Enum abstract interface

abstract interface class Enum

Annotations: @Since.new("2.14")

An enumerated value.

This class is implemented by all types and values introduced using an enum declaration. Non-platform classes cannot extend or mix in this class. Concrete classes cannot implement the interface.

The identifier used to name an enum value is available as a String, via the EnumName.name extension property on the enum value.

Available Extensions

• EnumName

Properties

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

external int get hashCode;

index no setter

int get index

A numeric identifier for the enumerated value.

The values of a single enumeration are numbered consecutively from zero to one less than the number of values. This is also the index of the value in the enumerated type's static values list.

Implementation

int get index;

name extension no setter

String get name

The name of the enum value.

The name is a string containing the source identifier used to declare the enum value.

For example, given a declaration like:

enum MyEnum {
  value1,
  value2
}

the result of MyEnum.value1.name is the string "value1".

Available on Enum, provided by the EnumName extension

Implementation

String get name => _name;

runtimeType no setter inherited

Type get runtimeType

A representation of the runtime type of the object.

Inherited from Object.

Implementation

external Type get runtimeType;

Methods

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:

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:

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

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

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

external String toString();

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

external bool operator ==(Object other);

Static Methods

compareByIndex()

int compareByIndex<T extends Enum>(T value1, T value2)

Compares two enum values by their index.

A generic Comparator function for enum types which orders enum values by their index value, which corresponds to the source order of the enum element declarations in the enum declaration.

Example:

enum Season { spring, summer, autumn, winter }

void main() {
  var relationByIndex =
      Enum.compareByIndex(Season.spring, Season.summer); // < 0
  relationByIndex =
      Enum.compareByIndex(Season.summer, Season.spring); // > 0
  relationByIndex =
      Enum.compareByIndex(Season.spring, Season.winter); // < 0
  relationByIndex =
      Enum.compareByIndex(Season.winter, Season.spring); // > 0
}

Implementation

@Since("2.15")
static int compareByIndex<T extends Enum>(T value1, T value2) =>
    value1.index - value2.index;

compareByName()

int compareByName<T extends Enum>(T value1, T value2)

Compares enum values by name.

The EnumName.name of an enum value is a string representing the source name used to declare that enum value.

This Comparator compares two enum values by comparing their names, and can be used to sort enum values by their names. The comparison uses String.compareTo, and is therefore case sensitive.

Example:

enum Season { spring, summer, autumn, winter }

void main() {
  var seasons = [...Season.values]..sort(Enum.compareByName);
  print(seasons);
  // [Season.autumn, Season.spring, Season.summer, Season.winter]
}

Implementation

@Since("2.15")
static int compareByName<T extends Enum>(T value1, T value2) =>
    value1.name.compareTo(value2.name);