Function (Java SE 25 & JDK 25)
Description: declaration: module: java.base, package: java.util.function, interface: Function
Keywords: java.util.function.Function interface

# Interface Function<T,R>

Type Parameters:
  `T` - the type of the input to the function
  `R` - the type of the result of the function

All Known Subinterfaces:
  `UnaryOperator<T>`

Functional Interface:
  This is a functional interface and can therefore be used as the assignment target for a lambda expression or method reference.

@FunctionalInterface public interface Function<T,R>
Represents a function that accepts one argument and produces a result.

This is a functional interface whose functional method is `apply(Object)`.

Since:
  1.8

- ## Method Summary

  Modifier and Type
  Method
  Description
  `default <V> Function<T,V>`
  `andThen(Function<? super R, ? extends V> after)`
  Returns a composed function that first applies this function to its input, and then applies the `after` function to the result.
  `R`
  `apply(T t)`
  Applies this function to the given argument.
  `default <V> Function<V,R>`
  `compose(Function<? super V, ? extends T> before)`
  Returns a composed function that first applies the `before` function to its input, and then applies this function to the result.
  `static <T> Function<T,T>`
  `identity()`
  Returns a function that always returns its input argument.

- ## Method Details

  - ### apply

    R apply(T t)
    Applies this function to the given argument.
    Parameters:
      `t` - the function argument
    Returns:
      the function result

  - ### compose

    default <V> Function<V,R> compose(Function<? super V, ? extends T> before)
    Returns a composed function that first applies the `before` function to its input, and then applies this function to the result. If evaluation of either function throws an exception, it is relayed to the caller of the composed function.
    Type Parameters:
      `V` - the type of input to the `before` function, and to the composed function
    Parameters:
      `before` - the function to apply before this function is applied
    Returns:
      a composed function that first applies the `before` function and then applies this function
    Throws:
      `NullPointerException` - if before is null
    See Also:
      - `andThen(Function)`

  - ### andThen

    default <V> Function<T,V> andThen(Function<? super R, ? extends V> after)
    Returns a composed function that first applies this function to its input, and then applies the `after` function to the result. If evaluation of either function throws an exception, it is relayed to the caller of the composed function.
    Type Parameters:
      `V` - the type of output of the `after` function, and of the composed function
    Parameters:
      `after` - the function to apply after this function is applied
    Returns:
      a composed function that first applies this function and then applies the `after` function
    Throws:
      `NullPointerException` - if after is null
    See Also:
      - `compose(Function)`

  - ### identity

    static <T> Function<T,T> identity()
    Returns a function that always returns its input argument.
    Type Parameters:
      `T` - the type of the input and output objects to the function
    Returns:
      a function that always returns its input argument