What Is the Reason Behind "Non-Static Method Cannot Be Referenced from a Static Context"

What is the reason behind non-static method cannot be referenced from a static context?

You can't call something that doesn't exist. Since you haven't created an object, the non-static method doesn't exist yet. A static method (by definition) always exists.

non-static method cannot be referenced from a **static context**. What is static content here?

Your main-method is the static context, and you are trying to call the non-static method display() from it. That doesn't work even if it is in the same class. To have the disply method non-static you have to do this.

public static void main(String[] args) throws FileNotFoundException{
    ReverseLL r = new ReverseLL();
    r.display(head);

}

public void display(Node head){
    ...
}

Non-Static method cannot be referenced from a static context with methods and variables

You need to make both your method - printMenu() and getUserChoice() static, as you are directly invoking them from your static main method, without creating an instance of the class, those methods are defined in. And you cannot invoke a non-static method without any reference to an instance of the class they are defined in.

Alternatively you can change the method invocation part to:

BookStoreApp2 bookStoreApp = new BookStoreApp2();
bookStoreApp.printMenu();
bookStoreApp.getUserChoice();

non static method cannot be referenced from a static context JPA Java

The problem is that you're trying to call the instance method createStudent() from a static context in main(). If you change your createStudent() method to be static, you should be good to go:

public static Student createStudent(String student_id, Date dob, String gender, String nationality, Type_Name name) {
    // ... And so on 
}

EDIT: OP pointed out that this change alone gives him another error when accessing the variables em and emf. To fix that, you'd need to make those variables static, too:

static EntityManager em;
static EntityManagerFactory emf;

At that point, everything in your class is static. Assuming this is a simple one-off or example -- which I'm comfortable assuming since the class is called Main -- making everything static is just fine. In all, the code would look like this:

package grade_db;

import bean.Student;
import bean.Type_Name;
import bean.University;
import java.util.Date;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
/**
 *
 * @author Sam
 */

public class Main {

    static EntityManager em;
    static EntityManagerFactory emf;

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        // TODO code application logic here

        EntityManagerFactory emf = Persistence.createEntityManagerFactory("db/grades.odb");

        EntityManager em;
        em = emf.createEntityManager();

        createStudent("stu00001", new Date(631152000000)), "m", "WB", new Type_Name("Bob", "", "Smith"));

        em.close();
        emf.close();
    }

    public static Student createStudent(String student_id, Date dob, String gender, String nationality, Type_Name name){
        Student stu = new Student();
        stu.setDob(dob);
        stu.setGender(gender);
        stu.setName(name);
        stu.setNationality(nationality);

        stu.setCampus_id("cam00001");
        stu.setCourse_id(null);
        stu.setStudent_id(student_id);

        em.persist(stu);
        return stu;
    }
}

---

Related Topics