java - Understanding how to decouple -

i've been trying keep coupling down in code, think may not understand it. basic understanding coupling "how dependent classes on each other , know behavior of each other." know dependency injection 1 way reduce coupling , ioc.

the following quick example came of student, professor, , course. course has list of students , professor. have controller (using mvc) injects student , professor objects. following still considered coupled, or tightly coupled? example of di, correct?

student class

public class student { private string firstname; private string lastname; private int studentid; private int address; private int telephone; public string getfirstname() {     return firstname; } public void setfirstname(string firstname) {     this.firstname = firstname; } public string getlastname() {     return lastname; } public void setlastname(string lastname) {     this.lastname = lastname; } public int getstudentid() {     return studentid; } public void setstudentid(int studentid) {     this.studentid = studentid; } public int getaddress() {     return address; } public void setaddress(int address) {     this.address = address; } public int gettelephone() {     return telephone; } public void settelephone(int telephone) {     this.telephone = telephone; } } 

professor class

public class professor { private string firstname; private string lastname; private int professorid; private int address; private int telephone; private int salary; public string getfirstname() {     return firstname; } public void setfirstname(string firstname) {     this.firstname = firstname; } public string getlastname() {     return lastname; } public void setlastname(string lastname) {     this.lastname = lastname; } public int getprofessorid() {     return professorid; } public void setprofessorid(int professorid) {     this.professorid = professorid; } public int getaddress() {     return address; } public void setaddress(int address) {     this.address = address; } public int gettelephone() {     return telephone; } public void settelephone(int telephone) {     this.telephone = telephone; } public int getsalary() {     return salary; } public void setsalary(int salary) {     this.salary = salary; } } 

course class

import java.util.list;  public class course { private list<student> students; private professor professor; public professor getprofessor() {     return professor; } public void setprofessor(professor professor) {     this.professor = professor; } public list<student> getstudents() {     return students; } public void setstudents(list<student> students) {     this.students = students; }  } 

i have controller (using mvc) injects student , professor objects. following still considered coupled, or tightly coupled?

since references classes, have tightly coupled design. approach use interfaces in code. allow change implementation time want , not affect rest of application.

this example of di, correct?

if course, professor , student configured beans, , specify somewhere how inject instances during bean instantiation, di example. 3 pojo classes.