How to convert List<Object> to Map<String , List<String>> using java 8 streams?

Question

I have Employee object which has rowId, skillId and student as below

[0, 20,"John"],

[1, 30,"Amy"],

[2, 20,"Tom"]

and I need response like this:

Map<skillId, List<student>>

20 , ["John","Tom"]

30 , [Amy]

Can anyone help?

Answer 1

Try use groupingBy and mapping

list.stream()
     .collect(Collectors.groupingBy(Employee::getSkillId,
                    Collectors.mapping(Employee:getStudent, Collectors.toList())));

Answer 2

Please use the below,

Map<Integer, List<Employee>> collect = employeeList.stream()
                .collect(Collectors.groupingBy(Employee::getSkillId, Collectors.toList()));

Answer 3

Map<Integer, List<String>> skillAndList = list.stream().collect(Collectors.groupingBy(Employee::getSkillId, Collectors.mapping(Employee::getStudent, Collectors.toList())));
System.out.println(skillAndList);

The output is the same as you expected -

{30=[Amy], 20=[John,Tom]}