一、方法介绍

表示接受一个参数并产生结果的函数。

参数类型

T - 函数输入的类型
R - 函数的结果类型

方法介绍

R apply(T t)
将此函数应用于给定的参数。
default Function<V, R> compose(Function<? super V, ? extends T> before)
返回一个组合函数，首先将before函数应用于其输入，然后将此函数应用于结果。如果任一函数的评估引发异常，则将其转发给组合函数的调用者。
default Function<T, V> andThen(Function<? super R, ? extends V> after)
返回一个组合函数，首先将此函数应用于其输入，然后将after函数应用于结果。如果任一函数的评估引发异常，则将其转发给组合函数的调用者。
static Function<T, T> identity()
返回一个总是返回其输入参数的函数。

源码

@FunctionalInterface
public interface Function<T, R> {

    /**
     * Applies this function to the given argument.
     *
     * @param t the function argument
     * @return the function result
     */
    R apply(T t);

    /**
     * Returns a composed function that first applies the {@code 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.
     *
     * @param <V> the type of input to the {@code before} function, and to the
     *           composed function
     * @param before the function to apply before this function is applied
     * @return a composed function that first applies the {@code before}
     * function and then applies this function
     * @throws NullPointerException if before is null
     *
     * @see #andThen(Function)
     */
    default <V> Function<V, R> compose(Function<? super V, ? extends T> before) {
        Objects.requireNonNull(before);
        return (V v) -> apply(before.apply(v));
    }

    /**
     * Returns a composed function that first applies this function to
     * its input, and then applies the {@code after} function to the result.
     * If evaluation of either function throws an exception, it is relayed to
     * the caller of the composed function.
     *
     * @param <V> the type of output of the {@code after} function, and of the
     *           composed function
     * @param after the function to apply after this function is applied
     * @return a composed function that first applies this function and then
     * applies the {@code after} function
     * @throws NullPointerException if after is null
     *
     * @see #compose(Function)
     */
    default <V> Function<T, V> andThen(Function<? super R, ? extends V> after) {
        Objects.requireNonNull(after);
        return (T t) -> after.apply(apply(t));
    }

    /**
     * Returns a function that always returns its input argument.
     *
     * @param <T> the type of the input and output objects to the function
     * @return a function that always returns its input argument
     */
    static <T> Function<T, T> identity() {
        return t -> t;
    }
}

二、demo

public class Test {
    public static void main(String[] args) throws Exception {
        Function<Integer, Integer> add = p -> p + 10;
        Integer result = add.apply(10);
        // 这里会输出 20，因为这个函数定义的操作时把参数加上 10 后返回
        System.out.println(result);


        Function<Integer, Integer> multiplyTen = a -> a * 10;
        Function<Integer, Integer> addTen = a -> a + 10;
        // 先增加 10，然后再乘 10，输出结果 110
        Function<Integer, Integer> addTenThenMultiplyTen = multiplyTen.compose(addTen);
        System.out.println(addTenThenMultiplyTen.apply(1));


        // 先乘 10，然后再加 10，输出结果 20
        Function<Integer, Integer> multiplyTenAddTenThen = multiplyTen.andThen(addTen);
        System.out.println(multiplyTenAddTenThen.apply(1));
    }


}