介绍一下Java的BiFunction

@FunctionalInterface
public interface BiFunction<T, U, R> {

    /**
     * Applies this function to the given arguments.
     *
     * @param t the first function argument
     * @param u the second function argument
     * @return the function result
     */
    R apply(T t, U u);

    /**
     * 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
     */
    default <V> BiFunction<T, U, V> andThen(Function<? super R, ? extends V> after) {
        Objects.requireNonNull(after);
        return (T t, U u) -> after.apply(apply(t, u));
    }
}

BiFunction<String, String, String> biDemo = (s1, s2) -> s1 + s2;
System.out.println(biDemo.apply("hello ", "world"));

Function<String, Integer> funcDemo = String::length;
System.out.println(biDemo.andThen(funcDemo).apply("hello ", "world"));

public <T1, T2, R1, R2> R2 combination(T1 t1, T2 t2, BiFunction<T1, T2, R1> biFunction, Function<R1, R2> function) {
    return biFunction.andThen(function).apply(t1, t2);
}