Optional
public final class Optional
extends Object
| java.lang.Object | |
| ↳ | java.util.Optional<T> |
A container object which may or may not contain a non-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.
Summary
Public methods | |
|---|---|
static <T> Optional<T> | empty() Returns an empty |
boolean | equals(Object obj) Indicates whether some other object is "equal to" this |
Optional<T> | filter(Predicate<? super T> predicate) If a value is present, and the value matches the given predicate, returns an |
<U> Optional<U> | flatMap(Function<? super T, ? extends Optional<? extends U>> mapper) If a value is present, returns the result of applying the given |
T | get() If a value is present, returns the value, otherwise throws |
int | hashCode() Returns the hash code of the value, if present, otherwise |
void | ifPresent(Consumer<? super T> action) If a value is present, performs the given action with the value, otherwise does nothing. |
void | ifPresentOrElse(Consumer<? super T> action, Runnable emptyAction) 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 |
boolean | isPresent() If a value is present, returns |
<U> Optional<U> | map(Function<? super T, ? extends U> mapper) If a value is present, returns an |
static <T> Optional<T> | of(T value) Returns an |
static <T> Optional<T> | ofNullable(T value) Returns an |
Optional<T> | or(Supplier<? extends Optional<? extends T>> supplier) If a value is present, returns an |
T | orElse(T other) If a value is present, returns the value, otherwise returns |
T | orElseGet(Supplier<? extends T> supplier) If a value is present, returns the value, otherwise returns the result produced by the supplying function. |
<X extends Throwable> T | orElseThrow(Supplier<? extends X> exceptionSupplier) If a value is present, returns the value, otherwise throws an exception produced by the exception supplying function. |
T | orElseThrow() If a value is present, returns the value, otherwise throws |
Stream<T> | stream() If a value is present, returns a sequential |
String | toString() Returns a non-empty string representation of this |
Inherited methods | |
|---|---|
Public methods
empty
public static Optional<T> empty ()
Returns an empty Optional instance. No value is present for this Optional.
API Note:
- Though it may be tempting to do so, avoid testing if an object is empty by comparing with
==or!=against instances returned byOptional.empty(). There is no guarantee that it is a singleton. Instead, useisEmpty()orisPresent().
| Returns | |
|---|---|
Optional<T> | an empty Optional |
equals
public boolean equals (Object obj)
Indicates whether some other object is "equal to" this Optional. The other object is considered equal if:
- it is also an
Optionaland; - both instances have no value present or;
- the present values are "equal to" each other via
equals().
| Parameters | |
|---|---|
obj | Object: an object to be tested for equality |
| Returns | |
|---|---|
boolean | true if the other object is "equal to" this object otherwise false |
filter
public Optional<T> filter (Predicate<? super T> predicate)
If a value is present, and the value matches the given predicate, returns an Optional describing the value, otherwise returns an empty Optional.
| Parameters | |
|---|---|
predicate | Predicate: the predicate to apply to a value, if present |
| Returns | |
|---|---|
Optional<T> | an Optional describing the value of this Optional, if a value is present and the value matches the given predicate, otherwise an empty Optional |
| Throws | |
|---|---|
NullPointerException | if the predicate is null |
flatMap
public Optional<U> flatMap (Function<? super T, ? extends Optional<? extends U>> mapper)
If a value is present, returns the result of applying the given Optional-bearing mapping function to the value, otherwise returns an empty Optional.
This method is similar to map(java.util.function.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.
| Parameters | |
|---|---|
mapper | Function: the mapping function to apply to a value, if present |
| Returns | |
|---|---|
Optional<U> | the result of applying an Optional-bearing mapping function to the value of this Optional, if a value is present, otherwise an empty Optional |
| Throws | |
|---|---|
NullPointerException | if the mapping function is null or returns a null result |
get
public T get ()
If a value is present, returns the value, otherwise throws NoSuchElementException.
API Note:
- The preferred alternative to this method is
orElseThrow().
| Returns | |
|---|---|
T | the non-null value described by this Optional |
| Throws | |
|---|---|
NoSuchElementException | if no value is present |
hashCode
public int hashCode ()
Returns the hash code of the value, if present, otherwise 0 (zero) if no value is present.
| Returns | |
|---|---|
int | hash code value of the present value or 0 if no value is present |
ifPresent
public void ifPresent (Consumer<? super T> action)
If a value is present, performs the given action with the value, otherwise does nothing.
| Parameters | |
|---|---|
action | Consumer: the action to be performed, if a value is present |
| Throws | |
|---|---|
NullPointerException | if value is present and the given action is null |
ifPresentOrElse
public void ifPresentOrElse (Consumer<? super T> action, Runnable emptyAction)
If a value is present, performs the given action with the value, otherwise performs the given empty-based action.
| Parameters | |
|---|---|
action | Consumer: the action to be performed, if a value is present |
emptyAction | Runnable: the empty-based action to be performed, if no value is present |
| Throws | |
|---|---|
NullPointerException | if a value is present and the given action is null, or no value is present and the given empty-based action is null. |
isEmpty
public boolean isEmpty ()
If a value is not present, returns true, otherwise false.
| Returns | |
|---|---|
boolean | true if a value is not present, otherwise false |
isPresent
public boolean isPresent ()
If a value is present, returns true, otherwise false.
| Returns | |
|---|---|
boolean | true if a value is present, otherwise false |
map
public Optional<U> map (Function<? super T, ? extends U> mapper)
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.
