java - Convert List<Object> in Map<attributeA,List<attributeB> -

public class test {    public void foo() {    arraylist<person> persons = new arraylist<>();    persons.add(new person("marco", "bianchi"));   persons.add(new person("marco", "rossi"));   persons.add(new person("marco", "verdi"));   persons.add(new person("giacomo", "bianchi"));   persons.add(new person("giacomo", "rossi"));   persons.add(new person("giacomo", "verdi"));    map<string, list<string>> map = ?? }   class person {     string name;     string surname;      public string getname() {        return name;     }     public string getsurname() {        return surname;  } 

i need have map<name, list<surname>> when name equal , surname in list. expected result:

{ marco=[bianchi, rossi, verdi], giacomo=[bianchi, rossi, verdi] }

can me?

i need lambda function solution.

if using java 8 take @ stream api. here example how easy solve task:

public class mapexample {      public static void main(string[] args) {         arraylist<person> people = new arraylist<>();          people.add(new person("marco", "bianchi"));         people.add(new person("marco", "rossi"));         people.add(new person("marco", "verdi"));         people.add(new person("giacomo", "bianchi"));         people.add(new person("giacomo", "rossi"));         people.add(new person("giacomo", "verdi"));          map<string, list<string>> map = people.stream()             .collect(                     collectors.groupingby(person::getname,                     collectors.mapping(person::getsurname, collectors.tolist()))             );     } } 

with static import looks more declarative:

import static java.util.stream.collectors.*;  // ....  map<string, list<string>> map = people.stream()         .collect(                 groupingby(person::getname, mapping(person::getsurname, tolist()))         );