What is the controller in Java Swing?

Question

I would like to apply the MVC design to my Java application using Swing in a meaningful way. Therefore my question is, how the controllers would be structured in Java Swing?

I have two options in mind:

1. Every Component Listener is an own class, as part of the controller package
2. Every Component Listener is an anonymous class inside the view package which delegates its call to a class with controller methods.

Is both possible? Is it a question of preference, or is it clearly defined?

Answer 1

The Controller makes up the other half of the component interface, mainly the interaction half. The Controller takes care of mouse and keyboard events.

in Swing components like JButton etc are the controllers. and all listener classes redirect events to model that have your business logic

example :

main program

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);
    }
}

View

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);
    }
}

controller

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
}

Model

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();
    }
}

Answer 2

Platzhirsch,

Dude, this a SERIOUSLY heavy-weight question... one which nobody is going to answer adequately in a forum post. This artice Java SE Application Design With MVC by Robert Eckstein (one of the Java Gods) discusses the problem at length.

Personally, I ended-up using a "variant" of MVC which I called MBVC (Model Business View Controller), which is actually pretty close to MVVMC... MVVM is widely used in .NET cirles; adding a controller made sense to me, as I had some web MVC experience. I wish I'd read the above article BEFORE I set-out to MVC-ise my app. You can still read my rather perplexed posts on Sun's (now Oracle's) Java forums.

Cheers. Keith.