Java8 Stream详解~归约(reduce)

归约，也称缩减，顾名思义，是把一个流缩减成一个值，能实现对集合求和、求乘积和求最值操作。

「案例一：求Integer集合的元素之和、乘积和最大值。」

public class StreamTest {
 public static void main(String[] args) {
  List<Integer> list = Arrays.asList(1, 3, 2, 8, 11, 4);
  // 求和方式1
  Optional<Integer> sum = list.stream().reduce((x, y) -> x + y);
  // 求和方式2
  Optional<Integer> sum2 = list.stream().reduce(Integer::sum);
  // 求和方式3
  Integer sum3 = list.stream().reduce(0, Integer::sum);
  
  // 求乘积
  Optional<Integer> product = list.stream().reduce((x, y) -> x * y);

  // 求最大值方式1
  Optional<Integer> max = list.stream().reduce((x, y) -> x > y ? x : y);
  // 求最大值写法2
  Integer max2 = list.stream().reduce(1, Integer::max);

  System.out.println("list求和：" + sum.get() + "," + sum2.get() + "," + sum3);
  System.out.println("list求积：" + product.get());
  System.out.println("list求和：" + max.get() + "," + max2);
 }
}

「案例二：求所有员工的工资之和和最高工资。」

public class StreamTest {
 public static void main(String[] args) {
  List<Person> personList = new ArrayList<Person>();
  personList.add(new Person("Tom", 8900, 23, "male", "New York"));
  personList.add(new Person("Jack", 7000, 25, "male", "Washington"));
  personList.add(new Person("Lily", 7800, 21, "female", "Washington"));
  personList.add(new Person("Anni", 8200, 24, "female", "New York"));
  personList.add(new Person("Owen", 9500, 25, "male", "New York"));
  personList.add(new Person("Alisa", 7900, 26, "female", "New York"));

  // 求工资之和方式1：
  Optional<Integer> sumSalary = personList.stream().map(Person::getSalary).reduce(Integer::sum);
  // 求工资之和方式2：
  Integer sumSalary2 = personList.stream().reduce(0, (sum, p) -> sum += p.getSalary(),
    (sum1, sum2) -> sum1 + sum2);
  // 求工资之和方式3：
  Integer sumSalary3 = personList.stream().reduce(0, (sum, p) -> sum += p.getSalary(), Integer::sum);

  // 求最高工资方式1：
  Integer maxSalary = personList.stream().reduce(0, (max, p) -> max > p.getSalary() ? max : p.getSalary(),
    Integer::max);
  // 求最高工资方式2：
  Integer maxSalary2 = personList.stream().reduce(0, (max, p) -> max > p.getSalary() ? max : p.getSalary(),
    (max1, max2) -> max1 > max2 ? max1 : max2);

  System.out.println("工资之和：" + sumSalary.get() + "," + sumSalary2 + "," + sumSalary3);
  System.out.println("最高工资：" + maxSalary + "," + maxSalary2);
 }
}