Mediator

Diagram

Explain

The mediator pattern acts as a mediator by directing the communication between objects. This allows for lower coupling and more flexability and ease of modification if the interaction between objects is ever changed. As seen in the Diagram above the mediator pattern generally involves a mediator object which hold serveral colleagues. The mediator is used to relay messages between these colleagues.

Example

General Java Example

This example is a general example taken from [Execpc]

//
// Mediator.java
//
import java.io.*;

abstract class Mediator
{

public abstract void colleagueChanged(Colleague theChangedColleague);

public static void main(String args[])
{

ConcreteMediator aConcreteMediator = new ConcreteMediator();
aConcreteMediator.createConcreteMediator();

(aConcreteMediator.getOneColleage()).Display();
(aConcreteMediator.getNextColleague()).Display();

OneColleague newColleague = new OneColleague(aConcreteMediator, "OneColleague");

aConcreteMediator.colleagueChanged( (OneColleague)newColleague );

(aConcreteMediator.getOneColleage()).Display();
(aConcreteMediator.getNextColleague()).Display();

}

}
	//
// Colleague.java
//
abstract class Colleague
{
private Mediator _aMediator;

public Colleague(Mediator m)
{
_aMediator = m;
}

public void changed()
{
_aMediator.colleagueChanged(this);
}

public Mediator getMediator()
{
return(_aMediator);
}

public abstract void Display();
}
//
// ConcreteMediator.java
//
class ConcreteMediator extends Mediator
{
private OneColleague _aOneColleague;
private NextColleague _aNextColleague;

public void colleagueChanged( Colleague theChangedColleague )
{

if ((( OneColleague)theChangedColleague).getSelection().equals( _aOneColleague.getSelection() ) )
{
_aNextColleague.setText( _aOneColleague.getSelection() );
}

}

public void createConcreteMediator()
{

_aOneColleague = new OneColleague(this, "OneColleague");
_aNextColleague = new NextColleague(this, "NextColleague");

}

public OneColleague getOneColleage()
{
return(_aOneColleague);
}

public NextColleague getNextColleague()
{
return(_aNextColleague);
}
}
	//
// OneColleague.java
//
class OneColleague extends Colleague
{
private String _text;

public OneColleague(Mediator m, String t)
{
super( m );
_text = t;
}

public void setText(String text)
{
_text = text;
}

public String getSelection()
{
return(_text);
}

public void Display()
{
System.out.println("OneColleague = " + _text);
}
}

LimeWire

LimeWire uses several mediators in their coding. This Mediator is used to control the tip of the day window in limewire.

    	
package com.limegroup.gnutella.gui;

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Container;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.Font;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Enumeration;
import java.util.List;
import java.util.ResourceBundle;

import javax.swing.BorderFactory;
import javax.swing.Box;
import javax.swing.BoxLayout;
import javax.swing.JButton;
import javax.swing.JCheckBox;
import javax.swing.JComponent;
import javax.swing.JDialog;
import javax.swing.JEditorPane;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.SwingUtilities;
import javax.swing.WindowConstants;

import com.limegroup.gnutella.gui.themes.ThemeFileHandler;
import com.limegroup.gnutella.gui.themes.ThemeMediator;
import com.limegroup.gnutella.gui.themes.ThemeObserver;
import com.limegroup.gnutella.settings.StartupSettings;
import com.limegroup.gnutella.util.CommonUtils;


public final class TipOfTheDayMediator implements ThemeObserver {

    /**
     * The instance of this class.
     */
    private static TipOfTheDayMediator _instance;

    /**
     * The title for the TOTD window.
     */
    private static final String TOTD_TITLE =
        GUIMediator.getStringResource("TOTD_TITLE");

    /**
     * The 'Did You Know' intro.
     */
    private static final String TOTD_INTRO =
        GUIMediator.getStringResource("TOTD_INTRODUCTION");

    /**
     * The 'Show Tips At Startup' string
     */
    private static final String TOTD_STARTUP =
        GUIMediator.getStringResource("TOTD_SHOW_AT_STARTUP");

    /**
     * 'Next'.
     */
    private static final String TOTD_NEXT =
        GUIMediator.getStringResource("TOTD_NEXT");

    /**
     * 'Previous'.
     */
    private static final String TOTD_PREVIOUS =
        GUIMediator.getStringResource("TOTD_PREVIOUS");

    /**
     * 'Close'.
     */
    private static final String TOTD_CLOSE =
        GUIMediator.getStringResource("TOTD_CLOSE");

    /**
     * The actual TOTD JDialog.
     */
    private final JDialog _dialog = new JDialog();

