Java Stream 用法（java stream函数）

JDK8之后Lambda表达式，极大的简化了集合的操作，最近项目需要用到，特整理了一下几种用法。

先来一个对象

@Data
@AllArgsConstructor
public class Student {
    private int no;
    private String name;
    private String classNo;
    private int core;
}

添加maven 配置

<dependency>
    <groupId>org.projectlombok</groupId>
    <artifactId>lombok</artifactId>
    <version>1.18.20</version>
    <scope>provided</scope>
</dependency>
<dependency>
    <groupId>ch.qos.logback</groupId>
    <artifactId>logback-classic</artifactId>
    <version>1.2.5</version>
</dependency>

测试类

import lombok.extern.slf4j.Slf4j;

import java.util.*;
import java.util.stream.Collectors;
@Slf4j
public class TestStream {
    public static void main(String [] args){
        List<Student> studentList=new ArrayList<>();
        studentList.addAll(Arrays.asList(
                 new Student(1,"111","12",30)
                ,new Student(2,"111","12",40)
                ,new Student(3,"111","12",50)
                ,new Student(4,"111","12",60)
                ,new Student(5,"333","33",70)
                ,new Student(6,"333","33",80)
                ,new Student(7,"333","44",90)
                ));
        //按照年纪分类统计
       Map<Optional<String>,List<Student>> map1= studentList.stream()
               .collect(Collectors.groupingBy(o->Optional.ofNullable(o.getClassNo())));
       printInfo(map1);
			//按照年级统计数量
       Map<Optional<String>,Long> map2= studentList.stream()
               .collect(Collectors.groupingBy(o->Optional.ofNullable(o.getClassNo()),Collectors.counting()));
       printInfo(map2);
      //按照年级计算求和，数量等
       Map<String, IntSummaryStatistics> map3= studentList.stream()
                .collect(Collectors.groupingBy(Student::getClassNo,Collectors.summarizingInt(Student::getCore)));
        printInfo(map3);
      //按照姓名统计数量
        Map<String, Map<String, Long>> map4= studentList.stream()
                .collect(Collectors.groupingBy(Student::getClassNo,Collectors.groupingBy(Student::getName,Collectors.counting())));
        printInfo(map4);
      //按照姓名统计
        Map<String, Map<String, List<Student>>> map5= studentList.stream()
                .collect(Collectors.groupingBy(Student::getClassNo,Collectors.groupingBy(Student::getName)));
        printInfo(map5);
      	//按照学号统计分数
        Map<Integer,Integer> map7=studentList.stream().collect(Collectors.toMap(Student::getNo,Student::getCore));
        printInfo(map7);
      	//按照学号统计分数，生成新的Map，指定Value的取值
        Map<String,Integer> map8=studentList.stream().collect(Collectors.toMap(Student::getClassNo,Student::getCore,(v1,v2)->v1));
        printInfo(map8);
       //处理Key值位空的情况
        studentList.add(new Student(3,"333",null,90));
        Map<Optional<String>,Long> map6= studentList.stream()
                .collect(Collectors.groupingBy(o->Optional.ofNullable(o.getClassNo()),Collectors.counting()));
        printOptionInfo(map6);
      //Optional null/空处理
       log.info("option:{}", Optional.ofNullable(null).orElse("xx"));
       log.info("option:{}", Optional.ofNullable("aa").orElse("xx"));

    }
	//打印方法
    private  static void printOptionInfo(Map<Optional<String>,Long> map) {
        map.forEach((key, value) -> {
            log.info("{}+{}+{}+{}",key.isPresent(), key.isPresent(),key.isEmpty(),key, value);
        });
    }
    //打印方法
    private  static void printInfo(Map map) {
        map.forEach((key, value) -> {
            log.info("{}-{}", key, value);
        });
    }

}

欢迎交流，后续持续更新