Map example in java

June 16, 2022

I would like to write lastnamesByFirstname method.
In the people lists, there are bunch of people, and each person have two field the firstname and the second name. I would like to write a method which will turn back in the following way, key value would be the firstname, and the value list would be those name which firstnames are the same. I tried with groupingby wiht sorting buts somehow never worked.

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

my attempt look like this, but the type would be not matching with the return type.

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

>Solution :

In the second parameter of groupingBy, you can transform List<Person> to List<String> using mapping:

var peopleStream = people.stream()
        .collect(groupingBy(Person::getFirstName,
                mapping(Person::getLastName, Collectors.toList())));