Uses of Class java.util.Optional (Java Platform SE 8 )
Date: 2020-03-26

## Uses of Class
java.util.Optional

- Packages that use Optional
  Package | Description
  --- | ---
  java.util | Contains the collections framework, legacy collection classes, event model, date and time facilities, internationalization, and miscellaneous utility classes (a string tokenizer, a random-number generator, and a bit array).
  java.util.stream | Classes to support functional-style operations on streams of elements, such as map-reduce transformations on collections.

- - ### Uses of Optional in java.util

    Methods in java.util that return Optional
    Modifier and Type | Method and Description
    --- | ---
    `static <T> Optional<T>` | Optional.`empty()` Returns an empty `Optional` instance.
    `Optional<T>` | Optional.`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>` | Optional.`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`.
    `<U> Optional<U>` | Optional.`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>` | Optional.`of(T value)` Returns an `Optional` with the specified present non-null value.
    `static <T> Optional<T>` | Optional.`ofNullable(T value)` Returns an `Optional` describing the specified value, if non-null, otherwise returns an empty `Optional`.

    Method parameters in java.util with type arguments of type Optional
    Modifier and Type | Method and Description
    --- | ---
    `<U> Optional<U>` | Optional.`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`.

  - ### Uses of Optional in java.util.stream

    Methods in java.util.stream that return Optional
    Modifier and Type | Method and Description
    --- | ---
    `Optional<T>` | Stream.`findAny()` Returns an `Optional` describing some element of the stream, or an empty `Optional` if the stream is empty.
    `Optional<T>` | Stream.`findFirst()` Returns an `Optional` describing the first element of this stream, or an empty `Optional` if the stream is empty.
    `Optional<T>` | Stream.`max(Comparator<? super T> comparator)` Returns the maximum element of this stream according to the provided `Comparator`.
    `Optional<T>` | Stream.`min(Comparator<? super T> comparator)` Returns the minimum element of this stream according to the provided `Comparator`.
    `Optional<T>` | Stream.`reduce(BinaryOperator<T> accumulator)` Performs a reduction on the elements of this stream, using an associative accumulation function, and returns an `Optional` describing the reduced value, if any.

    Methods in java.util.stream that return types with arguments of type Optional
    Modifier and Type | Method and Description
    --- | ---
    `static <T> Collector<T,?,Optional<T>>` | Collectors.`maxBy(Comparator<? super T> comparator)` Returns a `Collector` that produces the maximal element according to a given `Comparator`, described as an `Optional<T>`.
    `static <T> Collector<T,?,Optional<T>>` | Collectors.`minBy(Comparator<? super T> comparator)` Returns a `Collector` that produces the minimal element according to a given `Comparator`, described as an `Optional<T>`.
    `static <T> Collector<T,?,Optional<T>>` | Collectors.`reducing(BinaryOperator<T> op)` Returns a `Collector` which performs a reduction of its input elements under a specified `BinaryOperator`.

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