[Java知识库]List＜E＞转Map＜StringList＜E＞＞

实体类?

package com.example.demo.entity;

public class Person {
    private Integer age;
    private String name;

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Person(Integer age, String name) {
        this.age = age;
        this.name = name;
    }

    @Override
    public String toString() {
        return "Person{" +
                "age=" + age +
                ", name='" + name + '\'' +
                '}';
    }
}

测试类

 @Test
    void contextLoads() {
        List<Person> personList = new ArrayList<>();
        personList.add(new Person(1,""));

        personList.add(new Person(3,""));
        personList.add(new Person(4,"小红"));

        Map<String, List<Person>> groups = personList.stream().collect(Collectors.groupingBy(ele -> ele.getName(), Collectors.mapping(Function.identity(), Collectors.toList())));

        groups.forEach((k,v)->{
            if(k.isEmpty()){
                System.out.println(k+"ok===" + v);
            }else{
                System.out.println(k+"===" + v);
            }
        });
        System.out.println(groups);
        System.out.println("============================");

        List<Person> people = new ArrayList<>();
        people.add(new Person(1,null));

        people.add(new Person(3,""));
        people.add(new Person(4,"小红"));

        Map<Optional<String>, List<Person>> group = people.stream().collect(Collectors.groupingBy(ele -> Optional.ofNullable(ele.getName()), Collectors.mapping(Function.identity(), Collectors.toList())));

        group.forEach((k,v)->{
            if(k.filter(e->e.isEmpty()).isPresent()){
                System.out.println(k+"ok===" + v);
            }else{
                System.out.println(k+"===" + v);
            }
        });
        System.out.println(group);
    }

?注意：

使用groupBy的时候一定要确保分组的字段不是null的，可以是empty但是不能是null，否则会报错，如下图

?