Module java.base
Package java.util

Class 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; 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.

    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

      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.
      <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 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.

    • 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 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

      • 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 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

      • 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