Group the list of Objects and convert to a map using java8

Question

`Hi,

I have a list of Employees employeeList (assume this list has some values present)

List<Employee> employeeList ;

Say, My Employee DTO is something like below :

Employee {
List<Book> bookList;
}

Book DTO:

Book {
private String bookId;
private String bookName;
}

I have to stream through the employeeList and group the employees with same book Id and convert to a map having key as the BookId and value as list of Employees. I want to achieve this using java8 concepts. Please help!

I tried to use Collectors.groupingBy but I have to traverse through another List of Books inside list of Employees. Could not figure out how?`

Answer 1

You can flatten the Employee list to pairs of Employee, Book ID and then collect to the Map you want.

Something like this (not tested):

Map<String,List<Employee>> map =
    employeeList.stream()
                .flatMap(e -> e.getBookList()
                               .stream()
                               .map(book -> new SimpleEntry<String,Employee>(book.getID (), e)))
                .collect (Collectors.groupingBy (Map.Entry::getKey,
                                                 Collectors.mapping(Map.Entry::getValue,
                                                                    Collectors.toList())));