HashMap: multiple values as list

I have the list as mentioned below.

        List<Employees> lt = new ArrayList<Employees>(); 
		lt.add(new Employees("111", "Grade1")); //111 is the employee id
        lt.add(new Employees("222", "Grade2")); 
        lt.add(new Employees("333", "Grade2"));
		lt.add(new Employees("444", "Grade3"));
		lt.add(new Employees("555", "Grade3"));
		lt.add(new Employees("666", "Grade1"));
        ..............................

Am trying to create a map as mentioned below with the help of java8 with minimum coding from the list lt .
Here Grade is the designation of the employee. So for a particular grade I have to find out list of employee ids.

Map<String, List<String>> dataMap= new HashMap<>();
dataMap.put("Grade1", List1);  List contains "111" and "666"
dataMap.put("Grade2", List2);  List contains "222" and "333"
dataMap.put("Grade3", List3);  List contains "444" and "555"

...............................

Thank you in advance for any help you can provide

You can do this using the Collectors.groupingBy collector. Assuming that your Employees class has getGrade and getEmployeeID methods, you can write:

    Map<String, List<String>> dataMap = lt.stream().collect(Collectors.groupingBy(
        Employees::getGrade,
        LinkedHashMap::new,
        Collectors.mapping(Employees::getEmployeeID, Collectors.toList())));

The output is a Map with:

{Grade1=[111, 666], Grade2=[222, 333], Grade3=[444, 555]}

You don't need the map. You can do this using streams. To get Grade1, you could say:

List<Employees> grade1Employees = lt.stream().filter(e -> e.getGrade() == "Grade1").collect(Collectors.toList());

This would filter the list for where the employees are in "Grade1" and return the employee objects for them. This creates the grade1Employees variable. Then we could just use a for loop to get their IDs.