Re: How to implement Global variables in java which can be shared across several files ?

Mark Space <>
Thu, 11 Jun 2009 15:24:18 -0700
aks_java wrote:

Yes, now its working but I just can't figure out how to avoid global

Ah, well that is a good question. First, you might want to decide what
your business logic is, and what the model is. See MVC:


A typical controller has a model and a view, but they are not global,
they are instance variables:

public class Controller {
   Model model;
   View view; //JFrame or JPanel

   public Controller( Model model, View view ) {

I would build these objects on the EDT, along with the gui. It's normal
to need the controller and the view at the same time. Here's a new main
method / entry point:

public class Calculator {

   public static void main( String... args ) {
      javax.swing.SwingUtilities.invokeLater( new Runnable () {
         public void run() {
   private void createAndShowGui() {
     View v = new MyView();
     Model m = new MyModel();
     Controller c = new Controller( m, v);
     //... more later

I think your model could be simple for a calculator program. Mostly you
just manipulate the value on the screen. So maybe a simple double value
is enough.

public class MyModel {
   private double screenValue;
   public double getValue() { return screenValue; }
   public void setValue(double d) {screenValue = d;}

Now you need to connect the View to the Model via the controller.
Here's where that "more later" bit in the comment above comes in

  v.addListener( new Controller.CalcActionListner() );

Where CalcActionListener is

public class Controller {

  .... same as above, plus....

static class CalcActionListener implements ActionListener {

   public void actionPerformed( ActionEvent e ) {
      String ac = e.getActionCommand();
      if( "+".eqauls(ac) ) {
        // add something and update Model
        double sum = model.getValue() + model.getValue();
        model.setValue( sum );
        view.updateScreen( sum );
      else if ...

Etc. There I've shown one way of binding the objects together. Now
that I think about it, one double isn't enough, because you need two
values, not one, for most math operations. And you need a way to
accumulate digits (characters?) as they are typed. So the thing is
going to be a bit more complicated than shown here.

Also a real model updates (via observer pattern) a view itself, not
usually the controller (action listener), but this model is so simple
you can skip that if you like.

This should be something you can break up into smaller bits and start
testing. That's what MVC is for, is to chunk the parts up so they can
be worked on and tested separately. I hope this gets you started.

Note: no code here was checked for correct syntax or semantics, but it
should be close.

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