Question

So I am trying to find an example online about creating a preparedStatement that has an sql query referencing multiple tables.

For e.g. The examples I’ve encountered so far are always

e.g.

s = conn.prepareStatement ("DELETE FROM Users WHERE id_user = ?");
s.setInt (1, 2);

where there is only one table involved, and the method exists in the same class of the database table. E.g. User.class , user table in database.

The query that I have requires me to set the place holder from another table/class. In this case, my method exists in the User.class, however, it requires a the binding from a Group object.

SELECT DISTINCT *
FROM usuarios 
WHERE NOT EXISTS
(SELECT * FROM usuarios_grupos 
 WHERE usuarios_grupos.id_grupo = ? 
 AND usuarios_grupos.id_usuario = usuarios.id_usuario);

Will the method be the following:

public List<Usuarious> list(Grupos groups) throws DAOExceptions {
        Connection connection = null;
        PreparedStatement preparedStatement = null;
        ResultSet resultSet = null;
        List<Usuarious> users = new ArrayList<Usuarious>();

        try {
            connection = daoFactory.getConnection();
            preparedStatement = connection.prepareStatement(SQL_LIST_ALL);
            preparedStatement.setInt(1, groups.getId_grupo());
            resultSet = preparedStatement.executeQuery();
            while (resultSet.next()) {
                users.add(mapUser(resultSet));
            }
        } catch (SQLException e) {
            throw new DAOExceptions(e);
        } finally {
            close(connection, preparedStatement, resultSet);
        }

        return users;
    }

or will it be written differently?? because I seem to be getting a NPE with this, and from the examples I’ve seen online. The query always reference 1 table. Is what I’m doing here wrong?

okay here is my method for groups.getId_grupo(), which exists in my Group.class:

public class Grupos {

Integer id_grupo;
String descricao;

public Grupos() {

}

public Grupos(Integer id_grupo, String descricao) {
    this.id_grupo = id_grupo;
    this.descricao = descricao;
}

public Grupos(Integer id_grupo) {
    this.id_grupo = id_grupo;
}


public String getDescricao() {
    return descricao;
}

public void setDescricao(String descricao) {
    this.descricao = descricao;
}

public Integer getId_grupo() {
    return id_grupo;
}

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

}

I am calling my List list(Grupos groups) method in my ManagedBean

public class UsuariousGruposBean implements Serializable {

private Usuarious user = new Usuarious();
private Grupos grps = new Grupos();
private UsuariousGrupos userGroups = new UsuariousGrupos();

protected final UsuariousDAO userDAO = daoFactory.getUserDAO();
protected final GruposDAO grpDAO = daoFactory.getGruposDAO();
protected final UsuariousGruposDAO userGrpDAO = daoFactory.getUsuariousGruposDAO();

    private List<Usuarious> listOfUsuarios;
    private List<Grupos> listOfGrps;
    private List<UsuariousGrupos> listOfUserGroups;

public UsuariousGruposBean() {
    }

    public List<Usuarious> getListOfUsuarios() throws DAOExceptions {
        List<Usuarious> usuariosList = userDAO.list(grps);
        listOfUsuarios = usuariosList;
        return listOfUsuarios;
    }

Answer 1

First instance in your code can throw NPE is at:

preparedStatement = connection.prepareStatement(SQL_LIST_ALL);

if your connection is null, your connection factory didnt return you one, check if you have a valid connection

Second place :

groups.getId_grupo()

Check if your groups is null or not

If these are not the reasons then please post your stacktrace.