四时宝库

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。