java 8 group a collections by 2 property

In Java 8 and later, you can use the Collectors.groupingBy method of the Collectors class to group the elements of a Collection by one or more properties.

To group a Collection by two properties, you can use the Collectors.groupingBy method in combination with the Collectors.mapping method, which allows you to specify a function that maps the elements of the Collection to a new value that is used for grouping.

Here is an example of how you can use the Collectors.groupingBy and Collectors.mapping methods to group a List of Person objects by their name and age:

import java.util.List;
import java.util.Arrays;
import java.util.Map;
import java.util.stream.Collectors;

public class MyClass {
    public static void main(String[] args) {
        List<Person> people = Arrays.asList(
            new Person("Alice", 25),
            new Person("Bob", 30),
            new Person("Charlie", 35),
            new Person("Bob", 35)
        );

        Map<String, Map<Integer, List<Person>>> result = people.stream()
        .collect(Collectors.groupingBy(Person::getName,
                        Collectors.groupingBy(Person::getCity)));
    }
}