UMBC CMSC 331 Principles of Programming Languages

Model-View-Controller Example Code

adapted from an example by David Matuszek

This is a simple example of Model-View-Controller, with special emphasis on the Observer/Observable technique (highlighted in blue). All it does is put up a black circle that gradually turns red, then the program quits.

The Model does the work. Whenever it does something that another class might be interested in, it "broadcasts" the fact to all other classes that might be interested. Note that the Model does not need to know or care anything about which other classes, if any, might be interested, so it remains completely independent of those classes.

Model.java

import java.awt.*;
import java.util.*;
    
public class Model extends Observable {
    
    public Color color = new Color(0x010000);
    private int count = 10;
    
    void doSomething() {
        if (--count <= 0) {
            color = color.brighter();
            count = 10;
            // 
            setChanged();
            notifyObservers();

        }
    }
}

The View is interested in observing the Model class; it does this by implementing a version of the update method, which will automatically be called any time a class that it is observing broadcasts something.

View.java

import java.awt.*;
import java.util.*;

public class View extends Canvas implements Observer {
    
    Color color;
    
    public void paint(Graphics g) {
        if (color ==  null) return;
        g.setColor(color);
        g.fillOval(0, 0, getWidth(), getHeight());
    }
    
    public void update(Observable observable, Object arg) {
        color = ((Model)observable).color;
        repaint();
    }
}

The Controller in this program doesn't actually have any controls. However, it does set up the GUI, create the Model and the View, and register the View as an observer of the Model.

Controller.java

import java.awt.*;

public class Controller extends Frame {
    
    static Model model = new Model();
    static View view = new View();

    public static void main(String args[]) {
        Frame f = new Frame();
        f.add(view);
        f.setSize(200, 220);
        f.setVisible(true);
        
        model.addObserver(view);
        
        for (int i = 0; i < 200; i++) {
            model.doSomething();
            try { Thread.sleep(20); } catch (InterruptedException e) {}
        }
        f.dispose();
    }
}

This code is available as an executable jar file and also as a BlueJ project as a zip file.