Optional (Java Platform SE 8 )
Keywords: java.util.Optional class
Date: 2015-05-24

compact1, compact2, compact3
java.util

## Class Optional<T>

- java.lang.Object
- - java.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()` (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

- - ### Method Summary

    All Methods Static Methods Instance Methods Concrete Methods
    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> consumer)` If a value is present, invoke the specified consumer with the value, otherwise do nothing.
    `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.
    `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> 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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