Java之stream流求字段累计和的3种方法总结

public class Person {
    private String name;
    private int age;
 
    // 构造函数
    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }
 
    // Getter 和 Setter
    public String getName() {
        return name;
    }
 
    public int getAge() {
        return age;
    }
}

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
 
public class Main {
    public static void main(String[] args) {
        List<Person> people = Arrays.asList(
            new Person("Alice", 30),
            new Person("Bob", 25),
            new Person("Charlie", 35)
        );
        
        int totalAge = people.stream()
                             .mapToInt(Person::getAge) // 将Person对象转换为int流，提取年龄字段
                             .sum(); // 计算总和
        
        System.out.println("Total age: " + totalAge); // 输出Total age: 90
    }
}

int totalAge = people.stream()
                     .map(Person::getAge) // 将Person对象映射为年龄值
                     .reduce(0, Integer::sum); // 使用reduce方法累加年龄值，初始值为0
System.out.println("Total age: " + totalAge); // 输出Total age: 90

int totalAge = people.stream()
                     .collect(Collectors.summingInt(Person::getAge)); // 使用Collectors.summingInt收集器计算总和
System.out.println("Total age: " + totalAge); // 输出Total age: 90

BigDecimal reduceMorePayAmount = indemnityPayment.getRiPolicyPayeeIndemnitySet().stream()
                    .filter(riPolicyPayeeIndemnity -> riPolicyPayeeIndemnity.getIndemnityType().equals(IndemnityType.MORE_PAY.getValue()))
                    .map(RiPolicyPayeeIndemnity::getFeeAmount).reduce(BigDecimal.ZERO, BigDecimal::add);