Resolved: Map example in java

* {
     *  "Mary" -> ["Smith", "Silver"]
     *  "Joe" -> ["Smith"]
     * }
     */
    static Map<String, List<String>> lastnamesByFirstname(List<Person> people){
        
}

var peopleStream= people.stream()
                .collect(groupingBy(Person::getFirstName));

static Map<String, List<String>> lastnamesByFirstname(List<Person> people){
        people.stream()
              .collect(Collectors.groupingBy(Person::getFirstName, // Get first name as key of the map
                  Collectors.mapping(
                      Person::getLastName, //Get lastName as value of the list
                      Collectors.toList()))); //Specify that you want to collect as a list.
}