java8 对象转Map时重复 key Duplicate key xxxx的解决

@Getter
@Setter
@AllArgsConstructor
public class Student{
    private String className;
    private String studentName;
 
    public static void main(String[] args) {
 List<Student> list = new ArrayList<>();
 list.add(new Student("一年级二班", "小明"));
 list.add(new Student("一年级二班", "小芳"));
 list.add(new Student("一年级二班", "小华"));
 list.add(new Student("一年级三班", "翠花"));
 list.add(new Student("一年级三班", "香兰"));
 // 集合中对象属性转map 
 Map<String, String> map = list.stream().collect(Collectors.toMap(Student :: getClassName, Student :: getStudentName));
 System.out.println(map);
     } 
}

Collectors.toMap(keyMapper, valueMapper, mergeFunction)

{@link Map#merge(Object, Object, BiFunction)}

Map<String, String> map = list.stream().collect(Collectors.toMap(Student :: getClassName, Student :: getStudentName, 
 (value1, value2 )->{ 
            return value2; 
 }));

Map<String, String> map = list.stream().collect(Collectors.toMap(Student :: getClassName, Student :: getStudentName, 
 (key1 , key2)-> key2 ));

Map<String, String> map = list.stream().collect(Collectors.toMap(Student :: getClassName, Student :: getStudentName, 
 (key1 , key2)-> key1 + "," + key2 ));

Map<String, List<String>> map = list.stream().collect(Collectors.toMap(Student :: getClassName, 
    // 此时的value 为集合，方便重复时操作
    s ->  {
 List<String> studentNameList = new ArrayList<>();
 studentNameList.add(s.getStudentName());
 return studentNameList;
    }, 
    // 重复时将现在的值全部加入到之前的值内
    (List<String> value1, List<String> value2) -> {
 value1.addAll(value2);
 return value1;
    }
));