    /**
     * The JTextComponent that displays the tip.
     */
    private final JEditorPane _tipPane = new JEditorPane();

    /**
     * The 'Previous' JButton.  Global so it can be
     * enabled/disabled.
     */
    private final JButton _previous;

    /**
     * The prefix to use for general tips.
     */
    private static final String GENERAL = "GENERAL_";

    /**
     * The prefix to use for OSX tips.
     */
    private static final String OSX = "OSX_";

    /**
     * The prefix to use for Windows tips.
     */
    private static final String WINDOWS = "WINDOWS_";

    /**
     * The prefix to use for Linux tips.
     */
    private static final String LINUX = "LINUX_";

    /**
     * The prefix to use for other all other OS' tips.
     */
    private static final String OTHER = "OTHER_";

    /**
     * The prefix to use for non OSX tips.
     */
    private static final String NOT_OSX = "NOT_OSX_";

    /**
     * The prefix to use for Pro tips.
     */
    private static final String PRO = "PRO_";

    /**
     * The prefix to use for Free tips.
     */
    private static final String FREE = "FREE_";

    /**
     * The list of keys that are valid in the resource bundle.
     */
    private static final List<String> KEYS = new ArrayList<String>();

    /**
     * The index of the current tip.
     */
    private static int _currentTip;

    /**
     * The foreground color to use for text.
     */
    private static Color _foreground;

    /**
     * Whether or not we can display the TOTD dialog.
     */
    private boolean _canDisplay = true;


    /**
     * Private constructor that initiates the appropriate things for the TOTD.
     */
    private TipOfTheDayMediator() {
        retrieveKeys();

        _dialog.setModal(false);
        _dialog.setResizable(false);
        _dialog.setTitle(TOTD_TITLE);
        GUIUtils.addHideAction((JComponent)_dialog.getContentPane());

        // Previous' listener must be added here instead of
        // in constructDialog because otherwise multiple
        // listeners will be added when the theme changes.
        _previous = new JButton(TOTD_PREVIOUS);
        _previous.addActionListener(new PreviousTipListener());
        constructDialog();
        ThemeMediator.addThemeObserver(this);
    }

    /**
     * Returns the sole instance of this class.
     */
    public static synchronized TipOfTheDayMediator instance() {
        if  (_instance == null)
            _instance = new TipOfTheDayMediator();
        return _instance;
    }

    /**
     * Redraws the whole dialog upon theme change.
     */
    public void updateComponentTreeUI() {
        SwingUtilities.updateComponentTreeUI(_dialog);
    }

    /**
     * Causes the TOTD window to become visible.
     */
    public void displayTipWindow() {
        GUIMediator.safeInvokeLater(new Runnable() {
            public void run() {
                if (!_canDisplay)
                    return;

                if (_dialog.isShowing()) {
                    _dialog.setVisible(false);
                    _dialog.setVisible(true);
                    _dialog.toFront();
                    return;
                }

                if (GUIMediator.isAppVisible())
                    _dialog.setLocationRelativeTo(GUIMediator.getAppFrame());
                else
                    _dialog.setLocation(GUIMediator.getScreenCenterPoint(_dialog));

                _dialog.setVisible(true);

                if (!"text/html".equals(_tipPane.getContentType())) {
                    SwingUtilities.invokeLater(new Runnable() {
                        public void run() {
                            _tipPane.setContentType("text/html");
                            setText(getRandomTip());
                        }
                    });
                }

                _dialog.toFront();
            }
        });
    }

    /**
     * Hides the TOTD dialogue window.
     */
    public void hide() {
        _dialog.setVisible(false);
    }

    /**
     * Sets the text of the tip to a new tip.
     */
    private void setText(String tip) {
        int r = _foreground.getRed();
        int g = _foreground.getGreen();
        int b = _foreground.getBlue();
        String foreHex = toHex(r) + toHex(g) + toHex(b);
        _tipPane.setText("<html><body text='#" + foreHex + "'>" + tip + "</html>");
        _tipPane.setCaretPosition(0);
    }

    /**
     * Returns the int as a hex string.
     */
    private String toHex(int i) {
        String hex = Integer.toHexString(i).toUpperCase();
        if(hex.length() == 1)
            return "0" + hex;
        else
            return hex;
    }

