T
- the type of valuepublic final class Optional<T> extends Object
null
value. If a value is present, isPresent()
returns true
. If no value is present, the object is considered empty and isPresent()
returns false
. Additional methods that depend on the presence or absence of a contained value are provided, such as orElse()
(returns a default value if no value is present) and ifPresent()
(performs an action if a value is present).
This is a value-based class; programmers should treat instances that are equal as interchangeable and should not use instances for synchronization, or unpredictable behavior may occur. For example, in a future release, synchronization may fail.
Optional
is primarily intended for use as a method return type where there is a clear need to represent "no result," and where using null
is likely to cause errors. A variable whose type is Optional
should never itself be null
; it should always point to an Optional
instance.Modifier and Type | Method | Description |
---|---|---|
static <T> Optional |
empty() |
Returns an empty Optional instance. |
boolean |
equals |
Indicates whether some other object is "equal to" this Optional . |
Optional |
filter |
If a value is present, and the value matches the given predicate, returns an Optional describing the value, otherwise returns an empty Optional . |
<U> Optional |
flatMap |
If a value is present, returns the result of applying the given Optional -bearing mapping function to the value, otherwise returns an empty Optional . |
T |
get() |
If a value is present, returns the value, otherwise throws NoSuchElementException . |
int |
hashCode() |
Returns the hash code of the value, if present, otherwise 0 (zero) if no value is present. |
void |
ifPresent |
If a value is present, performs the given action with the value, otherwise does nothing. |
void |
ifPresentOrElse |
If a value is present, performs the given action with the value, otherwise performs the given empty-based action. |
boolean |
isEmpty() |
If a value is not present, returns true , otherwise false . |
boolean |
isPresent() |
If a value is present, returns true , otherwise false . |
<U> Optional |
map |
If a value is present, returns an Optional describing (as if by ofNullable(T) ) the result of applying the given mapping function to the value, otherwise returns an empty Optional . |
static <T> Optional |
of |
Returns an Optional describing the given non-null value. |
static <T> Optional |
ofNullable |
Returns an Optional describing the given value, if non-null , otherwise returns an empty Optional . |
Optional |
or |
If a value is present, returns an Optional describing the value, otherwise returns an Optional produced by the supplying function. |
T |
orElse |
If a value is present, returns the value, otherwise returns other . |
T |
orElseGet |
If a value is present, returns the value, otherwise returns the result produced by the supplying function. |
T |
orElseThrow() |
If a value is present, returns the value, otherwise throws NoSuchElementException . |
<X extends Throwable> |
orElseThrow |
If a value is present, returns the value, otherwise throws an exception produced by the exception supplying function. |
Stream |
stream() |
If a value is present, returns a sequential Stream containing only that value, otherwise returns an empty Stream . |
String |
toString() |
Returns a non-empty string representation of this Optional suitable for debugging. |
public static <T> Optional<T> empty()
Optional
instance. No value is present for this Optional
.==
or !=
against instances returned by Optional.empty()
. There is no guarantee that it is a singleton. Instead, use isEmpty()
or isPresent()
.T
- The type of the non-existent valueOptional
public static <T> Optional<T> of(T value)
Optional
describing the given non-null
value.T
- the type of the valuevalue
- the value to describe, which must be non-null
Optional
with the value presentNullPointerException
- if value is null
public static <T> Optional<T> ofNullable(T value)
Optional
describing the given value, if non-null
, otherwise returns an empty Optional
.T
- the type of the valuevalue
- the possibly-null
value to describeOptional
with a present value if the specified value is non-null
, otherwise an empty Optional
public T get()
NoSuchElementException
.orElseThrow()
.null
value described by this Optional
NoSuchElementException
- if no value is presentpublic boolean isPresent()
true
, otherwise false
.true
if a value is present, otherwise false
public boolean isEmpty()
true
, otherwise false
.true
if a value is not present, otherwise false
public void ifPresent(Consumer<? super T> action)
action
- the action to be performed, if a value is presentNullPointerException
- if value is present and the given action is null
public void ifPresentOrElse(Consumer<? super T> action, Runnable emptyAction)
action
- the action to be performed, if a value is presentemptyAction
- the empty-based action to be performed, if no value is presentNullPointerException
- if a value is present and the given action is null
, or no value is present and the given empty-based action is null
.public Optional<T> filter(Predicate<? super T> predicate)
Optional
describing the value, otherwise returns an empty Optional
.predicate
- the predicate to apply to a value, if presentOptional
describing the value of this Optional
, if a value is present and the value matches the given predicate, otherwise an empty Optional
NullPointerException
- if the predicate is null
public <U> Optional<U> map(Function<? super T,? extends U> mapper)
Optional
describing (as if by ofNullable(T)
) the result of applying the given mapping function to the value, otherwise returns an empty Optional
. If the mapping function returns a null
result then this method returns an empty Optional
.
