6.java8流的使用 stream的归约与收集

本文主要是介绍6.java8流的使用 stream的归约与收集，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

1.关键代码

代码类Stream12SpecificationsAndCollect

import com.netease.streamlearningbyjava.bean.Employee2;import org.junit.Test;import java.util.Arrays;
import java.util.DoubleSummaryStatistics;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Optional;
import java.util.Set;
import java.util.stream.Collectors;/*** 创建日期：2021/11/8 10:58** @author tony.sun* 类说明：规约与收集*/public class Stream12SpecificationsAndCollect {List<Employee2> employee2s = Arrays.asList(new Employee2("张三", 18, 9999.99, Employee2.Status.FREE),new Employee2("李四", 58, 5555.55, Employee2.Status.BUSY),new Employee2("王五", 26, 3333.33, Employee2.Status.VOCATION),new Employee2("赵六", 36, 6666.66, Employee2.Status.FREE),new Employee2("赵六", 36, 6666.66, Employee2.Status.FREE),new Employee2("田七", 12, 8888.88, Employee2.Status.BUSY));/*** 规约* reduce（T identity，BinaryOperator）/reduce(BinaryOperator)--可以将流中的元素反复结合起来，得到一个值。*/@Testpublic void test3(){List<Integer> list= Arrays.asList(1,2,3,4,5,6,7,8,9,10);Integer sum = list.stream().reduce(0, (x, y) -> x + y);//这个0是起始值，第一个值，y是上面的1,2,3,4,5,6,7,8,9,10，每次都加一次，加了10次System.out.println(sum);//55System.out.println("----------------------------------");Optional<Double> optional = employee2s.stream().map(Employee2::getSalary).reduce(Double::sum);System.out.println(optional);}/*** 收集* collect--将流转化为其他形式，接收一个Collector接口实现，用于给Stream中元素做汇总的方法*/@Testpublic void test4(){//提取老的list里面的值，变成一个新的list（只有name），然后打印List<String> list = employee2s.stream().map(Employee2::getName).collect(Collectors.toList());list.forEach(System.out::println);//结果：//张三//李四//王五//赵六//赵六//田七//去重System.out.println("--------------------------------");Set<String> set = employee2s.stream().map(Employee2::getName).collect(Collectors.toSet());set.forEach(System.out::println);//结果：//李四//张三//王五//赵六//田七/*** 其他格式*/HashSet<String> hashSet = employee2s.stream().map(Employee2::getName).collect(Collectors.toCollection(HashSet::new));hashSet.forEach(System.out::println);}@Testpublic void test5(){/*** 总数*/Long count = employee2s.stream().collect(Collectors.counting());System.out.println(count);/*** 平均值*/Double avg = employee2s.stream().collect(Collectors.averagingDouble(Employee2::getSalary));System.out.println(avg);/*** 总和*/DoubleSummaryStatistics sum = employee2s.stream().collect(Collectors.summarizingDouble(Employee2::getSalary));System.out.println(sum);/*** 最大值*/Optional<Employee2> max = employee2s.stream().collect(Collectors.maxBy((e1, e2) -> Double.compare(e1.getSalary(), e2.getSalary())));System.out.println(max);/*** 最小值*/Optional<Double> min = employee2s.stream().map(Employee2::getSalary).collect(Collectors.minBy(Double::compare));System.out.println(min.get());/*** 分组*/Map<Employee2.Status, List<Employee2>> group = employee2s.stream().collect(Collectors.groupingBy(Employee2::getStatus));System.out.println(group);}}

bean类Employee2

import java.util.Objects;/*** 创建日期：2021/10/29 14:01** @author tony.sun* 类说明：*/public class Employee2 {private String name;private Integer age;private Double salary;private Status Status;public Employee2(String name, Integer age, Double salary, Employee2.Status status) {this.name = name;this.age = age;this.salary = salary;Status = status;}/*** 闲着，忙碌，休假*/public enum Status{FREE,BUSY,VOCATION;}public String getName() {return name;}public void setName(String name) {this.name = name;}public Integer getAge() {return age;}public void setAge(Integer age) {this.age = age;}public Double getSalary() {return salary;}public void setSalary(Double salary) {this.salary = salary;}public Employee2.Status getStatus() {return Status;}public void setStatus(Employee2.Status status) {Status = status;}@Overridepublic String toString() {return "Employee2{" +"name='" + name + '\'' +", age=" + age +", salary=" + salary +", Status=" + Status +'}';}@Overridepublic boolean equals(Object o) {if (this == o) return true;if (o == null || getClass() != o.getClass()) return false;Employee2 employee2 = (Employee2) o;return Objects.equals(name, employee2.name) &&Objects.equals(age, employee2.age) &&Objects.equals(salary, employee2.salary) &&Status == employee2.Status;}@Overridepublic int hashCode() {return Objects.hash(name, age, salary, Status);}}

2.步骤与讲解

1.reduce收汁浓缩，这里起到一个合并值得左右，既1+2+3……+10；

List<Integer> list= Arrays.asList(1,2,3,4,5,6,7,8,9,10);Integer sum = list.stream().reduce(0, (x, y) -> x + y);//这个0是起始值，x是第一个值（计算后的值），y是上面的1,2,3,4,5,6,7,8,9,10，每次都加一次，加了10次System.out.println(sum);//55

//下面map表示要计算某个值，这里是Salary

System.out.println("----------------------------------");Optional<Double> optional = employee2s.stream().map(Employee2::getSalary).reduce(Double::sum);System.out.println(optional);

2.收集

1.获取list里面的值，然后提取成一个新的list

提取老的list里面的值，变成一个新的list（只有name），然后打印

//提取老的list里面的值，变成一个新的list（只有name），然后打印List<String> list = employee2s.stream().map(Employee2::getName).collect(Collectors.toList());list.forEach(System.out::println);//结果：//张三//李四//王五//赵六//赵六//田七

2.去重

set是一个不包含重复元素的 collection。确切地说，set 不包含满足 e1.equals(e2) 的元素对 e1 和 e2，并且最多包含一个 null 元素

System.out.println("--------------------------------");Set<String> set = employee2s.stream().map(Employee2::getName).collect(Collectors.toSet());set.forEach(System.out::println);//结果：//李四//张三//王五//赵六//田七

3.总数，平均值，总和，最大值，最小值

/*** 总数*/Long count = employee2s.stream().collect(Collectors.counting());System.out.println(count);/*** 平均值*/Double avg = employee2s.stream().collect(Collectors.averagingDouble(Employee2::getSalary));System.out.println(avg);/*** 总和*/DoubleSummaryStatistics sum = employee2s.stream().collect(Collectors.summarizingDouble(Employee2::getSalary));System.out.println(sum);/*** 最大值*/Optional<Employee2> max = employee2s.stream().collect(Collectors.maxBy((e1, e2) -> Double.compare(e1.getSalary(), e2.getSalary())));System.out.println(max);/*** 最小值*/Optional<Double> min = employee2s.stream().map(Employee2::getSalary).collect(Collectors.minBy(Double::compare));System.out.println(min.get());

4.分組groupingBy

/*** 分组*/Map<Employee2.Status, List<Employee2>> group = employee2s.stream().collect(Collectors.groupingBy(Employee2::getStatus));System.out.println(group);

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