The simplest Model View Controller (MVC) Java example

//Model.java
//(C) Joseph Mack 2011, jmack (at) wm7d (dot) net, released under GPL v3 (or any later version)

//inspired by Joseph Bergin's MVC gui at http://csis.pace.edu/~bergin/mvc/mvcgui.html

//Model holds an int counter (that's all it is).
//Model is an Observable
//Model doesn't know about View or Controller

public class Model extends java.util.Observable {	
	
	private int counter;	//primitive, automatically initialised to 0

	public Model(){

		System.out.println("Model()");

		/**
		Problem initialising both model and view:

		On a car you set the speedometer (view) to 0 when the car (model) is stationary.
		In some circles, this is called calibrating the readout instrument.
		In this MVC example, you would need two separate pieces of initialisation code,
			in the model and in the view. If you changed the initialisation value in one
			you'd have to remember (or know) to change the initialisation value in the other.
			A recipe for disaster.

		Alternately, when instantiating model, you could run  

		setValue(0);

		as part of the constructor, sending a message to the view. 
		This requires the view to be instantiated before the model,
		otherwise the message will be send to null (the unitialised value for view).
		This isn't a particularly onerous requirement, and is possibly a reasonable approach.

		Alternately, have RunMVC tell the view to intialise the model.
		The requires the view to have a reference to the model.
		This seemed an unneccesary complication.

		I decided instead in RunMVC, to instantiate model, view and controller, make all the connections,
		then since the Controller already has a reference to the model (which it uses to alter the status of the model),
		to initialise the model from the controller and have the model automatically update the view.
		*/

	} //Model()

	//uncomment this if View is using Model Pull to get the counter
	//not needed if getting counter from notifyObservers()
	//public int getValue(){return counter;}

	//notifyObservers()
	//model sends notification to view because of RunMVC: myModel.addObserver(myView)
	//myView then runs update()
	//
	//model Push - send counter as part of the message
	public void setValue(int value) {

		this.counter = value;
		System.out.println("Model init: counter = " + counter);
		setChanged();
		//model Push - send counter as part of the message
		notifyObservers(counter);
		//if using Model Pull, then can use notifyObservers()
		//notifyObservers()

	} //setValue()
	
	public void incrementValue() {

		++counter;
		System.out.println("Model     : counter = " + counter);
		setChanged();
		//model Push - send counter as part of the message
		notifyObservers(counter);
		//if using Model Pull, then can use notifyObservers()
		//notifyObservers()

	} //incrementValue()
	
} //Model

//View.java
//(C) Joseph Mack 2011, jmack (at) wm7d (dot) net, released under GPL v3 (or any later version)

//inspired by Joseph Bergin's MVC gui at http://csis.pace.edu/~bergin/mvc/mvcgui.html

//View is an Observer

import java.awt.Button;
import java.awt.Panel;
import java.awt.Frame;
import java.awt.TextField;
import java.awt.Label;
import java.awt.event.WindowEvent;	//for CloseListener()
import java.awt.event.WindowAdapter;	//for CloseListener()
import java.lang.Integer;		//int from Model is passed as an Integer
import java.util.Observable;		//for update();

class View implements java.util.Observer {

	//attributes as must be visible within class
	private TextField myTextField;
	private Button button; 

	//private Model model;		//Joe: Model is hardwired in, 
					//needed only if view initialises model (which we aren't doing)
	
	View() {
		System.out.println("View()");	
		
		//frame in constructor and not an attribute as doesn't need to be visible to whole class
		Frame frame 		= new Frame("simple MVC");
		frame.add("North", new Label("counter"));

		myTextField 		= new TextField();
		frame.add("Center", myTextField);

		//panel in constructor and not an attribute as doesn't need to be visible to whole class
		Panel panel 		= new Panel();
		button	 		= new Button("PressMe");
		panel.add(button);
		frame.add("South", panel);		

		frame.addWindowListener(new CloseListener());	
		frame.setSize(200,100);
		frame.setLocation(100,100);
		frame.setVisible(true);

	} //View()

	// Called from the Model
    	public void update(Observable obs, Object obj) {

		//who called us and what did they send?
		//System.out.println ("View      : Observable is " + obs.getClass() + ", object passed is " + obj.getClass());

		//model Pull 
		//ignore obj and ask model for value, 
		//to do this, the view has to know about the model (which I decided I didn't want to do)
		//uncomment next line to do Model Pull
    		//myTextField.setText("" + model.getValue());

		//model Push 
		//parse obj
		myTextField.setText("" + ((Integer)obj).intValue());	//obj is an Object, need to cast to an Integer

    	} //update()

	//to initialise TextField
	public void setValue(int v){
    		myTextField.setText("" + v);
	} //setValue()
    	
	public void addController(Controller controller){
		System.out.println("View      : adding controller");
		button.addActionListener(controller);	//need controller before adding it as a listener 
	} //addController()

	//uncomment to allow controller to use view to initialise model	
	//public void addModel(Model m){
	//	System.out.println("View      : adding model");
	//	this.model = m;
	//} //addModel()
	
	public static class CloseListener extends WindowAdapter {
		public void windowClosing(WindowEvent e) {
			e.getWindow().setVisible(false);
			System.exit(0);
		} //windowClosing()
	} //CloseListener

} //View

        public void addController(Controller controller){
                button.addActionListener(controller);
        } //addController()

import java.awt.event.ActionListener;
.
.
        public void addController(ActionListener controller){
                button.addActionListener(controller);
        } //addController()

addActionListener(java.awt.event.ActionListener) in java.awt.Button cannot be applied to (java.util.EventListener)

