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java流分组求和 Java 流分组

2023-03-07 05:20 Java教程

java流分组求和 Java 流分组

java流分组求和

Java流分组求和是一种常见的数据处理任务，它可以帮助我们快速地对数据进行分组，并计算出每个分组的总和。它可以用于多种应用场景，例如财务分析、数据可视化、机器学习等。

Java 8中引入了流API，使得我们能够使用函数式编程来实现流分组求和。首先，我们需要将原始数据集转换为Stream对象：

List<Data> dataList = ...;  // 原始数据集
Stream<Data> dataStream = dataList.stream();  // 转为Stream对象

然后，我们可以使用collectors.groupingBy()方法将Stream对象根据特定的字段进行分组：

Map<String, List<Data>> groupedData = dataStream.collect(Collectors.groupingBy(Data::getGroup));

最后，我们可以使用mapToLong()方法将分组后的数据映射为LongStream对象，并使用sum()方法来计算出所有分组的总和：

Map<String, Long> sumMap = groupedData.entrySet().stream()  // 返回 Stream 对象   .collect(Collectors.toMap(   // 返回 Map 对象   Map.Entry::getKey,   e -> e.getValue().stream().mapToLong(Data::getValue).sum()));

上述代码中的sumMap就是所有分组的总和。通过使用Java 8中新引入的流API，我们能够快速地实现流分组求和功能。此外，Java 8中还引入了一些新特性来优化代码书写体验（例如Lambda表达式、Optional API、断言、时间/日期API 等）。因此，在Java 8之后开发者可以大大减少代码量并提升代码可读性。

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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