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

测试类

 @Testvoid 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