Module java.base
Package java.util

# 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 java.lang.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`

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