was removed, or false if the queue contained no element equal to value.

Implementation

dart

bool remove(Object? value);

removeFirst()

E removeFirst()

Removes and returns the first element of this queue.

The queue must not be empty when this method is called.

Implementation

dart

E removeFirst();

removeLast()

E removeLast()

Removes and returns the last element of the queue.

The queue must not be empty when this method is called.

Implementation

dart

E removeLast();

removeWhere()

void removeWhere(bool Function(E element) test)

Removes all elements matched by test from the queue.

The test function must not throw or modify the queue.

Implementation

dart

void removeWhere(bool test(E element));

retainWhere()

void retainWhere(bool Function(E element) test)

Removes all elements not matched by test from the queue.

The test function must not throw or modify the queue.

Implementation

dart

void retainWhere(bool test(E element));

singleWhere() inherited

E singleWhere(bool Function(E element) test, {(E Function())? orElse})

The single element that satisfies test.

Checks elements to see if test(element) returns true. If exactly one element satisfies test, that element is returned. If more than one matching element is found, throws StateError. If no matching element is found, returns the result of orElse. If orElse is omitted, it defaults to throwing a StateError.

Example:

dart

final numbers = <int>[2, 2, 10];
var result = numbers.singleWhere((element) => element > 5); // 10

When no matching element is found, the result of calling orElse is returned instead.

dart

result = numbers.singleWhere((element) => element == 1,
    orElse: () => -1); // -1

There must not be more than one matching element.

dart

result = numbers.singleWhere((element) => element == 2); // Throws Error.

Inherited from Iterable.

Implementation

dart

E singleWhere(bool test(E element), {E orElse()?}) {
  var iterator = this.iterator;
  E result;
  do {
    if (!iterator.moveNext()) {
      if (orElse != null) return orElse();
      throw IterableElementError.noElement();
    }
    result = iterator.current;
  } while (!test(result));
  while (iterator.moveNext()) {
    if (test(iterator.current)) throw IterableElementError.tooMany();
  }
  return result;
}

skip() inherited

Iterable<E> skip(int count)

Creates an Iterable that provides all but the first count elements.

When the returned iterable is iterated, it starts iterating over this, first skipping past the initial count elements. If this has fewer than count elements, then the resulting Iterable is empty. After that, the remaining elements are iterated in the same order as in this iterable.

Some iterables may be able to find later elements without first iterating through earlier elements, for example when iterating a List. Such iterables are allowed to ignore the initial skipped elements.

Example:

dart

final numbers = <int>[1, 2, 3, 5, 6, 7];
final result = numbers.skip(4); // (6, 7)
final skipAll = numbers.skip(100); // () - no elements.

The count must not be negative.

Inherited from Iterable.

Implementation

dart

Iterable<E> skip(int count) => SkipIterable<E>(this, count);

skipWhile() inherited

Iterable<E> skipWhile(bool Function(E element) test)

Creates an Iterable that skips leading elements while test is satisfied.

The filtering happens lazily. Every new Iterator of the returned iterable iterates over all elements of this.

The returned iterable provides elements by iterating this iterable, but skipping over all initial elements where test(element) returns true. If all elements satisfy test the resulting iterable is empty, otherwise it iterates the remaining elements in their original order, starting with the first element for which test(element) returns false.

Example:

dart

final numbers = <int>[1, 2, 3, 5, 6, 7];
var result = numbers.skipWhile((x) => x < 5); // (5, 6, 7)
result = numbers.skipWhile((x) => x != 3); // (3, 5, 6, 7)
result = numbers.skipWhile((x) => x != 4); // ()
result = numbers.skipWhile((x) => x.isOdd); // (2, 3, 5, 6, 7)

Inherited from Iterable.

Implementation

dart

Iterable<E> skipWhile(bool test(E value)) => SkipWhileIterable<E>(this, test);

take() inherited

Iterable<E> take(int count)

Creates a lazy iterable of the count first elements of this iterable.

The returned Iterable may contain fewer than count elements, if this contains fewer than count elements.

The elements can be computed by stepping through iterator until count elements have been seen.

The count must not be negative.

