Java 流分组.pdfVIP

Java 流分组 Java流分组 Collectors类中的groupingBy()⽅法返回⼀个收集器，⽤于在将数据收集到Map之前对 数据进⾏分组。 它类似于SQL 中的“group by”⼦ 。 groupingBy()⽅法是重载的，它有三个版本: groupingBy(Function? super T,? extends K classifier) groupingBy(Function? super T,? extends K classifier, Collect groupingBy(Function? super T,? extends K classifier, Supplier 在前两个版本中，收集器负责为您创建Map对象。第三个版本允许我们指定⼀个⽤作 ⼯⼚的Supplier来获取Map对象。 classifier 函数⽣成地图中的键。 collector 对与每个键相关联的值执⾏简化 操作。 以下代码显⽰了如何按性别和计算每个组中的⼈数。 Collectors类的counting()⽅法返回要计数的Collector流中的元素数。 import java.time.LocalDate; import java.time.Month; import java.util.Arrays; import java.util.List; import java.util.Map; import java.util.stream.Collectors; public class Main { public static void main(String[] args) { MapEmployee.Gender, Long countByGender = Employee.persons() .stream() .collect(Collectors.groupingBy(Employee::getGender, Collec System.out.println(countByGender); } } class Employee { public static enum Gender { MALE, FEMALE } private long id; private String name; private Gender gender; private LocalDate dob; private double income; public Employee(long id, String name, Gender gender, LocalDate d double income) { this.id = id; = name; this.gender = gender; this.dob = dob; this.income = income; } public Gender getGender() { return gender; } public static ListEmployee persons() { Employee p1 = ne Employee(1, Jake, Gender.MALE, LocalDate.o Month.JANUARY, 1), 2343.0); Employee p2 = ne Employee(2, Jack, Gender.MALE, LocalDate.o Month.JULY, 21), 7100.0); Employee p3 = ne Employee(3, Jane, Gender.FEMALE, LocalDate Month.MAY, 29), 5455.0); Employee p4 = ne Employee(4, Jode, Gender.MALE, LocalDate.o Month.OCTOBER, 16), 1800.0); Employee p5 = ne Employee(5, Jeny, Gender.FEMALE, LocalDate Month.DECEMBER, 13), 1234 .0); Employee p6 =