import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
 
class Ideone {
    public static void main(String[] args) {
        Department a = new Department("a");
        Department b = new Department("b");
        Department c = new Department("c");
 
        Employee e1 = new Employee("e1", List.of(a, b));
        Employee e2 = new Employee("e2", List.of(c, b));
        Employee e3 = new Employee("e3", List.of(c, a));
        Employee e4 = new Employee("e4", List.of(a, b, c));
 
        List<Employee> employees = List.of(e1, e2, e3, e4);
        Map<Department,List<Employee>> result=employees.stream()
            .flatMap(employee->employee.getDepartments().stream()
                .map(department->new Pair(department,employee)))
            .collect(Collectors.groupingBy(pair->pair.d,
                                           Collectors.mapping(pair->pair.e,
                                                              Collectors.toList())));
        System.out.println(result);
    }
 
    static class Department{final String name;Department(String name){this.name=name;}public String toString(){return name;}}
    static class Employee{final String name;final List<Department> departments;Employee(String name,List<Department> departments){this.name=name;this.departments=departments;}List<Department> getDepartments(){return departments;}public String toString() {return name;}}
    // this is just a helper class instead of AbstractMap.whatever
    static class Pair{final Department d;final Employee e;Pair(Department d,Employee e){this.d=d;this.e=e;}}
}

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