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Showing content from https://docs.oracle.com/javase/9/docs/api/java/util/Optional.html below:

Optional (Java SE 9 & JDK 9 )

- Method Summary All Methods Static Methods Instance Methods Concrete 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.
  Optional 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 isPresent()
 If a value is present, returns true, otherwise false.
  Optional 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.
 <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 inherited from class java.lang.Objectclone, 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 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 methods orElse and orElseGet are generally preferable to this method, as they return a substitute value if the value is absent, instead of throwing an exception.
 
 Returns:
 
 the non-null value described by this Optional
 
 Throws:
 
 NoSuchElementException - if no value is present
 
 See Also:
 
 isPresent()
 - isPresent
```
public boolean isPresent()
```
 If a value is present, returns true, otherwise false.
 
 Returns:
 
 true if a value is present, otherwise false
 - 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  Optional 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  Optional 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 <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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