Collections – java8 converts lists to map >

I have a list < person > object and want to convert it to map < integer. The keyword of the list < person > > map represents a person's property level It may have multiple person objects with the same level in the source list. In this case, I want to group them all into the list to compare the corresponding levels in the resulting map

So far, I have the following code

public class PersonMain
{
   public static void main(String[] args)
   {
    Person person1 = new Person();
    person1.setName("person1");
    person1.setGrade(1);

    Person person2 = new Person();
    person2.setName("person2");
    person2.setGrade(2);

    Person person3 = new Person();
    person3.setName("person3");
    person3.setGrade(1);

    List<Person> persons = Arrays.asList(person1,person2,person3);

    Map<Integer,List<Person>> personsMap = persons.stream()
            .collect(Collectors.toMap(Person::getGrade,PersonMain::getPerson,PersonMain::combinePerson));

    System.out.println(personsMap);
}
private static List<Person> getPerson(Person p)
{
    List<Person> persons = new ArrayList<>();
    persons.add(p);
    return persons;
}
private static List<Person> combinePerson(List<Person> oldVal,List<Person> newVal)
{
        oldVal.addAll(newVal);
        return oldVal;
    }
}

Is there a better way to achieve this?

Solution

Your current solution is good, but using the groupingby collector is more intuitive:

Map<Integer,List<Person>> personsMap = 
        persons.stream()
                .collect(Collectors.groupingBy(Person::getGrade));

This overload of the groupingby collector is very simple, because it only requires a classifier (person:: getgrade) function, which will extract the key to group the streaming objects