Static context cannot access non-static in Collectors

collect, java, java-8, java-stream, method-reference

Solution

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())));

Problem

I have group of students. First I want to group them by the marks. Then I want to further group those sets into same name students together. ``` Map<Integer,Map<String,List<String>>> groupping = students.stream() .collect(Collectors.groupingBy(Student::getMarks, Collectors.mapping(Student::getName,Collectors.toList()))); ``` I am getting an error saying, Non-static method cannot be refered from a static context. Yes. I am pretty much aware that I cannot refer to a non-static method without having an instance. But with all these stream operations, I'm a bit confused about what has gone wrong really. Rather than how to fix this; I really want to know what's going on here. Any of your inputs is appreciated! Because If I write the below grouping is completely valid; ``` Map<Integer,List<Student>> m = students.stream(). collect(Collectors.groupingBy(Student::getMarks)); ``` Here is my Student.java class (In case if you need it) ``` public class Student { private String name; private int marks; // getters, setters, constructor and toString } ```

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