Thread: Teaching myself Java – how to pass on an object to an event listener?

I’m teaching myself Java. I am a fairly proficient programmer in other languages, but this is the first OO language I’m doing.

I have a question that is a bit hard to summarize. Or it should be: how can I pass on an object (or variable) to an event listener?

I am writing an application in which you can play a Sudoku game. I have separated the “logic” or the “model”(the classes with Sudoku data structures and methods to manipulate them) from the presentation (the view and the controller).

The main method starts off as follows:

SudokuModel model = new SudokuModel();
SudokuView viewController = new SudokuViewController(model);

The first line creates class for the logic and the second line creates the class for the view and the controller. Since the view and the controller need access to the business logic, the model is passed on to the ViewController class.

The SudokuViewController class creates the user interface in Swing and it handles the user input. For the user input I have created a number of listeners, like this:

table.addKeyListener(this);

Now these listeners need access to the model since they update it. However, as far as I’m aware the only parameter passed on to an event listener is the event itself. So these event listeners do not have direct access to the model, even though it is passed on to the constructor of the class SudokuViewController.

To circumvent this, I made model2 an attribute (variable) of the class SudokuViewController. The constructor of the class sets this variable as follows:

model2 = model;

Now the event listeners have access to model2, which they can manipulate.

This works. However, I think it is an ugly solution, introducing an additional object (model2). How can I solve my problem without doing so? I’d like to pass on the object model to the event listener, but this doesn’t seem to be possible.

Your variable "model2" is not an additional Object. Its a variable. And in your case it references the same object as model.
Whether your solution is ugly or not depends on your definition of ugly. If your controller is supposed to control your model then it makes sense that the controller *knows* the model, that is, saves the model in a variable. Otherwise it would be impossible to manipulate / control it.

What might be ugly is that its not your controller that is manipulating the model, but in fact, its the Listeners that are manipulating the model. What you can do instead is make the actual Controller manipulate the model with several methods of its own, but these methods are called from the Listeners.
The end result is the very same, but the idea behind it makes much more sense to me. Listeners tell the controller what is happening and the controller is manipulating the model accordingly.
But its your decision in the end.

Thread moved.

Thanks both! Very useful information!

I think the clone() function may be of some use here.

If a variable points to an object and you do something to the variable, does it affect the object?

model2 = model;

Is there such a thing as a pointer in Java?

@Chroot: It's hard to tell if you're giving advice or starting a new topic. You might investigate pointers in Java and whether Java is pass by value, pass by reference, or capable of doing both. If you need more help with those topics, please start your own thread.

If a variable points to an object and you do something to the variable, does it affect the object?

Yes, it does. For example when you create a new JFrame:

public class JFrameExample {
 
    private static JFrame frame;                        //<-------- We declare the frame-variable, which soon will hold the JFrame
 
    public static void main(String[] args) {
        frame = new JFrame("Title");                     //<------- We create the JFrame (and add it to the frame-variable)
 
        frame.setSize(400, 300);                      //<------ We set the size of the frame, which affects the object (the newly created JFrame) that is contained in the frame-variable.
    }
}

/TheDDestroyer12

I don't do either. I was too fast to post it. Sorry. Ignore it.

What package did you use in the jsdk 1.4.3 or something like that on your web start or JVM at home?