java8.util

Class Optional<T>

java.lang.Object

java8.util.Optional<T>

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() will return true and get() will return the value.

Additional methods that depend on the presence or absence of a contained value are provided, such as orElse() (return a default value if value not present) and ifPresent() (perform an action if the 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.

Since:

1.8

Method Summary

Modifier and Type	Method and 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, return an Optional describing the value, otherwise return an empty Optional.
<U> Optional<U>	flatMap(Function<? super T,Optional<U>> mapper) If a value is present, apply the provided Optional-bearing mapping function to it, return that result, otherwise return an empty Optional.
T	get() If a value is present in this Optional, returns the value, otherwise throws NoSuchElementException.
int	hashCode() Returns the hash code value of the present value, if any, or 0 (zero) if no value is present.
void	ifPresent(Consumer<? super T> action) If a value is present, perform the given action with the value, otherwise do nothing.
void	ifPresentOrElse(Consumer<? super T> action, Runnable emptyAction) If a value is present, perform the given action with the value, otherwise perform the given empty-based action.
boolean	isPresent() Return true if there is a value present, otherwise false.
<U> Optional<U>	map(Function<? super T,? extends U> mapper) If a value is present, apply the provided mapping function to it, and if the result is non-null, return an Optional describing the result.
static <T> Optional<T>	of(T value) Returns an Optional with the specified present non-null value.
static <T> Optional<T>	ofNullable(T value) Returns an Optional describing the specified value, if non-null, otherwise returns an empty Optional.
T	orElse(T other) Return the value if present, otherwise return other.
T	orElseGet(Supplier<? extends T> other) Return the value if present, otherwise invoke other and return the result of that invocation.
<X extends Throwable> T	orElseThrow(Supplier<? extends X> exceptionSupplier) Return the contained value, if present, otherwise throw an exception to be created by the provided supplier.
Stream<T>	stream() If a value is present return a sequential Stream containing only that value, otherwise return an empty Stream.
String	toString() Returns a non-empty string representation of this Optional suitable for debugging.

Methods inherited from class java.lang.Object

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

Method Detail

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 Option.empty(). There is no guarantee that it is a singleton. Instead, use isPresent().

Type Parameters:

T - Type of the non-existent value

Returns:

an empty Optional

of

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

Returns an Optional with the specified present non-null value.

Type Parameters:

T - the class of the value

Parameters:

value - the value to be present, 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 specified value, if non-null, otherwise returns an empty Optional.

Type Parameters:

T - the class 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 in this Optional, returns the value, otherwise throws NoSuchElementException.

Returns:

the non-null value held by this Optional

Throws:

NoSuchElementException - if there is no value present

See Also:

isPresent()

isPresent

public boolean isPresent()

Return true if there is a value present, otherwise false.

Returns:

true if there is a value present, otherwise false

ifPresent

public void ifPresent(Consumer<? super T> action)

If a value is present, perform the given action with the value, otherwise do nothing.

Parameters:

action - the action to be performed if a value is present

Throws:

NullPointerException - if a value is present and action is null

ifPresentOrElse

public void ifPresentOrElse(Consumer<? super T> action,
                            Runnable emptyAction)

If a value is present, perform the given action with the value, otherwise perform 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 a value is not present

Throws:

NullPointerException - if a value is present and action is null, or a value is not present and emptyAction is null.

Since:

1.9

filter

public Optional<T> filter(Predicate<? super T> predicate)

If a value is present, and the value matches the given predicate, return an Optional describing the value, otherwise return an empty Optional.

Parameters:

predicate - a predicate to apply to the 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, apply the provided mapping function to it, and if the result is non-null, return an Optional describing the result. Otherwise return 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 file names, selects one that has not yet been processed, and then opens that file, returning an Optional<FileInputStream>:

     Optional<FileInputStream> fis =
         names.stream().filter(name -> !isProcessedYet(name))
                       .findFirst()
                       .map(name -> new FileInputStream(name));
 

Here, findFirst returns an Optional<String>, and then map returns an Optional<FileInputStream> for the desired file if one exists.

Type Parameters:

U - The type of the result of the mapping function

Parameters:

mapper - a mapping function to apply to the 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,Optional<U>> mapper)

If a value is present, apply the provided Optional-bearing mapping function to it, return that result, otherwise return an empty Optional. This method is similar to map(Function), but the provided mapper is one whose result is already an Optional, and if invoked, flatMap does not wrap it with an additional Optional.

Type Parameters:

U - The type parameter to the Optional returned by

Parameters:

mapper - a mapping function to apply to the value, if present the mapping function

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

stream

public Stream<T> stream()

If a value is present return a sequential Stream containing only that value, otherwise return 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:

1.9

orElse

public T orElse(T other)

Return the value if present, otherwise return other.

Parameters:

other - the value to be returned if there is no value present, may be null

Returns:

the value, if present, otherwise other

orElseGet

public T orElseGet(Supplier<? extends T> other)

Return the value if present, otherwise invoke other and return the result of that invocation.

Parameters:

other - a Supplier whose result is returned if no value is present

Returns:

the value if present otherwise the result of other.get()

Throws:

NullPointerException - if value is not present and other is null

orElseThrow

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

Return the contained value, if present, otherwise throw an exception to be created by the provided supplier.

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 supplier which will return the exception to be thrown

Returns:

the present value

Throws:

X - if there is no value present

NullPointerException - if no value is present and exceptionSupplier 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

hashCode

public int hashCode()

Returns the hash code value of the present value, if any, or 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

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.

Overrides:

toString in class Object

Returns:

the string representation of this instance