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Class 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() (execute a block of code 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

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 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> consumer)

If a value is present, invoke the specified consumer with the value, otherwise do nothing.

Parameters:
consumer - block to be executed if a value is present
Throws:
NullPointerException - if value is present and consumer is null

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

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:
{code 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 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
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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Documentation extracted from Debian's OpenJDK Development Kit package.
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