package lambda; import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors; public class LambdaMapReduce { private static List<User> users = Arrays.asList( new User(1, "张三", 12,User.Sex.MALE),  new User(2, "李四", 21, User.Sex.FEMALE),  new User(3,"王五", 32, User.Sex.MALE),  new User(4, "赵六", 32, User.Sex.FEMALE)); public static void main(String[] args) { reduceAvg(); reduceSum(); //与stream.reduce方法不同，Stream.collect修改现存的值，而不是每处理一个元素，创建一个新值 //获取所有男性用户的平均年龄 Averager averageCollect = users.parallelStream() .filter(p -> p.getGender() == User.Sex.MALE) .map(User::getAge) .collect(Averager::new, Averager::accept, Averager::combine); System.out.println("Average age of male members: " + averageCollect.average()); //获取年龄大于12的用户列表 List<User> list = users.parallelStream().filter(p -> p.age > 12) .collect(Collectors.toList()); System.out.println(list); //按性别统计用户数 Map<User.Sex, Integer> map = users.parallelStream().collect( Collectors.groupingBy(User::getGender, Collectors.summingInt(p -> 1))); System.out.println(map); //按性别获取用户名称 Map<User.Sex, List<String>> map2 = users.stream() .collect( Collectors.groupingBy( User::getGender, Collectors.mapping(User::getName, Collectors.toList()))); System.out.println(map2); //按性别求年龄的总和 Map<User.Sex, Integer> map3 = users.stream().collect( Collectors.groupingBy(User::getGender, Collectors.reducing(0, User::getAge, Integer::sum))); System.out.println(map3); //按性别求年龄的平均值 Map<User.Sex, Double> map4 = users.stream().collect( Collectors.groupingBy(User::getGender, Collectors.averagingInt(User::getAge))); System.out.println(map4); } // 注意，reduce操作每处理一个元素总是创建一个新值， // Stream.reduce适用于返回单个结果值的情况 //获取所有用户的平均年龄 private static void reduceAvg() { // mapToInt的pipeline后面可以是average,max,min,count,sum double avg = users.parallelStream().mapToInt(User::getAge)  .average().getAsDouble(); System.out.println("reduceAvg User Age: " + avg); } //获取所有用户的年龄总和 private static void reduceSum() { double sum = users.parallelStream().mapToInt(User::getAge) .reduce(0, (x, y) -> x + y); // 可以简写为.sum()
 System.out.println("reduceSum User Age: " + sum); }
}