Java 流分區(qū)

Java流 - Java流分區(qū)

分區(qū)是分組的一種特殊情況。

分組數(shù)據(jù)基于從函數(shù)返回的鍵。可能有很多組。

分區(qū)僅處理基于謂詞的兩個(gè)組。評(píng)估為true的值為一個(gè)組，false為另一個(gè)組。

partitioningBy()方法，它收集映射中的數(shù)據(jù)，其鍵的總是布爾類型，在兩個(gè)版本中重載。

partitioningBy(Predicate<? super T> predicate)
partitioningBy(Predicate<? super T> predicate,  Collector<? super T,A,D> downstream)

例子

第一個(gè)版本的partitioningBy()方法返回基于謂詞執(zhí)行分區(qū)的收集器。

以下代碼顯示如何根據(jù)性別對(duì)employee進(jìn)行分區(qū)。

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<Boolean,  List<Employee>>  partionedByMaleGender = 
    Employee.persons()
            .stream()
            .collect(Collectors.partitioningBy(Employee::isMale)); 
    System.out.println(partionedByMaleGender);
  }
}

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 boolean isMale() {
    return this.gender == Gender.MALE;
  }
  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;
  }

  @Override
  public String toString() {
    String str = String.format("(%s, %s,  %s,  %s,  %.2f)\n", id, name, gender,
        dob, income);
    return str;
  }
}

上面的代碼生成以下結(jié)果。

例2

第二個(gè)版本允許我們指定另一個(gè)可以執(zhí)行還原操作的收集器。

以下代碼顯示如何按性別分區(qū)員工，并以逗號(hào)分隔的字符串收集其名稱:

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<Boolean,String> partionedByMaleGender = Employee.persons()
        .stream()
        .collect(Collectors.partitioningBy(Employee::isMale, 
                   Collectors.mapping(Employee::getName, Collectors.joining(", "))));
    System.out.println(partionedByMaleGender);
  }
}

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 boolean isMale() {
    return this.gender == Gender.MALE;
  }
  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。