JDBC多对一映射/空指针异常错误

Question

Right so I have been working on this project for awhile, and I am using JDBC + JSF.

I currently have 3 DAOs, User, Groups, and UserGroups.

My question would be..how do you establish a relationship between the 3 classes , User, Groups and UserGroups??

In my database, UserGroup is a associative entity, which has IDUSer and IDGroup as a Primary Composite Key.

The relationship between User and UserGroup is 1 to Many. The relationship between Groups and UserGroup is also 1 to Many, with UserGroup having a Many to 1 relation with both User and Groups.

Would this be right if I want to create a relationship between User and UserGroup, Group and UserGroup??

public class UsuariousGrupos {

Integer id_usuario;
Integer id_grupo;
Grupos  groups;
Usuarious users;

public UsuariousGrupos() {
}

public UsuariousGrupos(Integer id_usuario, Integer id_grupo) {
    this.id_usuario = id_usuario;
    this.id_grupo = id_grupo;
}

public Integer getId_grupo() {
    return id_grupo;
}

public void setId_grupo(Integer id_grupo) {
    this.id_grupo = id_grupo;
}

public Integer getId_usuario() {
    return id_usuario;
}

public void setId_usuario(Integer id_usuario) {
    this.id_usuario = id_usuario;
}

public Grupos getGroups() {
    return groups;
}

public void setGroups(Grupos groups) {
    this.groups = groups;
}

public Usuarious getUsers() {
    return users;
}

public void setUsers(Usuarious users) {
    this.users = users;
}

Also, I tested my DAO connection, and they are working fine. I am able to display name/description from my User database table, and Groups database table. However, since my UserGroup table stores just IDs, I am only able to display the IDs. I want to display the name/description of the User/Group that is related to the ID. However, when I do the following:

 List<UsuariousGrupos> ugdList = ugd.list() ;
    System.out.println("List of users successfully queried: " + ugdList);
    System.out.println("Thus, amount of usergroups in database is: " + ugdList.size());
    for (int i = 0; i < ugdList.size(); i++) {
        System.out.println("Groups in database are: " + ugdList.get(i).getUsers().getNome());
        System.out.println("Users in database are: " + ugdList.get(i).getGroups().getDescricao());
    }

I get a Exception in thread "main" java.lang.NullPointerException. Apparently, I am unable to access the Users and Groups object from my UserGroup List : / , which leads me to think that I am missing some sort of relationship among the classes.

The null pointer exception occurs at these lines:

System.out.println("Groups in database are: " + ugdList.get(i).getUsers().getNome());
System.out.println("Users in database are: " + ugdList.get(i).getGroups().getDescricao()

apparently, ugdList.get(i).getUsers() and ugdList.get(i).getGroups() are null.

Any thoughts on this??

Answer 1

You need to map the name of each user and overload the toString method of the user class like this :

public String toString()
{
return String.format(""%s %s", this.name, this.lastName);
}

By the way, ugdList.get(i).getUsers().getNome()); seems wrong to me But using the toString method applied earlier, you can do this :

for (User user : ugdList.get(i).getUsers())
{
printf(user);
}

BUT you should test ugdList.get(i).getUsers() by steps before : check that get(i).getUsers != null then run the loop.

Hope this helps !