Spring-MVC CRUD project Static method error

I am working on a MVC CRUD tutorial. I created a controller service and main methods. But my IDE called non-static methods cannot be referenced by static method error which is in located in StudentController class addStudent and getStudent methods. Can anyone help me with that. Also structural advices are gladly accepted. Thanks for any help.

StudentController:

package com.example.demo;
import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.annotation.*;
import java.util.List;
@RestController
public class StudentController {
StudentService studentService = new StudentService();
@RequestMapping(value = "/students ", method = RequestMethod.GET, headers = "Accept=application/json")
public List<Student> getStudent() {
List<Student> studentList = studentService.getAllStudents();
return studentList;
}
@RequestMapping(value = "/students ", method = RequestMethod.GET, headers = "Accept=application/json")
public Student getStudentById(@PathVariable int id) {
return StudentService.getStudent(id);
}
@RequestMapping(value = "/students ", method = RequestMethod.GET, headers = "Accept=application/json")
public Student addStudent(@RequestBody Student student) {
return StudentService.addStudent(student);
}
@RequestMapping(value = "/students", method = RequestMethod.PUT, headers = "Accept=application/json")
public Student updateStudent(@RequestBody Student student) {
return studentService.updateStudent(student);
}
@RequestMapping(value = "/student/{id}", method = RequestMethod.DELETE, headers = "Accept=application/json")
public void deleteStudent(@PathVariable("id") int id) {
studentService.deleteStudent(id);
}
}
}

My StudentService class:

package com.example.demo;
import javax.persistence.Id;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
public class StudentService {
private static HashMap<Integer, Student> studentHashMap = getStudentHashMap();
public StudentService() {
super();
if (studentHashMap == null) {
studentHashMap = new HashMap<Integer, Student>();
Student student1 = new Student(1, "Erdem", "Akıncı", "Yazılım geliştirici");
Student student2 = new Student(2, "Atıf", "İmal", "Beden İşçisi");
Student student3 = new Student(3, "Salih", "Özdemir", "Kasiyer");
Student student4 = new Student(4, "Mustafa", "Şensoy", "Yazılım geliştirici");
studentHashMap.put(1, student1);
studentHashMap.put(2, student2);
studentHashMap.put(3, student3);
studentHashMap.put(4, student4);
}
}
public static int getMaximumId() {
int max = 0;
for (int id : studentHashMap.keySet()) {
if (max <= id) {
max = id;
}
}
return max;
}
public HashMap<Integer, Student> getStudentHashMap() {
return studentHashMap;
}
public List<Student> getAllStudents() {
List<Student> students = new ArrayList<Student>(studentHashMap.values());
return students;
}
public Student getStudent(int id) {
Student student = studentHashMap.get(id);
return student;
}
public Student addStudent(Student student) {
student.setId(getMaximumId() + 1);
getStudentHashMap().put(student.getId(), student);
return student;
}
public Student updateStudent(Student student) {
if (student.getId() <= 0)
return null;
studentHashMap.put(student.getId(), student);
return student;
}
public void deleteStudent(int id) {
studentHashMap.remove(id);
}
}

As the IDE tells you already, you cannot access a static method from a non-static context. You do this because you have a static method in your service class that you use in one of your instance methods. Remove the static keyword from the method getMaximumId, this is not necessary.

Otherwise I would recommend a beginner tutorial where you learn more about the Spring Annotation (service, controller, repository) and the concept of dependency injection.

Something like this: https://spring.io/guides/tutorials/rest/