//View.java
//(C) Joseph Mack 2011, jmack (at) wm7d (dot) net, released under GPL v3 (or any later version)

//inspired by Joseph Bergin's MVC gui at http://csis.pace.edu/~bergin/mvc/mvcgui.html

//View is an Observer

import java.awt.Button;
import java.awt.Panel;
import java.awt.Frame;
import java.awt.TextField;
import java.awt.Label;
import java.awt.event.WindowEvent;	//for CloseListener()
import java.awt.event.WindowAdapter;	//for CloseListener()
import java.lang.Integer;		//int from Model is passed as an Integer
import java.util.Observable;		//for update();
import java.awt.event.ActionListener;	//for addController()


class View implements java.util.Observer {

	//attributes as must be visible within class
	private TextField myTextField;
	private Button button; 

	//private Model model;		//Joe: Model is hardwired in, 
					//needed only if view initialises model (which we aren't doing)
	
	View() {
		System.out.println("View()");	
		
		//frame in constructor and not an attribute as doesn't need to be visible to whole class
		Frame frame 		= new Frame("simple MVC");
		frame.add("North", new Label("counter"));

		myTextField 		= new TextField();
		frame.add("Center", myTextField);

		//panel in constructor and not an attribute as doesn't need to be visible to whole class
		Panel panel 		= new Panel();
		button	 		= new Button("PressMe");
		panel.add(button);
		frame.add("South", panel);		

		frame.addWindowListener(new CloseListener());	
		frame.setSize(200,100);
		frame.setLocation(100,100);
		frame.setVisible(true);

	} //View()

	// Called from the Model
    	public void update(Observable obs, Object obj) {

		//who called us and what did they send?
		//System.out.println ("View      : Observable is " + obs.getClass() + ", object passed is " + obj.getClass());

		//model Pull 
		//ignore obj and ask model for value, 
		//to do this, the view has to know about the model (which I decided I didn't want to do)
		//uncomment next line to do Model Pull
    		//myTextField.setText("" + model.getValue());

		//model Push 
		//parse obj
		myTextField.setText("" + ((Integer)obj).intValue());	//obj is an Object, need to cast to an Integer

    	} //update()

	//to initialise TextField
	public void setValue(int v){
    		myTextField.setText("" + v);
	} //setValue()
    	
	public void addController(ActionListener controller){
		System.out.println("View      : adding controller");
		button.addActionListener(controller);	//need instance of controller before can add it as a listener 
	} //addController()

	//uncomment to allow controller to use view to initialise model	
	//public void addModel(Model m){
	//	System.out.println("View      : adding model");
	//	this.model = m;
	//} //addModel()
	
	public static class CloseListener extends WindowAdapter {
		public void windowClosing(WindowEvent e) {
			e.getWindow().setVisible(false);
			System.exit(0);
		} //windowClosing()
	} //CloseListener

} //View

//Controller.java
//(C) Joseph Mack 2011, jmack (at) wm7d (dot) net, released under GPL v3 (or any later version)

//inspired by Joseph Bergin's MVC gui at http://csis.pace.edu/~bergin/mvc/mvcgui.html

//Controller is a Listener

class Controller implements java.awt.event.ActionListener {

	//Joe: Controller has Model and View hardwired in
	Model model;
	View view;

	Controller() {	
		System.out.println ("Controller()");
	} //Controller()

	//invoked when a button is pressed
	public void actionPerformed(java.awt.event.ActionEvent e){
		//uncomment to see what action happened at view
		/*
		System.out.println ("Controller: The " + e.getActionCommand() 
			+ " button is clicked at " + new java.util.Date(e.getWhen())
			+ " with e.paramString " + e.paramString() );
		*/
		System.out.println("Controller: acting on Model");
		model.incrementValue();
	} //actionPerformed()

	//Joe I should be able to add any model/view with the correct API
	//but here I can only add Model/View
	public void addModel(Model m){
		System.out.println("Controller: adding model");
		this.model = m;
	} //addModel()

	public void addView(View v){
		System.out.println("Controller: adding view");
		this.view = v;
	} //addView()

	public void initModel(int x){
		model.setValue(x);
	} //initModel()

} //Controller

//RunMVC.java
//(C) Joseph Mack 2011, jmack (at) wm7d (dot) net, released under GPL v3 (or any later version)

public class RunMVC {

	//The order of instantiating the objects below will be important for some pairs of commands.
	//I haven't explored this in any detail, beyond that the order below works.

	private int start_value = 10;	//initialise model, which in turn initialises view

	public RunMVC() {

		//create Model and View
		Model myModel 	= new Model();
		View myView 	= new View();

		//tell Model about View. 
		myModel.addObserver(myView);
		/*	
			init model after view is instantiated and can show the status of the model
			(I later decided that only the controller should talk to the model
			and moved initialisation to the controller (see below).)
		*/
		//uncomment to directly initialise Model
		//myModel.setValue(start_value);	

		//create Controller. tell it about Model and View, initialise model
		Controller myController = new Controller();
		myController.addModel(myModel);
		myController.addView(myView);
		myController.initModel(start_value);

		//tell View about Controller 
		myView.addController(myController);
		//and Model, 
		//this was only needed when the view inits the model
		//myView.addModel(myModel);

	} //RunMVC()

} //RunMVC

//(C) Joseph Mack 2011, jmack (at) wm7d (dot) net, released under GPL v3 (or any later version)

public class Main{

	public static void main(String[] args){

		RunMVC mainRunMVC = new RunMVC();

	} //main()

} //Main

jar cfe RunMVC.jar Main *.class