# Class Optional<T>

java.lang.Object
java.util.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; 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.

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

- ## Method Summary

  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 Object

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

- ## Method Details

  - ### 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 `==` or `!=` against instances returned by `Optional.empty()`. There is no guarantee that it is a singleton. Instead, use `isEmpty()` or `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
    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` or produces a `null` result

  - ### 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(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 `Optional`s must be unambiguously differentiable.
    Returns:
      the string representation of this instance