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Java 流分组

智云科技 发布于 2023年8月17日 评论关闭 阅读(84)

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”子句。

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, Collector<? super T,A,D> downstream)

groupingBy(Function<? super T,? extends K> classifier, Supplier<M> mapFactory, Collector<? super T,A,D> downstream)

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) {
 Map<Employee.Gender, Long> countByGender = Employee.persons()
 .stream()
 .collect(Collectors.groupingBy(Employee::getGender, Collectors.counting())); 
 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 dob,
 double income) {
 this.id = id;
 this.name = name;
 this.gender = gender;
 this.dob = dob;
 this.income = income;
 }
 public Gender getGender() {
 return gender;
 }
 public static List<Employee> persons() {
 Employee p1 = new Employee(1, "Jake", Gender.MALE, LocalDate.of(1971,
 Month.JANUARY, 1), 2343.0);
 Employee p2 = new Employee(2, "Jack", Gender.MALE, LocalDate.of(1972,
 Month.JULY, 21), 7100.0);
 Employee p3 = new Employee(3, "Jane", Gender.FEMALE, LocalDate.of(1973,
 Month. MAY , 29), 5455.0);
 Employee p4 = new Employee(4, "Jode", Gender.MALE, LocalDate.of(1974,
 Month.OCTOBER, 16), 1800.0);
 Employee p5 = new Employee(5, "Jeny", Gender.FEMALE, LocalDate.of(1975,
 Month.DECEMBER, 13), 1234.0);
 Employee  p6  = new Employee(6, "Jason", Gender.MALE, LocalDate.of(1976,
 Month.JUNE, 9), 3211.0);
 List<Employee> persons = Arrays.asList(p1, p2, p3, p4, p5, p6);
 return persons;
 }
}

上面的代码生成以下结果。

例2

以下代码显示如何按性别对person的名称进行分组: 

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) {
 Map<Employee.Gender, String> namesByGender = Employee.persons()
 .stream()
 .collect(Collectors.groupingBy(Employee::getGender, 
 Collectors.mapping(Employee::getName, Collectors.joining(", "))));
 System.out.println(namesByGender);
 }
}
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 dob,
 double income) {
 this.id = id;
 this.name = name;
 this.gender = gender;
 this.dob = dob;
 this.income = income;
 }
 public String getName() {
 return name;
 }
 public Gender getGender() {
 return gender;
 }
 public static List<Employee> persons() {
 Employee p1 = new Employee(1, "Jake", Gender.MALE, LocalDate.of(1971,
 Month.JANUARY, 1), 2343.0);
 Employee p2 = new Employee(2, "Jack", Gender.MALE, LocalDate.of(1972,
 Month.JULY, 21), 7100.0);
 Employee p3 = new Employee(3, "Jane", Gender.FEMALE, LocalDate.of(1973,
 Month.MAY, 29), 5455.0);
 Employee p4 = new Employee(4, "Jode", Gender.MALE, LocalDate.of(1974,
 Month.OCTOBER, 16), 1800.0);
 Employee p5 = new Employee(5, "Jeny", Gender.FEMALE, LocalDate.of(1975,
 Month.DECEMBER, 13), 1234.0);
 Employee p6 = new Employee(6, "Jason", Gender.MALE, LocalDate.of(1976,
 Month.JUNE, 9), 3211.0);
 List<Employee> persons = Arrays.asList(p1, p2, p3, p4, p5, p6);
 return persons;
 }
}

上面的代码生成以下结果。

例3

以下代码显示了如何在列表中收集员工姓名。 

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) {
 Map<Employee.Gender, List<String>> namesByGender = 
 Employee.persons()
 .stream()
 .collect(Collectors.groupingBy(Employee::getGender, 
 Collectors.mapping(Employee::getName, Collectors.toList())));
 System.out.println(namesByGender);
 }
}
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 dob,
 double income) {
 this.id = id;
 this.name = name;
 this.gender = gender;
 this.dob = dob;
 this.income = income;
 }
 public String getName() {
 return name;
 }
 public Gender getGender() {
 return gender;
 }
 public static List<Employee> persons() {
 Employee p1 = new Employee(1, "Jake", Gender.MALE, LocalDate.of(1971,
 Month.JANUARY, 1), 2343.0);
 Employee p2 = new Employee(2, "Jack", Gender.MALE, LocalDate.of(1972,
 Month.JULY, 21), 7100.0);
 Employee p3 = new Employee(3, "Jane", Gender.FEMALE, LocalDate.of(1973,
 Month.MAY, 29), 5455.0);
 Employee p4 = new Employee(4, "Jode", Gender.MALE, LocalDate.of(1974,
 Month.OCTOBER, 16), 1800.0);
 Employee p5 = new Employee(5, "Jeny", Gender.FEMALE, LocalDate.of(1975,
 Month.DECEMBER, 13), 1234.0);
 Employee p6 = new Employee(6, "Jason", Gender.MALE, LocalDate.of(1976,
 Month.JUNE, 9), 3211.0);
 List<Employee> persons = Arrays.asList(p1, p2, p3, p4, p5, p6);
 return persons;
 }
}

