Question: Employee contains list of departments and we have a list of employees now how to obtain a map like Map<Department,List of Employees> from employee list.

The below code does the trick but I wanted to know how to use streams api effectively instead of for loop.

    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);
    Set<Department> departments = employees.stream().flatMap(employee ->
            employee.getDepartments().stream()).collect(Collectors.toSet());

    for (Department d : departments) {
        for (Employee employee : employees) {
            if (employee.getDepartments().contains(d)) {
                if (!result.containsKey(d)) {
                    result.put(d, new ArrayList<Employee>());
                }
                result.get(d).add(employee);
            }
        }
    }
    return result;

CodePudding user response：

You can do it as follows:

Map<Department, List<Employee>> map = 
    departments.stream()
            .collect(Collectors.toMap(
                        Function.identity(),
                        d -> employees.stream()
                                    .filter(e -> e.getDepartments().contains(d))
                                    .collect(Collectors.toList())
                    )
            );

ONLINE DEMO

CodePudding user response：

Sorry, on mobile. Cannot add sample code at the moment

1. Stream all employees
2. Flat map to Pair (Apache) or Map Entry (JDK AbstractMap Simple immutable Entry)
3. Collect to map. Key mapping is the department, value mapping. This is done by grouping by department, storing the employees as list (see related question: https://stackoverflow.com/a/49889657/1870799 )