Java: Model-View-Controller (MVC) Structure

// stucture/calc-mvc/CalcMVC.java -- Calculator in MVC pattern.
// Fred Swartz -- December 2004

import javax.swing.*;

public class CalcMVC {
    //... Create model, view, and controller.  They are
    //    created once here and passed to the parts that
    //    need them so there is only one copy of each.
    public static void main(String[] args) {
        
        CalcModel      model      = new CalcModel();
        CalcView       view       = new CalcView(model);
        CalcController controller = new CalcController(model, view);
        
        view.setVisible(true);
    }
}

// structure/calc-mvc/CalcView.java - View component
//    Presentation only.  No user actions.
// Fred Swartz -- December 2004

import java.awt.*;
import javax.swing.*;
import java.awt.event.*;

class CalcView extends JFrame {
    //... Constants
    private static final String INITIAL_VALUE = "1";
    
    //... Components
    private JTextField m_userInputTf = new JTextField(5);
    private JTextField m_totalTf     = new JTextField(20);
    private JButton    m_multiplyBtn = new JButton("Multiply");
    private JButton    m_clearBtn    = new JButton("Clear");
    
    private CalcModel m_model;
    
    //======================================================= constructor
    /** Constructor */
    CalcView(CalcModel model) {
        //... Set up the logic
        m_model = model;
        m_model.setValue(INITIAL_VALUE);
        
        //... Initialize components
        m_totalTf.setText(m_model.getValue());
        m_totalTf.setEditable(false);
        
        //... Layout the components.      
        JPanel content = new JPanel();
        content.setLayout(new FlowLayout());
        content.add(new JLabel("Input"));
        content.add(m_userInputTf);
        content.add(m_multiplyBtn);
        content.add(new JLabel("Total"));
        content.add(m_totalTf);
        content.add(m_clearBtn);
        
        //... finalize layout
        this.setContentPane(content);
        this.pack();
        
        this.setTitle("Simple Calc - MVC");
        // The window closing event should probably be passed to the 
        // Controller in a real program, but this is a short example.
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }
    
    void reset() {
        m_totalTf.setText(INITIAL_VALUE);
    }
    
    String getUserInput() {
        return m_userInputTf.getText();
    }
    
    void setTotal(String newTotal) {
        m_totalTf.setText(newTotal);
    }
    
    void showError(String errMessage) {
        JOptionPane.showMessageDialog(this, errMessage);
    }
    
    void addMultiplyListener(ActionListener mal) {
        m_multiplyBtn.addActionListener(mal);
    }
    
    void addClearListener(ActionListener cal) {
        m_clearBtn.addActionListener(cal);
    }
}

// stucture/calc-mvc/CalcController.java - Controller
//    Handles user interaction with listeners.
//    Calls View and Model as needed.
// Fred Swartz -- December 2004

import java.awt.event.*;

public class CalcController {
    //... The Controller needs to interact with both the Model and View.
    private CalcModel m_model;
    private CalcView  m_view;
    
    //========================================================== constructor
    /** Constructor */
    CalcController(CalcModel model, CalcView view) {
        m_model = model;
        m_view  = view;
        
        //... Add listeners to the view.
        view.addMultiplyListener(new MultiplyListener());
        view.addClearListener(new ClearListener());
    }
    
    
    ////////////////////////////////////////// inner class MultiplyListener
    /** When a mulitplication is requested.
     *  1. Get the user input number from the View.
     *  2. Call the model to mulitply by this number.
     *  3. Get the result from the Model.
     *  4. Tell the View to display the result.
     * If there was an error, tell the View to display it.
     */
    class MultiplyListener implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            String userInput = "";
            try {
                userInput = m_view.getUserInput();
                m_model.multiplyBy(userInput);
                m_view.setTotal(m_model.getValue());
                
            } catch (NumberFormatException nfex) {
                m_view.showError("Bad input: '" + userInput + "'");
            }
        }
    }//end inner class MultiplyListener
    
    
    //////////////////////////////////////////// inner class ClearListener
    /**  1. Reset model.
     *   2. Reset View.
     */    
    class ClearListener implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            m_model.reset();
            m_view.reset();
        }
    }// end inner class ClearListener
}

// structure/calc-mvc/CalcModel.java
// Fred Swartz - December 2004
// Model
//     This model is completely independent of the user interface.
//     It could as easily be used by a command line or web interface.

import java.math.BigInteger;

public class CalcModel {
    //... Constants
    private static final String INITIAL_VALUE = "0";
    
    //... Member variable defining state of calculator.
    private BigInteger m_total;  // The total current value state.
    
    //============================================================== constructor
    /** Constructor */
    CalcModel() {
        reset();
    }
    
    //==================================================================== reset
    /** Reset to initial value. */
    public void reset() {
        m_total = new BigInteger(INITIAL_VALUE);
    }
    
    //=============================================================== multiplyBy
    /** Multiply current total by a number.
    *@param operand Number (as string) to multiply total by.
    */
    public void multiplyBy(String operand) {
        m_total = m_total.multiply(new BigInteger(operand));
    }
    
    //================================================================= setValue
    /** Set the total value. 
    *@param value New value that should be used for the calculator total. 
    */
    public void setValue(String value) {
        m_total = new BigInteger(value);
    }
    
    //================================================================= getValue
    /** Return current calculator total. */
    public String getValue() {
        return m_total.toString();
    }
}