If the mapping function returns a null result then this method returns an empty Optional.
API Note:
- This method supports post-processing on
Optionalvalues, 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 anOptional<Path>:Optional<Path> p = uris.stream().filter(uri -> !isProcessedYet(uri)) .findFirst() .map(Paths::get);findFirstreturns anOptional<URI>, and thenmapreturns anOptional<Path>for the desired URI if one exists.
| Parameters | |
|---|---|
mapper | Function: the mapping function to apply to a value, if present |
| Returns | |
|---|---|
Optional<U> | an Optional describing the result of applying a mapping function to the value of this Optional, if a value is present, otherwise an empty Optional |
| Throws | |
|---|---|
NullPointerException | if the mapping function is null |
of
public static Optional<T> of (T value)
Returns an Optional describing the given non-null value.
| Parameters | |
|---|---|
value | T: the value to describe, which must be non-null |
| Returns | |
|---|---|
Optional<T> | an Optional with the value present |
| Throws | |
|---|---|
NullPointerException | if value is null |
ofNullable
public static Optional<T> ofNullable (T value)
Returns an Optional describing the given value, if non-null, otherwise returns an empty Optional.
| Parameters | |
|---|---|
value | T: the possibly-null value to describe |
| Returns | |
|---|---|
Optional<T> | an Optional with a present value if the specified value is non-null, otherwise an empty Optional |
or
public Optional<T> or (Supplier<? extends Optional<? extends T>> supplier)
If a value is present, returns an Optional describing the value, otherwise returns an Optional produced by the supplying function.
| Parameters | |
|---|---|
supplier | Supplier: the supplying function that produces an Optional to be returned |
| Returns | |
|---|---|
Optional<T> | returns an Optional describing the value of this Optional, if a value is present, otherwise an Optional produced by the supplying function. |
| Throws | |
|---|---|
NullPointerException | if the supplying function is null or produces a null result |
orElse
public T orElse (T other)
If a value is present, returns the value, otherwise returns other.
| Parameters | |
|---|---|
other | T: the value to be returned, if no value is present. May be null. |
| Returns | |
|---|---|
T | the value, if present, otherwise other |
orElseGet
public T orElseGet (Supplier<? extends T> supplier)
If a value is present, returns the value, otherwise returns the result produced by the supplying function.
| Parameters | |
|---|---|
supplier | Supplier: the supplying function that produces a value to be returned |
| Returns | |
|---|---|
T | the value, if present, otherwise the result produced by the supplying function |
| Throws | |
|---|---|
NullPointerException | if no value is present and the supplying function is null |
orElseThrow
public T orElseThrow (Supplier<? extends X> exceptionSupplier)
If a value is present, returns the value, otherwise throws an exception produced by the exception supplying function.
API Note:
- A method reference to the exception constructor with an empty argument list can be used as the supplier. For example,
IllegalStateException::new
| Parameters | |
|---|---|
exceptionSupplier | Supplier: the supplying function that produces an exception to be thrown |
| Returns | |
|---|---|
T | the value, if present |
| Throws | |
|---|---|
| if no value is present |
NullPointerException | if no value is present and the exception supplying function is null |
Throwable | |
orElseThrow
public T orElseThrow ()
If a value is present, returns the value, otherwise throws NoSuchElementException.
| Returns | |
|---|---|
T | the non-null value described by this Optional |
| Throws | |
|---|---|
NoSuchElementException | if no value is present |
stream
public Stream<T> stream ()
If a value is present, returns a sequential Stream containing only that value, otherwise returns an empty Stream.
API Note:
- This method can be used to transform a
Streamof optional elements to aStreamof present value elements:Stream<Optional<T>> os = .. Stream<T> s = os.flatMap(Optional::stream)
| Returns | |
|---|---|
Stream<T> | the optional value as a Stream |
toString
public String toString ()
Returns a non-empty string representation of this Optional suitable for debugging. The exact presentation format is unspecified and may vary between implementations and versions.
Implementation Requirements:
- If a value is present the result must include its string representation in the result. Empty and present
Optionals must be unambiguously differentiable.
| Returns | |
|---|---|
String | the string representation of this instance |