[移动开发]java 8 之函数编程BiFunction

文章对java 8 BiFunction接口使用及源码做解释，在java 8 后interface 支持方法体，但是需要用default 修饰

1. 我们先来看下源码

import java.util.Objects;

/**
 * Represents a function that accepts two arguments and produces a result.
 * This is the two-arity specialization of {@link Function}.
 *
 * <p>This is a <a href="package-summary.html">functional interface</a>
 * whose functional method is {@link #apply(Object, Object)}.
 *
 * @param <T> the type of the first argument to the function
 * @param <U> the type of the second argument to the function
 * @param <R> the type of the result of the function
 *
 * @see Function
 * @since 1.8
 */
@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));
    }
}

1.@FunctionalInterface 函数接口注解
2. apply 函数参数说明：将此函数应用于给定的参数。
参数：T– 第一个函数参数, U– 第二个函数参数 返回：函数结果，R返回函数执行完结果
3. andThen先来解释下入参：
R是第一个函数执行完的结果参数，V返回参数
该函数先执行R函数运行完结果在执行函数after函数

return (T t, U u) -> after.apply(apply(t, u));

2. 来个举例：

import java.util.function.BiFunction;
import java.util.function.Function;

/**
 * @author zhaoyy
 * @version 1.0
 * @description TODO
 * @date 2022/4/14
 **/
public class BiFunctionTest {

    public static void main(String[] args) {
        System.out.println("--------------Integer----------------------------");
        Integer r = consumer(2, 3, (x, y) -> x * y);
        System.out.println(r);
        System.out.println("-------------字符串-----------------------------");
        String r1 = consumer(new StringBuffer("sfda"), new StringBuffer("TTT"), (x, y) -> x.append(y).toString());
        System.out.println(r1);
        System.out.println("----------------AndThen--------------------------");
        Integer r2 = consumerAndThen(2, 3, (x, y) -> x * y, y -> y * y);
        System.out.println(r2);
        System.out.println("----------------AndThen-字符串-------------------------");
        String r3 = consumerAndThen("abc", "def", (x, y) -> x + y, y -> y + y);
        System.out.println(r3);
    }

    public static <D, T, R> R consumer(D a, T b, BiFunction<D, T, R> function) {
        return function.apply(a, b);
    }

    public static <D, T, R, V> V consumerAndThen(D a, T b, BiFunction<D, T, R> function, Function<R, V> after) {
        return function.andThen(after).apply(a, b);
    }
}

输出结果：

--------------Integer----------------------------
6
-------------字符串-----------------------------
sfdaTTT
----------------AndThen--------------------------
36
----------------AndThen-字符串-------------------------
abcdefabcdef

输出解释：
第一次输出：x *y =2 * 3=6 输出结果：6
第二次输出：使用字符串，x+y=afds+TTT=afdsTTT 输出结果：afdsTTT
第三次输出：使用方法 addThen , x * y=R ，R * R =V 输出结果：36 。计算步骤（2 * 3=6， 6 * 6=36）
第四次输出：同第三输出步骤一样，结果输出abcdefabcdef

备注：方便扩展使用，举例使用乏类模式出入参数，你也可以使用具体参数类型测试

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