Example:

dart

final numbers = <int>[1, 2, 3, 5, 6, 7];
final result = numbers.take(4); // (1, 2, 3, 5)
final takeAll = numbers.take(100); // (1, 2, 3, 5, 6, 7)

Inherited from Iterable.

Implementation

dart

Iterable<E> take(int count) => TakeIterable<E>(this, count);

takeWhile() inherited

Iterable<E> takeWhile(bool Function(E element) test)

Creates a lazy iterable of the leading elements satisfying test.

The filtering happens lazily. Every new iterator of the returned iterable starts iterating over the elements of this.

The elements can be computed by stepping through iterator until an element is found where test(element) is false. At that point, the returned iterable stops (its moveNext() returns false).

Example:

dart

final numbers = <int>[1, 2, 3, 5, 6, 7];
var result = numbers.takeWhile((x) => x < 5); // (1, 2, 3)
result = numbers.takeWhile((x) => x != 3); // (1, 2)
result = numbers.takeWhile((x) => x != 4); // (1, 2, 3, 5, 6, 7)
result = numbers.takeWhile((x) => x.isOdd); // (1)

Inherited from Iterable.

Implementation

dart

Iterable<E> takeWhile(bool test(E value)) => TakeWhileIterable<E>(this, test);

toList() inherited

List<E> toList({bool growable = true})

Creates a List containing the elements of this Iterable.

The elements are in iteration order. The list is fixed-length if growable is false.

Example:

dart

final planets = <int, String>{1: 'Mercury', 2: 'Venus', 3: 'Mars'};
final keysList = planets.keys.toList(growable: false); // [1, 2, 3]
final valuesList =
    planets.values.toList(growable: false); // [Mercury, Venus, Mars]

Inherited from Iterable.

Implementation

dart

List<E> toList({bool growable = true}) =>
    List<E>.of(this, growable: growable);

toSet() inherited

Set<E> toSet()

Creates a Set containing the same elements as this iterable.

The set may contain fewer elements than the iterable, if the iterable contains an element more than once, or it contains one or more elements that are equal. The order of the elements in the set is not guaranteed to be the same as for the iterable.

Example:

dart

final planets = <int, String>{1: 'Mercury', 2: 'Venus', 3: 'Mars'};
final valueSet = planets.values.toSet(); // {Mercury, Venus, Mars}

Inherited from Iterable.

Implementation

dart

Set<E> toSet() => Set<E>.of(this);

toString() inherited

String toString()

A string representation of this object.

Some classes have a default textual representation, often paired with a static parse function (like int.parse). These classes will provide the textual representation as their string representation.

Other classes have no meaningful textual representation that a program will care about. Such classes will typically override toString to provide useful information when inspecting the object, mainly for debugging or logging.

Inherited from Object.

Implementation

dart

external String toString();

where() inherited

Iterable<E> where(bool Function(E element) test)

Creates a new lazy Iterable with all elements that satisfy the predicate test.

The matching elements have the same order in the returned iterable as they have in iterator.

This method returns a view of the mapped elements. As long as the returned Iterable is not iterated over, the supplied function test will not be invoked. Iterating will not cache results, and thus iterating multiple times over the returned Iterable may invoke the supplied function test multiple times on the same element.

Example:

dart

final numbers = <int>[1, 2, 3, 5, 6, 7];
var result = numbers.where((x) => x < 5); // (1, 2, 3)
result = numbers.where((x) => x > 5); // (6, 7)
result = numbers.where((x) => x.isEven); // (2, 6)

Inherited from Iterable.

Implementation

dart

Iterable<E> where(bool test(E element)) => WhereIterable<E>(this, test);

whereType() inherited

Iterable<T> whereType<T>()

Creates a new lazy Iterable with all elements that have type T.

The matching elements have the same order in the returned iterable as they have in iterator.

This method returns a view of the mapped elements. Iterating will not cache results, and thus iterating multiple times over the returned Iterable may yield different results, if the underlying elements change between iterations.

Inherited from Iterable.

Implementation

dart

Iterable<T> whereType<T>() => WhereTypeIterable<T>(this);

Operators

operator ==() inherited

bool operator ==(Object other)

The equality operator.

The default behavior for all Objects is to return true if and only if this object and other are the same object.

