Optional<T> reduce(BinaryOperator<T> accumulator);

T reduce(T identity, BinaryOperator<T> accumulator);

<U> U reduce(U identity,
                 BiFunction<U, ? super T, U> accumulator,
                 BinaryOperator<U> combiner);

List<Person> javaProgrammers = new ArrayList<Person>() {
        {
            add(new Person("Elsdon", "Jaycob", "Java programmer", "male", 2000, 18));
            add(new Person("Tamsen", "Brittany", "Java programmer", "female", 2371, 55));
            add(new Person("Floyd", "Donny", "Java programmer", "male", 3322, 25));
            add(new Person("Sindy", "Jonie", "Java programmer", "female", 35020, 15));
            add(new Person("Vere", "Hervey", "Java programmer", "male", 2272, 25));
            add(new Person("Maude", "Jaimie", "Java programmer", "female", 2057, 87));
            add(new Person("Shawn", "Randall", "Java programmer", "male", 3120, 99));
            add(new Person("Jayden", "Corrina", "Java programmer", "female", 345, 25));
            add(new Person("Palmer", "Dene", "Java programmer", "male", 3375, 14));
            add(new Person("Addison", "Pam", "Java programmer", "female", 3426, 20));
        }
    };

 public interface BinaryOperator<T> extends BiFunction<T,T,T> {
      public static <T> BinaryOperator<T> minBy(Comparator<? super T> comparator) {
        Objects.requireNonNull(comparator);
        return (a, b) -> comparator.compare(a, b) <= 0 ? a : b;
    }
    public static <T> BinaryOperator<T> maxBy(Comparator<? super T> comparator) {
        Objects.requireNonNull(comparator);
        return (a, b) -> comparator.compare(a, b) >= 0 ? a : b;
    }
}

 @FunctionalInterface
public interface BiFunction<T, U, R> {
    R apply(T t, U u);//接收兩個(gè)參數(shù) t 和 u, 返回 R
}

     @Test
    public void Test() {
        int asInt = javaProgrammers.stream()
                                    .mapToInt(Person::getSalary)//返回?cái)?shù)值流，減少拆箱封箱操作，避免占用內(nèi)存  IntStream
                                    .reduce((x, y) -> x += y)// int
                                    .getAsInt(); //return int
        System.out.printf("方式一   reduce(BinaryOperator<T> accumulator)   求薪資測(cè)試結(jié)果："+asInt);
 
        /*解析：
             1. reduce(BinaryOperator<T> accumulator)    reduce方法接受一個(gè)函數(shù)，這個(gè)函數(shù)有兩個(gè)參數(shù)
             2. 第一個(gè)參數(shù)是上次函數(shù)執(zhí)行的返回值（也稱為中間結(jié)果），第二個(gè)參數(shù)是stream中的元素，這個(gè)函數(shù)把這兩個(gè)值相加，得到的和會(huì)被賦值給下次執(zhí)行這個(gè)函數(shù)的第一個(gè)參數(shù)
         *注意：
             1.第一次執(zhí)行的時(shí)候第一個(gè)參數(shù)的值是Stream的第一個(gè)元素，第二個(gè)參數(shù)是Stream的第二個(gè)元素
             2.方法返回值類型是Optional
         */
    }

     @Test
    public void test1(){
        int reduce = javaProgrammers.stream().mapToInt(Person::getSalary).reduce(10000, (x, y) -> x += y);
        System.out.printf("方式二  reduce(T identity, BinaryOperator<T> accumulator)   求薪資測(cè)試結(jié)果："+reduce);
 
        /*注意：
         *      1.與方式一相比設(shè)置了累加器的初始值，參數(shù)一（x）則不再是Stream中的第一個(gè)數(shù)據(jù)而是設(shè)置的初始值（10000）其他相同
         */
    }

@Test
    public void test2() {
        ArrayList<Integer> accResult_ = Stream.of(1, 2, 3, 4)
                //第一個(gè)參數(shù)，初始值為ArrayList
                .reduce(new ArrayList<Integer>(),
                        //第二個(gè)參數(shù)，實(shí)現(xiàn)了BiFunction函數(shù)式接口中apply方法，并且打印BiFunction
                        new BiFunction<ArrayList<Integer>, Integer, ArrayList<Integer>>() {
                            @Override
                            public ArrayList<Integer> apply(ArrayList<Integer> acc, Integer item) {
 
                                acc.add(item);
                                System.out.println("item: " + item);
                                System.out.println("acc+ : " + acc);
                                System.out.println("BiFunction");
                                return acc;
                            }
                            //第三個(gè)參數(shù)---參數(shù)的數(shù)據(jù)類型必須為返回?cái)?shù)據(jù)類型，改參數(shù)主要用于合并多個(gè)線程的result值
                            // （Stream是支持并發(fā)操作的，為了避免競(jìng)爭(zhēng)，對(duì)于reduce線程都會(huì)有獨(dú)立的result）
                        }, new BinaryOperator<ArrayList<Integer>>() {
                            @Override
                            public ArrayList<Integer> apply(ArrayList<Integer> acc, ArrayList<Integer> item) {
                                System.out.println("BinaryOperator");
                                acc.addAll(item);
                                System.out.println("item: " + item);
                                System.out.println("acc+ : " + acc);
                                System.out.println("--------");
                                return acc;
                            }
                        });
        System.out.println("accResult_: " + accResult_);
 
        System.out.println("------------------lambda優(yōu)化代碼-----------------");
 
        ArrayList<Integer> newList = new ArrayList<>();
 
        ArrayList<Integer> accResult_s = Stream.of(1,2,3,4)
                .reduce(newList,
                        (acc, item) -> {
                            acc.add(item);
                            System.out.println("item: " + item);
                            System.out.println("acc+ : " + acc);
                            System.out.println("BiFunction");
                            return acc;
                        }, (acc, item) -> null);
        System.out.println("accResult_s: " + accResult_s);
    }