Java 流分組

Java流分組

Collectors類中的groupingBy()方法返回一個(gè)收集器，用于在將數(shù)據(jù)收集到Map之前對(duì)數(shù)據(jù)進(jìn)行分組。 它類似于SQL中的“group by”子句。

groupingBy()方法是重載的，它有三個(gè)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)

在前兩個(gè)版本中，收集器負(fù)責(zé)為您創(chuàng)建Map對(duì)象。第三個(gè)版本允許我們指定一個(gè)用作工廠的Supplier來獲取Map對(duì)象。

classifier 函數(shù)生成地圖中的鍵。 collector 對(duì)與每個(gè)鍵相關(guān)聯(lián)的值執(zhí)行簡化操作。

以下代碼顯示了如何按性別和計(jì)算每個(gè)組中的人數(shù)。

Collectors類的counting()方法返回要計(jì)數(shù)的Collector流中的元素?cái)?shù)。

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;
  }
}

上面的代碼生成以下結(jié)果。

例2

以下代碼顯示如何按性別對(duì)person的名稱進(jìn)行分組:

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;
  }
}

上面的代碼生成以下結(jié)果。

例3

以下代碼顯示了如何在列表中收集員工姓名。

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;
  }
}

上面的代碼生成以下結(jié)果。

例4

以下代碼按性別分組人員。在每個(gè)性別組中，它創(chuàng)建另一個(gè)組他們出生的月份，并列出了在這個(gè)組出生的人的名字。

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;
  }
}

上面的代碼生成以下結(jié)果。

例5

以下代碼顯示如何按性別分組匯總收入統(tǒng)計(jì)。

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;
  }
}

上面的代碼生成以下結(jié)果。