MVC pattern Java Sample code.

[carl varola]

All,
Any sample java codes that implements the MVC pattern please?
Thanks.

[Cindy Glass]

Moving to Patterns.

[Frank Carver]

Something like the following is probably the simplest example:

class Model
{
  public int getValue() { return total; }
  public int add(int increment) { total += increment; }
  
  private int total;
}
 
class View
{
  public View(Model model) { this.model = model; }
  public void refresh() { display = model.getValue(); }
  
  private int display;
  private Model model;
}
 
class Controller
{
  public Controller(Model model, View view) 
  { 
    this,model = model;
    this.view = view; 
  }
  public void add(int increment) 
  {
    model.add(increment);
    view.refresh();
  }
  
  private Model model;
  private View view;
}

However, as it stands, that's not very useful. The whole point of patterns is that they are not just pre-built code libraries. So to give a sensible code example for you, we really need to know a bit more about your application.
Can you tell us a bit more about what you need ?

[carl varola]

Frank,
Thanks for the sample code.
All what I need is to show my students a working sample code to demonstrate MVC pattern.
I cannot see complete decoupling between those 3 layers in the sample code,would you please modified that codes in a way where I can see MVC better?
Usually the start point is the view...right?I mean I will put main() method there?
We have lots of discussing about MVC and seems to me we cannot see it very well,please any possible clear explanation?
Thanks.

[Frank Carver]

Well I'm still waiting to learn a bit more about your application. All you have told me is that you want to start it from "main". This tends to imply that it's not a servlet or applet based example you are looking for, as they don't have a "main". Please tell us more about what context you are thinking of applying this pattern in.
In the meanwhile, if you are more concerned about decoupling, consider this slightly more elaborate example:

import java.util.List;
import java.util.ArrayList;
import java.util.Iterator;
import java.io.InputStreamReader;
import java.io.BufferedReader;
import java.io.IOException;
interface Model
{
  public Object getValue();
}
class CalculatorModel implements Model
{
  public Object getValue() { return new Integer(total); }
  public void add(int increment) { total += increment; }
  public void subtract(int increment) { total -= increment; }
  private int total = 0;
}
interface View
{
  public void setModel(Model model);
  public void refresh();
}
class TextView implements View
{
  public TextView(int id) { this.id = id; }
  public void setModel(Model model) { this.model = model; }
  public void refresh() { System.out.println("View " + id + ": " + model.getValue()); }
  private Model model;
  private int id;
}
class CalculatorController
{
  public CalculatorController()
  {
    this.model = new CalculatorModel();
    this.views = new ArrayList();
  }
  public int addView(View view)
  {
    view.setModel(model);
    views.add(view);
    return views.size();
  }
  public void removeView(int index)
  {
    views.remove(index);
  }
  public void refresh()
  {
    Iterator it = views.iterator();
    while (it.hasNext())
    {
    ((View)it.next()).refresh();
    }
  }
  public void operation(String op, int increment)
  {
    boolean changed = false;
    if (increment != 0)
    {
      if ("+".equals(op))
      {
        model.add(increment);
        changed = true;
      }
      else if ("-".equals(op))
      {
        model.subtract(increment);
        changed = true;
      }
      else
      {
        System.out.println("unrecognized operation '" + op + "'");
      }
    }
    if (changed)
    {
      refresh();
    }
  }
  private CalculatorModel model;
  private List views;
}
public class Application
{
  public Application()
  {
    in = new BufferedReader(new InputStreamReader(System.in));
    cc = new CalculatorController();
    nViews = cc.addView(new TextView(0));
  }
  private String getLine()
    throws IOException
  {
    System.out.print("op: ");
    System.out.flush();
    return in.readLine();
  }
  private void run()
    throws IOException
  {
    for (String line = getLine(); line != null; line = getLine())
    {
      if (line.length() == 0) { System.out.println("use +|-|A|R|X {number}"); continue; }
      String op = line.substring(0,1);
      if ("X".equalsIgnoreCase(op))
      {
        System.out.println("Exit.  Goodbye.");
        break;
      }
      if ("A".equalsIgnoreCase(op))
      {
        View view = new TextView(nViews);
      System.out.println("created new view " + nViews);
      nViews = cc.addView(view);
      continue;
      }
      if ("=".equals(op))
      {
        cc.refresh();
        continue;
      }
      String tail = line.substring(1).trim();
      int number = 0;
      try
      {
        number = Integer.parseInt(tail);
      }
      catch(NumberFormatException e)
      {
        System.out.println("oops, '" + tail + "' is not a number, try again!");
      }
      if ("R".equalsIgnoreCase(op))
      {
        cc.removeView(number);
      }
      else
      {
        cc.operation(op, number);
      }
    }
  }
  public static void main(String[] args)
  throws java.io.IOException
  {
    new Application().run();
  }
  private BufferedReader in;
  private CalculatorController cc;
  private int nViews;
}

I've tested it so I know it will compile. Save the code block as "Application.java", Compile it with "javac Application.java", and run it with "java Application".
At the prompt, you can type one of the following commands:

X :- to exit the calculator

A :- to add a new view. The controller can have as many views as you like.

R followed by a number :- to remove the numbered view

+ followed by a number :- to add the number to the stored value

- followed by a number :- to subtract the number from the stored value

= to refresh all the views

try the following sequence, as an example

java Application
op: =
view 0: 0
op: A
created new view 1
op: =
view 0: 0
view 1: 0
op: +23
view 0: 23
view 1: 23
op: R0
op: =
view 1: 23
op: X
Exit!. Goodbye.

Does this offer any more help in understanding how MVC can help with decoupling?

[carl varola]

Frank,
Thanks alot for your time to develop that,i will study the code and let you know.
Thanks.