## Uses of Interface
java.util.function.ToLongFunction

- Packages that use ToLongFunction
  Package | Description
  --- | ---
  java.util |
  java.util.stream | Classes to support functional-style operations on streams of elements, such as map-reduce transformations on collections.

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

    Methods in java.util with parameters of type ToLongFunction
    Modifier and Type | Method and Description
    --- | ---
    `static <T> Comparator<T>` | Comparator.`comparingLong(ToLongFunction<? super T> keyExtractor)` Accepts a function that extracts a `long` sort key from a type `T`, and returns a `Comparator<T>` that compares by that sort key.
    `default Comparator<T>` | Comparator.`thenComparingLong(ToLongFunction<? super T> keyExtractor)` Returns a lexicographic-order comparator with a function that extracts a `long` sort key.

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

    Methods in java.util.stream with parameters of type ToLongFunction
    Modifier and Type | Method and Description
    --- | ---
    `static <T> Collector<T,?,Double>` | Collectors.`averagingLong(ToLongFunction<? super T> mapper)` Returns a `Collector` that produces the arithmetic mean of a long-valued function applied to the input elements.
    `LongStream` | Stream.`mapToLong(ToLongFunction<? super T> mapper)` Returns a `LongStream` consisting of the results of applying the given function to the elements of this stream.
    `static <T> Collector<T,?,LongSummaryStatistics>` | Collectors.`summarizingLong(ToLongFunction<? super T> mapper)` Returns a `Collector` which applies an `long`-producing mapping function to each input element, and returns summary statistics for the resulting values.
    `static <T> Collector<T,?,Long>` | Collectors.`summingLong(ToLongFunction<? super T> mapper)` Returns a `Collector` that produces the sum of a long-valued function applied to the input elements.

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