Lỗi non static variable cannot be referenced from a static context

[Tran_Quang1 (vippromanu)]

public class studentManagerment {
    Student [] list = new Student[100];
    int index = 0;

    // check group
    public boolean sameGroup(Student s1, Student s2){
        if (s1.getGroup().equals(s2.getGroup()) ) return true;
        else return false;
    }

    // them student
    public void addStudent(Student newStudent){
        list[index] = newStudent;
        this.index++;
        System.out.println();
    }

    //Sap xep
    public String studentByGroup(){
        String listStudent = "";
        for (int i=0;i<index-1;i++)
            for (int j=0;j<index-1;j++) {
                if (list[j].getGroup().compareTo(list[j+1].getGroup()) < 0){
                    Student temp = list[j];
                    list[j] = list [j+1];
                    list[j+1] = temp;
                }
            }
        for (int i=0;i<index;i++){
            if (i==0||!sameGroup(list[i],list[i-1])){
                listStudent +=  list[i].getGroup() + "\n";
            }
            listStudent += list[i].getinfo() + "n";
        }
        return listStudent;
    }

    public void removeStudent(String id){
        Student[] students = new Student[100];
        int k = 0;
        for (int i=0;i<index;i++){
            if (list[i].getId().equals(id)){
                students[k] = list[i];
                k++;
            }
        }
        list = students.clone();
        index--;
    }

    public static void main(String[] args){
        Student  s1 = new Student("Nguyen Van B","17021000","K63AA");

        addStudent(s1);
    }

}

Lỗi

Error:(59, 9) java: non-static method addStudent(com.company.Student) cannot be referenced
from a static context

[rogp10]

Bạn add vào list nào

p/s: Managerment

[Duc_Anh1 (Đức Anh)]

Do hàm method addStudent không phải static nhé