Java 流聚合

Java流 – Java流聚合

要计算数字流上的和，max，min，average等，我们可以将非数字流映射到数值流类型(IntStream，LongStream或DoubleStream)，然后使用专门的方法。

以下代码计算收入的总和。 mapToDouble()方法将 Stream<Employee> 转换为 DoubleStream 。对DoubleStream调用sum()方法。

import java.time.LocalDate;
import java.time.Month;
import java.util.Arrays;
import java.util.List;

public class Main {
  public static void main(String[] args) {
    double totalIncome = Employee.persons()
        .stream()
        .mapToDouble(Employee::getIncome)
        .sum(); 
    System.out.println("Total Income:  "  + totalIncome);
  }
}

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 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。

例2

要获取流的最小值和最大值，使用特定流的min()和max()方法。

import java.time.LocalDate;
import java.time.Month;
import java.util.Arrays;
import java.util.Comparator;
import java.util.List;
import java.util.Optional;

public class Main {
  public static void main(String[] args) {
    Optional<Employee> person = Employee.persons().stream()
        .max(Comparator.comparingDouble(Employee::getIncome));

    if (person.isPresent()) {
      System.out.println("Highest earner: " + person.get());
    } else {
      System.out.println("Could not  get   the   highest earner.");
    }
  }
}

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 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。

例3

下面的代码打印了最高的收入员工列表使用DoubleStream的max()方法:

import java.time.LocalDate;
import java.time.Month;
import java.util.Arrays;
import java.util.List;
import java.util.OptionalDouble;

public class Main {
  public static void main(String[] args) {
    OptionalDouble income = 
       Employee.persons()
               .stream()
               .mapToDouble(Employee::getIncome).max();

    if (income.isPresent()) {
      System.out.println("Highest income:   " + income.getAsDouble());
    } else {
      System.out.println("Could not  get   the   highest income.");
    }
  }
}

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 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。