上面的代码生成以下结果。

例4

以下代码按性别分组人员。在每个性别组中，它创建另一个组他们出生的月份，并列出了在这个组出生的人的名字。 

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) {
 Map personsByGenderAndDobMonth
 = Employee.persons()
 .stream()
 .collect(Collectors.groupingBy(Employee::getGender, 
 Collectors.groupingBy(p -> p.getDob().getMonth(), 
 Collectors.mapping(Employee::getName,Collectors.joining(", ")))));
 System.out.println(personsByGenderAndDobMonth);
 }
}
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 dob,
 double income) {
 this.id = id;
 this.name = name;
 this.gender = gender;
 this.dob = dob;
 this.income = income;
 }
 public String getName() {
 return name;
 }
 public Gender getGender() {
 return gender;
 }
 public void setGender(Gender gender) {
 this.gender = gender;
 }
 public LocalDate getDob() {
 return dob;
 }
 public static List<Employee> persons() {
 Employee p1 = new Employee(1, "Jake", Gender.MALE, LocalDate.of(1971,
 Month.JANUARY, 1), 2343.0);
 Employee p2 = new Employee(2, "Jack", Gender.MALE, LocalDate.of(1972,
 Month.JULY, 21), 7100.0);
 Employee p3 = new Employee(3, "Jane", Gender.FEMALE, LocalDate.of(1973,
 Month.MAY, 29), 5455.0);
 Employee p4 = new Employee(4, "Jode", Gender.MALE, LocalDate.of(1974,
 Month.OCTOBER, 16), 1800.0);
 Employee p5 = new Employee(5, "Jeny", Gender.FEMALE, LocalDate.of(1975,
 Month.DECEMBER, 13), 1234.0);
 Employee p6 = new Employee(6, "Jason", Gender.MALE, LocalDate.of(1976,
 Month.JUNE, 9), 3211.0);
 List<Employee> persons = Arrays.asList(p1, p2, p3, p4, p5, p6);
 return persons;
 }
}

上面的代码生成以下结果。

例5

以下代码显示如何按性别分组汇总收入统计。 

import java.time.LocalDate;
import java.time.Month;
import java.util.Arrays;
import java.util.DoubleSummaryStatistics;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
public class Main {
 public static void main(String[] args) {
 Map<Employee.Gender, DoubleSummaryStatistics> incomeStatsByGender = Employee.persons()
 .stream()
 .collect(Collectors.groupingBy(Employee::getGender, Collectors.summarizingDouble(Employee::getIncome)));
 System.out.println(incomeStatsByGender);
 }
}
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 dob,
 double income) {
 this.id = id;
 this.name = name;
 this.gender = gender;
 this.dob = dob;
 this.income = income;
 }
 public String getName() {
 return name;
 }
 public Gender getGender() {
 return gender;
 }
 public double getIncome() {
 return income;
 }
 public static List<Employee> persons() {
 Employee p1 = new Employee(1, "Jake", Gender.MALE, LocalDate.of(1971,
 Month.JANUARY, 1), 2343.0);
 Employee p2 = new Employee(2, "Jack", Gender.MALE, LocalDate.of(1972,
 Month.JULY, 21), 7100.0);
 Employee p3 = new Employee(3, "Jane", Gender.FEMALE, LocalDate.of(1973,
 Month.MAY, 29), 5455.0);
 Employee p4 = new Employee(4, "Jode", Gender.MALE, LocalDate.of(1974,
 Month.OCTOBER, 16), 1800.0);
 Employee p5 = new Employee(5, "Jeny", Gender.FEMALE, LocalDate.of(1975,
 Month.DECEMBER, 13), 1234.0);
 Employee p6 = new Employee(6, "Jason", Gender.MALE, LocalDate.of(1976,
 Month.JUNE, 9), 3211.0);
 List<Employee> persons = Arrays.asList(p1, p2, p3, p4, p5, p6);
 return persons;
 }
}

上面的代码生成以下结果。

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文章来源：智云一二三科技

文章标题：Java 流分组

文章地址：https://www.zhihuclub.com/180019.shtml

关于作者: 智云科技

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