Class Optional<T>

Type Parameters:
T - the type of value
public final class Optional<T>
extends Object

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; use of identity-sensitive operations (including reference equality (==), identity hash code, or synchronization) on instances of Optional may have unpredictable results and should be avoided.

API Note:
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.
Since:
1.8

Methods

Modifier and Type Method Description
static <T> Optional<T> empty()

Returns an empty Optional instance.
boolean equals​(Object obj)

Indicates whether some other object is "equal to" this Optional.
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.
<U> 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.
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​(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 true, otherwise false.
boolean isPresent()

If a value is present, returns true, otherwise false.
<U> 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.
static <T> Optional<T> of​(T value)

Returns an Optional describing the given non-null value.
static <T> Optional<T> ofNullable​(T value)

Returns an Optional describing the given value, if non-null, otherwise returns an empty Optional.
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.
T orElse​(T other)

If a value is present, returns the value, otherwise returns other.
T orElseGet​(Supplier<? extends T> supplier)

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>
T		 orElseThrow​(Supplier<? extends X> exceptionSupplier)

If a value is present, returns the value, otherwise throws an exception produced by the exception supplying function.
Stream<T> 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.

Methods declared in class java.lang.Object

clone, finalize, getClass, notify, notifyAll, wait, wait, wait

Methods

empty

public static <T> 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 == against instances returned by Optional.empty(). There is no guarantee that it is a singleton. Instead, use isPresent().
Type Parameters:
T - The type of the non-existent value
Returns:
an empty Optional

of

public static <T> Optional<T> of(T value)

Returns an Optional describing the given non-null value.

Type Parameters:
T - the type of the value
Parameters:
value - the value to describe, which must be non-null
Returns:
an Optional with the value present
Throws:
NullPointerException - if value is null

ofNullable

public static <T> Optional<T> ofNullable(T value)

Returns an Optional describing the given value, if non-null, otherwise returns an empty Optional.

Type Parameters:
T - the type of the value
Parameters:
value - the possibly-null value to describe
Returns:
an Optional with a present value if the specified value is non-null, otherwise an empty Optional

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:
the non-null value described by this Optional
Throws:
NoSuchElementException - if no value is present

isPresent

public boolean isPresent()

If a value is present, returns true, otherwise false.

Returns:
true if a value is present, otherwise false

isEmpty

public boolean isEmpty()

If a value is not present, returns true, otherwise false.

Returns:
true if a value is not present, otherwise false
Since:
11

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 - 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 - the action to be performed, if a value is present
emptyAction - 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.
Since:
9

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 - the predicate to apply to a value, if present
Returns:
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

map

public <U> 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 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.
Type Parameters:
U - The type of the value returned from the mapping function
Parameters:
mapper - the mapping function to apply to a value, if present
Returns:
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

flatMap

public <U> 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(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.

Type Parameters:
U - The type of value of the Optional returned by the mapping function
Parameters:
mapper - the mapping function to apply to a value, if present
Returns:
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

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 - the supplying function that produces an Optional to be returned
Returns:
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
Since:
9

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 Stream of optional elements to a Stream of present value elements: 
Stream<Optional<T>> os = ..
     Stream<T> s = os.flatMap(Optional::stream)
Returns:
the optional value as a Stream
Since:
9

orElse

public T orElse(T other)

If a value is present, returns the value, otherwise returns other.

Parameters:
other - the value to be returned, if no value is present. May be null.
Returns:
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 - the supplying function that produces a value to be returned
Returns:
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()

If a value is present, returns the value, otherwise throws NoSuchElementException.

Returns:
the non-null value described by this Optional
Throws:
NoSuchElementException - if no value is present
Since:
10

orElseThrow

public <X extends Throwable> T orElseThrow(Supplier<? extends X> exceptionSupplier)
                                    throws X extends Throwable

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
Type Parameters:
X - Type of the exception to be thrown
Parameters:
exceptionSupplier - the supplying function that produces an exception to be thrown
Returns:
the value, if present
Throws:
X - if no value is present
NullPointerException - if no value is present and the exception supplying function is null
X extends Throwable

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 Optional and;
  • both instances have no value present or;
  • the present values are "equal to" each other via equals().

Overrides:
equals in class Object
Parameters:
obj - an object to be tested for equality
Returns:
true if the other object is "equal to" this object otherwise false
See Also:
Object.hashCode(), HashMap

hashCode

public int hashCode()

Returns the hash code of the value, if present, otherwise 0 (zero) if no value is present.

Overrides:
hashCode in class Object
Returns:
hash code value of the present value or 0 if no value is present
See Also:
Object.equals(java.lang.Object), System.identityHashCode(java.lang.Object)

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.

Overrides:
toString in class Object
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:
the string representation of this instance

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https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/Optional.html