Static context cannot access non-static in Collectors
Unfortunately, the error message “Non-static method cannot be refered from a static context.” is just a place-holder for any type mismatch problem, when method references are involved. The compiler simply failed to determine the actual problem.
In your code, the target type Map<Integer, Map<String, List<String>>>
doesn’t match the result type of the combined collector which is Map<Integer, List<String>>
, but the compiler didn’t try to determine this (stand-alone) result type, as the (nested) generic method invocations incorporating method references requires the target type for resolving the method references. So it doesn’t report a type mismatch of the assignment, but a problem with resolving the method references.
The correct code simply is
Map<Integer, List<String>> groupping = students.stream()
.collect(Collectors.groupingBy(Student::getMarks,
Collectors.mapping(Student::getName, Collectors.toList())));
I think Holger has given a good explanation about the error and why it doesn't make much sense in one run.
Considering your goal, I think this is the solution you need to have.
Map<Integer, Map<String, List<Student>>> grouping = students.stream().collect(Collectors.groupingBy(Student::getMarks,
Collectors.groupingBy(Student::getName)));
This would simply give you a student list first grouped by marks, then by name. :))