摘要：函數(shù)式接口進(jìn)階與默認(rèn)方法詳解上一篇我們快速的借助示例演示了的簡(jiǎn)單應(yīng)用，體會(huì)到了使用對(duì)集合處理的便捷和其與函數(shù)式接口密不可分的關(guān)系，所以為了更高效的使用，有必要更熟練的掌握函數(shù)式接口。

Java8-5-函數(shù)式接口進(jìn)階與默認(rèn)方法詳解
上一篇我們快速的借助示例演示了stream api的簡(jiǎn)單應(yīng)用，體會(huì)到了使用stream api對(duì)集合處理的便捷和其與函數(shù)式接口密不可分的關(guān)系，所以為了更高效的使用stream api，有必要更熟練的掌握函數(shù)式接口。Java8中內(nèi)置了大量的函數(shù)式接口，接下來(lái)我們選擇一些比較常用的一起學(xué)習(xí)下。

Function接口
在之前的文章中，我們簡(jiǎn)單介紹過(guò)Function接口中apply方法的應(yīng)用，除了apply這個(gè)抽象方法，F(xiàn)unction接口中還內(nèi)置了兩個(gè)比較常用的默認(rèn)方法（接口中增加的有具體實(shí)現(xiàn)的方法，擴(kuò)展了接口功能，子類(lèi)默認(rèn)會(huì)繼承該實(shí)現(xiàn)），看下Function接口源碼

/**
 * Represents a function that accepts one argument and produces a result.
 *
 * 
This is a functional interface
 * whose functional method is {@link #apply(Object)}.
 *
 * @param  the type of the input to the function
 * @param  the type of the result of the function
 *
 * @since 1.8
 */
@FunctionalInterface
public interface Function {

    R apply(T t);

    /**
     * @return a composed function that first applies the {@code before}
     * function and then applies this function
     */
    default  Function compose(Function before) {
        Objects.requireNonNull(before);
        return (V v) -> apply(before.apply(v));
    }
    /**
     * @return a composed function that first applies this function and then
     * applies the {@code after} function
     */
    default  Function andThen(Function after) {
        Objects.requireNonNull(after);
        return (T t) -> after.apply(apply(t));
    }
    /**
     * 省略
     */
}

Function接口定義中有兩個(gè)泛型，按著接口文檔說(shuō)明第一個(gè)泛型是輸入類(lèi)型，第二泛型是結(jié)果類(lèi)型。
compose方法接收一個(gè)Function參數(shù)before，該方法說(shuō)明是返回一個(gè)組合的函數(shù)，首先會(huì)應(yīng)用before，然后應(yīng)用當(dāng)前對(duì)象，換句話(huà)說(shuō)就是先執(zhí)行before對(duì)象的apply，再執(zhí)行當(dāng)前對(duì)象的apply，將兩個(gè)執(zhí)行邏輯串起來(lái)。
andThen方法接收一個(gè)Function參數(shù)after，與compose方法相反，它是先執(zhí)行當(dāng)前對(duì)象的apply方法，再執(zhí)行after對(duì)象的方法?？匆粋€(gè)應(yīng)用示例

public class FunctionTest {
    public static void main(String[] args) {
        FunctionTest functionTest = new FunctionTest();
        System.out.println(functionTest.compute1(5,i -> i * 2,i -> i * i));//50
        System.out.println(functionTest.compute2(5,i -> i * 2,i -> i * i));//100
    }

    public int compute1(int i, Function after,Function before){
        return after.compose(before).apply(i);
    }

    public int compute2(int i, Function before,Function after){
        return before.andThen(after).apply(i);
    }
}

定義了compute1和compute2兩個(gè)方法，compute1方法第一個(gè)參數(shù)是要計(jì)算的數(shù)據(jù)，第二個(gè)參數(shù)是后執(zhí)行的函數(shù)，第一個(gè)是先執(zhí)行的函數(shù)，因?yàn)檩斎胼敵龆际菙?shù)字類(lèi)型，所以泛型都指定為Integer類(lèi)型，通過(guò)after.compose(before);將兩個(gè)函數(shù)串聯(lián)起來(lái)然后執(zhí)行組合后的Funtion方法apply(i)。當(dāng)調(diào)用compute1(5,i -> i 2,i -> i i)時(shí)，先平方再乘以2所以結(jié)果是50。而compute2方法對(duì)兩個(gè)Function的調(diào)用正好相反，所以結(jié)果是100。

BiFunction接口
接下來(lái)繼續(xù)看下另一個(gè)很常用的函數(shù)式接口BiFunction

/**
 * This is the two-arity specialization of {@link Function}.
 * @param  the type of the first argument to the function
 * @param  the type of the second argument to the function
 * @param  the type of the result of the function
 *
 * @see Function
 * @since 1.8
 */
@FunctionalInterface
public interface BiFunction {

    /**
     * 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  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  BiFunction andThen(Function after) {
        Objects.requireNonNull(after);
        return (T t, U u) -> after.apply(apply(t, u));
    }
}

public class BiFunctionTest {
    public static void main(String[] args) {
        BiFunctionTest2 biFunctionTest2 = new BiFunctionTest2();
        System.out.println(biFunctionTest2.compute(4,5,(a,b) -> a * b,a -> a * 2));
    }

    public int compute(int a, int b, BiFunction biFunction,
                       Function function){
        return biFunction.andThen(function).apply(a,b);
    }
}