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Francrt
Asked: 2020-04-16 23:13:24 +0800 CST 2020-04-16 23:13:24 +0800 CST

NullPointer when executing a PreparedStatement

  • 772

I am trying to generate a ArrayListusers from a DB, the case is that when executing the PreparedStatementit returns a NullPointerand tells me that the connection object is null, it is something that does not happen to me in other statements that I have and they work.

I leave the code of Arraythe and the code of the class Conecta(Objeto Conect):

      public class Usuario {

      private int id;
      private String nickName;
      private String password;    
      private String Email; 
      private ConectaDB conexion;

      public ArrayList<Usuario> getUsuarios(){
      Connection con = null;
      con = conexion.conecta();
      ArrayList<Usuario> Usuarios = new ArrayList();                    
      PreparedStatement getData = conexion.preparedStatement("SELECT IDUsuario, Nickname, Password, Email FROM usuario");           
      ResultSet data;

      try{            
      data = getData.executeQuery();
      while(data.next()){
        id = data.getInt(1);
        nickName = data.getString(2);
        password = data.getString(3);
        Email = data.getString(4);
      Usuario usser = new Usuario(data.getInt(1), data.getString(2), data.getString(3), data.getString(4));
      Usuarios.add(usser);
      }
       return Usuarios;
  } catch(SQLException e){              
    }  return Usuarios;
}

And here is the code for the Connect class:

  public class ConectaDB {

    public static Connection conecta(){

          Connection conecta=null;

          try{
              Class.forName("com.mysql.jdbc.Driver");
              String servidor="jdbc:mysql://xxxxxx:xxxx/bbdd_g1"; //conectamos a nuestra propia maquina
              String usuario="admin_g1";
              String password="a7586";
              //inicializamos objeto conecta, del tipo connection
              conecta = (Connection)DriverManager.getConnection(servidor, usuario, password);

          }catch(ClassNotFoundException e)
          {
              System.out.println(e.getMessage());
          }
            catch(Exception e) //capturar el resto de exceptions
          {
              System.out.println(e.getMessage());
          }finally{
              return conecta;//como el default de los case, pero se ejecuta si o si
          }

    }    

    public PreparedStatement preparedStatement(String x) {
        throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
    }
}

Thanks for the help!

java

2 Answers

  1. Best Answer

    Even though we don't see the object declaration conexion, it looks like it's giving you a NullPointer because you're not accessing the object with the connection.

    The connection is in conbut the preparedStatement is taken out of connection, which is an instance of ConectaDB.

    Edited: It should also be prepareStatement (without d in prepared)

    Edited: You can use the following code. I have prepared the connection method preparedStatement to do the prepareStatement, but surely you should choose one of the following strategies. - Make a static method that prepares the query for you, passing the connection to it. - Make ConectaDB an object that saves the connection and you only pass the query to it. - Remove the prepareStatement method from ConectaDB so as not to mess with the instance between the two objects.

    public class Usuario {

    private int id;
    private String nickName;
    private String password;
    private String Email;
    private ConectaDB conexion;

    public Usuario(int id, String nickName, String password, String email) {
        this.id = id;
        this.nickName = nickName;
        this.password = password;
        Email = email;
    }

    public ArrayList<Usuario> getUsuarios() {
        Connection con = null;
        con = conexion.conecta();
        ArrayList<Usuario> Usuarios = new ArrayList();

        try {
            PreparedStatement getData = con.prepareStatement("SELECT IDUsuario, Nickname, Password, Email FROM usuario");
            ResultSet data;
            data = getData.executeQuery();
            while (data.next()) {
                id = data.getInt(1);
                nickName = data.getString(2);
                password = data.getString(3);
                Email = data.getString(4);
                Usuario usser = new Usuario(data.getInt(1), data.getString(2), data.getString(3), data.getString(4));
                Usuarios.add(usser);
            }
            return Usuarios;
        } catch (SQLException e) {
        }
        return Usuarios;
    }
}

    Y

    public class ConectaDB {

    public static Connection conecta(){

        Connection conecta=null;

        try{
            Class.forName("com.mysql.jdbc.Driver");
            String servidor="jdbc:mysql://xxxxxx:xxxx/bbdd_g1"; //conectamos a nuestra propia maquina
            String usuario="admin_g1";
            String password="a7586";
            //inicializamos objeto conecta, del tipo connection
            conecta = DriverManager.getConnection(servidor, usuario, password);

        }catch(ClassNotFoundException e)
        {
            System.out.println(e.getMessage());
        }
        catch(Exception e) //capturar el resto de exceptions
        {
            System.out.println(e.getMessage());
        }finally{
            return conecta;//como el default de los case, pero se ejecuta si o si
        }

    }

    public static PreparedStatement preparedStatement(Connection con ,String sql) throws SQLException {
        return con.prepareStatement(sql);
    }
}
    • 1

  2. Why...?

     PreparedStatement getData = conexion.preparedStatement("SELECT IDUsuario, Nickname, Password, Email FROM usuario");

    When...

      con = conexion.conecta();
    • 0