Optional
values, without the need to explicitly check for a return status. For example, the following code traverses a stream of URIs, selects one that has not yet been processed, and creates a path from that URI, returning an Optional<Path>
:
Optional<Path> p =
uris.stream().filter(uri -> !isProcessedYet(uri))
.findFirst()
.map(Paths::get);
Here, findFirst
returns an Optional<URI>
, and then map
returns an Optional<Path>
for the desired URI if one exists.U
- The type of the value returned from the mapping functionmapper
- the mapping function to apply to a value, if presentOptional
describing the result of applying a mapping function to the value of this Optional
, if a value is present, otherwise an empty Optional
NullPointerException
- if the mapping function is null
public <U> Optional<U> flatMap(Function<? super T,? extends Optional<? extends U>> mapper)
Optional
-bearing mapping function to the value, otherwise returns an empty Optional
. This method is similar to map(Function)
, but the mapping function is one whose result is already an Optional
, and if invoked, flatMap
does not wrap it within an additional Optional
.
U
- The type of value of the Optional
returned by the mapping functionmapper
- the mapping function to apply to a value, if presentOptional
-bearing mapping function to the value of this Optional
, if a value is present, otherwise an empty Optional
NullPointerException
- if the mapping function is null
or returns a null
resultpublic Optional<T> or(Supplier<? extends Optional<? extends T>> supplier)
Optional
describing the value, otherwise returns an Optional
produced by the supplying function.supplier
- the supplying function that produces an Optional
to be returnedOptional
describing the value of this Optional
, if a value is present, otherwise an Optional
produced by the supplying function.NullPointerException
- if the supplying function is null
or produces a null
resultpublic Stream<T> stream()
Stream
containing only that value, otherwise returns an empty Stream
.Stream
of optional elements to a Stream
of present value elements:
Stream<Optional<T>> os = ..
Stream<T> s = os.flatMap(Optional::stream)
Stream
public T orElse(T other)
other
.other
- the value to be returned, if no value is present. May be null
.other
public T orElseGet(Supplier<? extends T> supplier)
supplier
- the supplying function that produces a value to be returnedNullPointerException
- if no value is present and the supplying function is null
public T orElseThrow()
NoSuchElementException
.null
value described by this Optional
NoSuchElementException
- if no value is presentpublic <X extends Throwable> T orElseThrow(Supplier<? extends X> exceptionSupplier) throws X
IllegalStateException::new
X
- Type of the exception to be thrownexceptionSupplier
- the supplying function that produces an exception to be thrownX
- if no value is presentNullPointerException
- if no value is present and the exception supplying function is null
public boolean equals(Object obj)
Optional
. The other object is considered equal if: Optional
and; equals()
. public int hashCode()
0
(zero) if no value is present.public String toString()
Optional
suitable for debugging. The exact presentation format is unspecified and may vary between implementations and versions.
© 1993, 2023, Oracle and/or its affiliates. All rights reserved.
Documentation extracted from Debian's OpenJDK Development Kit package.
Licensed under the GNU General Public License, version 2, with the Classpath Exception.
Various third party code in OpenJDK is licensed under different licenses (see Debian package).
Java and OpenJDK are trademarks or registered trademarks of Oracle and/or its affiliates.
https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/Optional.html