Override this method to specify a different equality relation on a class. The overriding method must still be an equivalence relation. That is, it must be:

Total: It must return a boolean for all arguments. It should never throw.
Reflexive: For all objects o, o == o must be true.
Symmetric: For all objects o1 and o2, o1 == o2 and o2 == o1 must either both be true, or both be false.
Transitive: For all objects o1, o2, and o3, if o1 == o2 and o2 == o3 are true, then o1 == o3 must be true.

The method should also be consistent over time, so whether two objects are equal should only change if at least one of the objects was modified.

If a subclass overrides the equality operator, it should override the hashCode method as well to maintain consistency.

Inherited from Object.

Implementation

dart

external bool operator ==(Object other);

Static Methods

castFrom() override

Queue<T> castFrom<S, T>(Queue<S> source)

Adapts source to be a Queue<T>.

Any time the queue would produce an element that is not a T, the element access will throw.

When a T value is stored into the adapted queue, the operation will throw unless the value is also an instance of S.

If all accessed elements of source are actually instances of T, and if all elements stored into the returned queue are actually instances of S, then the returned queue can be used as a Queue<T>.

Methods which accept Object? as argument, like contains and remove, will pass the argument directly to this queue's method without any checks.

Implementation

dart

static Queue<T> castFrom<S, T>(Queue<S> source) => CastQueue<S, T>(source);

Queue<E> abstract interface ​

Constructors ​

Queue() factory ​

Queue.from() factory ​

Queue.of() factory ​

Properties ​

first no setter inherited ​

firstOrNull extension no setter ​

hashCode no setter inherited ​

indexed extension no setter ​

isEmpty no setter inherited ​

isNotEmpty no setter inherited ​

iterator no setter inherited ​

last no setter inherited ​

lastOrNull extension no setter ​

length no setter inherited ​

nonNulls extension no setter ​

runtimeType no setter inherited ​

single no setter inherited ​

singleOrNull extension no setter ​

wait extension no setter ​

Methods ​

add() ​

addAll() ​

addFirst() ​

addLast() ​

any() inherited ​

asNameMap() extension ​

byName() extension ​

cast() override ​

clear() ​

contains() inherited ​

elementAt() inherited ​

elementAtOrNull() extension ​

every() inherited ​

expand() inherited ​

firstWhere() inherited ​

fold() inherited ​

followedBy() inherited ​

forEach() inherited ​

join() inherited ​

lastWhere() inherited ​

map() inherited ​

noSuchMethod() inherited ​

reduce() inherited ​

remove() ​

removeFirst() ​

removeLast() ​

removeWhere() ​

retainWhere() ​

singleWhere() inherited ​

skip() inherited ​

skipWhile() inherited ​

take() inherited ​

takeWhile() inherited ​

toList() inherited ​

toSet() inherited ​

toString() inherited ​

where() inherited ​

whereType() inherited ​

Operators ​

operator ==() inherited ​

Static Methods ​

castFrom() override ​

Queue<E> abstract interface

Constructors

Queue() factory

Queue.from() factory

Queue.of() factory

Properties

first no setter inherited

firstOrNull extension no setter

hashCode no setter inherited

indexed extension no setter

isEmpty no setter inherited

isNotEmpty no setter inherited

iterator no setter inherited

last no setter inherited

lastOrNull extension no setter

length no setter inherited

nonNulls extension no setter

runtimeType no setter inherited

single no setter inherited

singleOrNull extension no setter

wait extension no setter

Methods

add()

addAll()

addFirst()

addLast()

any() inherited

asNameMap() extension

byName() extension

cast() override

clear()

contains() inherited

elementAt() inherited

elementAtOrNull() extension

every() inherited

expand() inherited

firstWhere() inherited

fold() inherited

followedBy() inherited

forEach() inherited

join() inherited

lastWhere() inherited

map() inherited

noSuchMethod() inherited

reduce() inherited

remove()

removeFirst()

removeLast()

removeWhere()

retainWhere()

singleWhere() inherited

skip() inherited

skipWhile() inherited

take() inherited

takeWhile() inherited

toList() inherited

toSet() inherited

toString() inherited

where() inherited

whereType() inherited

Operators

operator ==() inherited

Static Methods

castFrom() override