grouping objects java 8

Question

I have something like the below :

public class MyClass {
private Long stackId
private Long questionId
}

A collection of say 100, where the

Answer 1

List inputs = Arrays.asList(
    new MyClass(1L, 100L),
    new MyClass(1L, 101L),
    new MyClass(1L, 102L),
    new MyClass(1L, 103L),
    new MyClass(2L, 200L),
    new MyClass(2L, 201L),
    new MyClass(2L, 202L),
    new MyClass(2L, 203L)
);

Map> result = inputs
    .stream()
    .collect(
      Collectors.groupingBy(MyClass::getStackId,
        Collectors.mapping(
          MyClass::getQuestionId, 
          Collectors.toList()
        )
      )
    );