Optional< T> class
A value that might be absent.
Use Optional as an alternative to allowing fields, parameters or return values to be null. It signals that a value is not required and provides convenience methods for dealing with the absent case.
- Inheritance
- Object
- Iterable<
T> - IterableBase<
T> - Optional
Constructors
- Optional.absent()
-
Constructs an empty Optional.
const
- Optional.fromNullable(T value)
-
Constructs an Optional of the given
value
. [...]const - Optional.of(T value)
-
Constructs an Optional of the given
value
. [...]
Properties
- hashCode → int
-
Delegates to the underlying value hashCode.
read-only, override
- isPresent → bool
-
Whether the Optional contains a value.
read-only
-
iterator
→ Iterator<
T> -
Returns a new
Iterator
that allows iterating the elements of thisIterable
. [...]read-only, override - orNull → T
-
Gets the Optional value, or
null
if there is none.read-only - value → T
-
Gets the Optional value. [...]
read-only
- first → T
-
Returns the first element. [...]
read-only, inherited
- isEmpty → bool
-
Returns
true
if there are no elements in this collection. [...]read-only, inherited - isNotEmpty → bool
-
Returns true if there is at least one element in this collection. [...]
read-only, inherited
- last → T
-
Returns the last element. [...]
read-only, inherited
- length → int
-
Returns the number of elements in this. [...]
read-only, inherited
- runtimeType → Type
-
A representation of the runtime type of the object.
read-only, inherited
- single → T
-
Checks that this iterable has only one element, and returns that element. [...]
read-only, inherited
Methods
-
ifAbsent(
void ifAbsent()) → void - Execution a function if the Optional value is absent.
-
ifPresent(
void ifPresent(T value)) → void - Executes a function if the Optional value is present.
-
or(
T defaultValue) → T - Gets the Optional value with a default. [...]
-
toString(
) → String -
Returns a string representation of (some of) the elements of
this
. [...]override -
transform<
S>( S transformer(T value)) → Optional< S> - Transforms the Optional value. [...]
-
transformNullable<
S>( S transformer(T value)) → Optional< S> - Transforms the Optional value. [...]
-
any(
bool test(E element)) → bool -
Checks whether any element of this iterable satisfies
test
. [...]inherited -
cast<
R>( ) → Iterable< R> -
Provides a view of this iterable as an iterable of
R
instances. [...]inherited -
contains(
Object element) → bool -
Returns true if the collection contains an element equal to
element
. [...]inherited -
elementAt(
int index) → T -
Returns the
index
th element. [...]inherited -
every(
bool test(E element)) → bool -
Checks whether every element of this iterable satisfies
test
. [...]inherited -
expand<
T>( Iterable< T> f(E element)) → Iterable< T> -
Expands each element of this Iterable into zero or more elements. [...]
inherited
-
firstWhere(
bool test(E element), { T orElse() }) → T -
Returns the first element that satisfies the given predicate
test
. [...]inherited -
fold<
T>( T initialValue, T combine(T previousValue, E element)) → T -
Reduces a collection to a single value by iteratively combining each
element of the collection with an existing value [...]
inherited
-
followedBy(
Iterable< T> other) → Iterable< T> -
Returns the lazy concatentation of this iterable and
other
. [...]inherited -
forEach(
void f(E element)) → void -
Applies the function
f
to each element of this collection in iteration order.inherited -
join(
[String separator = "" ]) → String -
Converts each element to a String and concatenates the strings. [...]
inherited
-
lastWhere(
bool test(E element), { T orElse() }) → T -
Returns the last element that satisfies the given predicate
test
. [...]inherited -
map<
T>( T f(E e)) → Iterable< T> -
Returns a new lazy Iterable with elements that are created by
calling
f
on each element of thisIterable
in iteration order. [...]inherited -
noSuchMethod(
Invocation invocation) → dynamic -
Invoked when a non-existent method or property is accessed. [...]
inherited
-
reduce(
T combine(E value, E element)) → T -
Reduces a collection to a single value by iteratively combining elements
of the collection using the provided function. [...]
inherited
-
singleWhere(
bool test(E element), { T orElse() }) → T -
Returns the single element that satisfies
test
. [...]inherited -
skip(
int count) → Iterable< T> -
Returns an Iterable that provides all but the first
count
elements. [...]inherited -
skipWhile(
bool test(E value)) → Iterable< T> -
Returns an
Iterable
that skips leading elements whiletest
is satisfied. [...]inherited -
take(
int count) → Iterable< T> -
Returns a lazy iterable of the
count
first elements of this iterable. [...]inherited -
takeWhile(
bool test(E value)) → Iterable< T> -
Returns a lazy iterable of the leading elements satisfying
test
. [...]inherited -
toList(
{bool growable: true }) → List< T> -
Creates a List containing the elements of this Iterable. [...]
inherited
-
toSet(
) → Set< T> -
Creates a Set containing the same elements as this iterable. [...]
inherited
-
where(
bool test(E element)) → Iterable< T> -
Returns a new lazy Iterable with all elements that satisfy the
predicate
test
. [...]inherited -
whereType<
T>( ) → Iterable< T> -
Returns a new lazy Iterable with all elements that have type
T
. [...]inherited
Operators
-
operator ==(
dynamic o) → bool -
Delegates to the underlying value operator==.
override