@PUT
@Consumes(MediaType.APPLICATION_JSON)
@Produces(MediaType.APPLICATION_JSON)
@Path("/")
public User updateCourses(User user) {
    MongoCollection usersCollection = MongoConnector.getUserCollection();
    UserService userService = new UserService(usersCollection);
    User updatedUser = userService.updateCourses(user);
    return updatedUser;
}

public class User {

private String email;
private String pass;
private String type;
private List<String> courses;


public User(){

}

public User(String email, String pass, String type, List<String> courses) {
    this.email = email;
    this.pass = pass;
    this.type = type;
    this.courses = courses;
}

public String getEmail() {
    return email;
}

public void setEmail(String email) {
    this.email = email;
}

public String getPass() {
    return pass;
}

public void setPass(String pass) {
    this.pass = pass;
}

public String getType() {
    return type;
}

public void setType(String type) {
    this.type = type;
}

public List<String> getCourses() {
    return courses;
}

public void setCourses(List<String> courses) {
    this.courses = courses;
}}

{
"email": "deneme@student.com",
"pass": "4",
"type": "student",
"courses": [
    "Deneme1",
    "Deneme2",
    "Deneme3",
    "Deneme4"
]

public User updateCourses (User user){

    userCollection.updateOne(eq("email", user.getEmail()),Updates.addToSet("courses", user.getCourses()));
    return user;
}