Instantiation order

Hello. I'm trying to populate a combo box using an MVC model, and am using either serialization, or a database based on the users preference. I'm not sure exactly what's going on, but it's a visibility issue between the listener, controller, and interface classes upon instantiation...

FROM THE LISTENER:

Code:

---

public Listener(Controller pc)
{
  this.pc = pc;
  foodChoices = pc.getFoodChoices
    (categoryList.getSelectedItem().toString());
  foodList = new JComboBox(foodChoices);
  ...
  con.add(foodList);
  ...
}

FROM THE CONTROLLER:

public class Controller()
{
  public static void main(String[] args)
  {                       
    Controller app = newController();
    app.initialize();
  }

public void initialize()               
{               
  pl = new Listener(this);
 dir = System.getProperty("user.dir");
 appPath = dir.substring(0,dir.lastIndexOf('\\'));
 while (check)
 {
    String daoAccess = JOptionPane.showInputDialog(null,
    "Enter S to read/write to text file, D to read/write to database");
       
  if (daoAccess != null)
 {
      if (daoAccess.equalsIgnoreCase("d"))
    {
        sd = new SQLDAO(this);
        pl = new Listener(this);
        check = false;
      }
      else if (daoAccess.equalsIgnoreCase("s"))
      {
        dir = System.getProperty("user.dir");
        appPath = dir.substring(0,dir.lastIndexOf('\\'));
        pl = new ProteinListener(this);
        sd = new SerialDAO(appPath);
        check = false;
      }
      else if (daoAccess == "")
      {
          System.out.println("You must enter a data access type");
          check = true;
      }
      else
      {
        Utility3.errorMessage("Invalid entry");
        check = true;
      }
  }
  else
    break;
  }           
 }
(METHOD FOR READING DATABASE/FILE FOR COMBO BOX POPULATION):
public String[] getFoodChoices(String category)
{
  return dao.getFoodChoices(category);
}

}

---

RUNTIME ERROR:
NullPointerException
@ Controller.getFoodChoices
@ Listener.<init>
@ Controller.initialize
@ Controller.main

Any ideas?

Thanks so much. this is a personal project I'm doing over the summer.

Quote:

---

Originally Posted by Jeremy720

Hello. I'm trying to populate a combo box using an MVC model, and am using either serialization, or a database based on the users preference. I'm not sure exactly what's going on, but it's a visibility issue between the listener, controller, and interface classes upon instantiation...

FROM THE LISTENER:
public Listener(Controller pc)
{
this.pc = pc;
foodChoices = pc.getFoodChoices
(categoryList.getSelectedItem().toString());
foodList = new JComboBox(foodChoices);
...
con.add(foodList);
...
}

FROM THE CONTROLLER:

public class Controller()
{
public static void main(String[] args)
{
Controller app = newController();
app.initialize();
}

public void initialize()
{
pl = new Listener(this);
dir = System.getProperty("user.dir");
appPath = dir.substring(0,dir.lastIndexOf('\\'));
while (check)
{
String daoAccess = JOptionPane.showInputDialog(null,
"Enter S to read/write to text file, D to read/write to database");

if (daoAccess != null)
{
if (daoAccess.equalsIgnoreCase("d"))
{
sd = new SQLDAO(this);
pl = new Listener(this);
check = false;
}
else if (daoAccess.equalsIgnoreCase("s"))
{
dir = System.getProperty("user.dir");
appPath = dir.substring(0,dir.lastIndexOf('\\'));
pl = new ProteinListener(this);
sd = new SerialDAO(appPath);
check = false;
}
else if (daoAccess == "")
{
System.out.println("You must enter a data access type");
check = true;
}
else
{
Utility3.errorMessage("Invalid entry");
check = true;
}
}
else
break;
}
}
(METHOD FOR READING DATABASE/FILE FOR COMBO BOX POPULATION):
public String[] getFoodChoices(String category)
{
return dao.getFoodChoices(category);
}

}

RUNTIME ERROR:
NullPointerException
@ Controller.getFoodChoices
@ Listener.<init>
@ Controller.initialize
@ Controller.main

Any ideas?

Thanks so much. this is a personal project I'm doing over the summer.

---

Can you post your code in CODE tags?
It makes it really difficult to see what's going on otherwise.

However, what is "dao" in food choices, and where is it initialised?
I'm guessing a bit, but I think this is the culprit:

Code:

---

pl = new Listener(this);

---

This calls the getFoodChoices, however that happens before the bulk of the Controller has been initialised...so you get your NPE.

I've edited the original post to add code tags. To the original poster, please use tags when posting code so that your code retains its formatting and is thus readable. To do this, highlight the posted code and press the code button in the forum editor. Good luck!

thank you. firstly...I will post with 'code' tags'
secondly...thanks for your help. I had to wait to instantiate the listener class until the remainder of the controller class had run.