    /**
     * Iterates through all the tips' keys and stores the ones
     * that are valid for this OS.
     */
    private void retrieveKeys() {
        ResourceBundle bundle = ResourceManager.getTOTDResourceBundle();
        Enumeration<String> e = bundle.getKeys();
        while(e.hasMoreElements()) {
            final String k = e.nextElement();
            if(k.startsWith(GENERAL))
                KEYS.add(k);
            else if(CommonUtils.isWindows() && k.startsWith(WINDOWS))
                KEYS.add(k);
            else if(CommonUtils.isMacOSX() && k.startsWith(OSX))
                KEYS.add(k);
            else if(CommonUtils.isLinux() && k.startsWith(LINUX))
                KEYS.add(k);
            else if(!CommonUtils.isWindows() &&
                    !CommonUtils.isMacOSX() &&
                    !CommonUtils.isLinux() &&
                    k.startsWith(OTHER))
                KEYS.add(k);
            else if(!CommonUtils.isMacOSX() && k.startsWith(NOT_OSX))
                KEYS.add(k);
            else if(CommonUtils.isPro() && k.startsWith(PRO))
                KEYS.add(k);
            else if(!CommonUtils.isPro() && k.startsWith(FREE))
                KEYS.add(k);
        }

        // randomize the list.
        Collections.shuffle(KEYS);
        _currentTip = -1;
    }

    /**
     * Retrieves a random tip and updates the _currentTip
     * index to that tip.
     */
    private String getRandomTip() {
        // If this is our last key, reshuffle them.
        if(_currentTip == KEYS.size() - 1) {
            Collections.shuffle(KEYS);
            _currentTip = -1;
        } else if(_currentTip < -1)
            _currentTip = -1;

        String k = KEYS.get(++_currentTip);

        if(_currentTip == 0)
            _previous.setEnabled(false);
        else
            _previous.setEnabled(true);

        ResourceBundle bundle = ResourceManager.getTOTDResourceBundle();
        return bundle.getString(k);
    }

    /**
     * Recreates the dialog box to update the theme.
     */
    public void updateTheme() {
        boolean wasShowing = _dialog.isShowing();

        _dialog.setVisible(false);
        _dialog.getContentPane().removeAll();
        // Lower the size of the font in the TIP because
        // it's going to get larger again.
        Font tipFont = new Font(
            _tipPane.getFont().getName(),
            _tipPane.getFont().getStyle(),
            _tipPane.getFont().getSize()-2);
        _tipPane.setFont(tipFont);
        constructDialog();
        _tipPane.setContentType("text/html");
        setText(getRandomTip());


        if (wasShowing) {
            _dialog.setVisible(true);
            _dialog.toFront();
        }
    }

    /**
     * Builds the TOTD dialog.
     */
    private void constructDialog() {
        JPanel imagePanel = new JPanel();
        imagePanel.setBorder(BorderFactory.createMatteBorder(0, 0, 0, 1,
                         ThemeFileHandler.TABLE_BACKGROUND_COLOR.getValue()));
        JLabel img = new JLabel(GUIMediator.getThemeImage("question"));
        imagePanel.add(img);

        JPanel didYouKnowPanel = new JPanel(new FlowLayout(FlowLayout.LEFT));
        JLabel didYouKnow = new JLabel(TOTD_INTRO);
        Font didYouKnowFont = new Font(
            "Dialog",
            didYouKnow.getFont().getStyle(),
            didYouKnow.getFont().getSize()+5);
        didYouKnow.setFont(didYouKnowFont);
        didYouKnowPanel.add(Box.createHorizontalStrut(3));
        didYouKnowPanel.add(didYouKnow);

        JPanel tipPanel = new JPanel();
        _foreground = didYouKnow.getForeground();
        tipPanel.setBorder(BorderFactory.createMatteBorder(1, 0, 0, 0,
                       ThemeFileHandler.TABLE_BACKGROUND_COLOR.getValue()));
        // THE HTML ENGINE TAKES TOO LONG TO LOAD, SO WE MUST LOAD AS TEXT.
        _tipPane.setContentType("text");
        _tipPane.setEditable(false);
        _tipPane.setBackground(tipPanel.getBackground());
        Font tipFont = new Font(
            "Dialog",
            _tipPane.getFont().getStyle(),
            _tipPane.getFont().getSize()+2);
        _tipPane.setFont(tipFont);
        _tipPane.addHyperlinkListener(GUIUtils.getHyperlinkListener());
        _tipPane.setText(GUIMediator.getStringResource("TOTD_LOADING_TIPS"));
        JScrollPane tipScroller = new JScrollPane(_tipPane);
        tipScroller.setPreferredSize(new Dimension(400, 100));
        tipScroller.setHorizontalScrollBarPolicy(
            JScrollPane.HORIZONTAL_SCROLLBAR_NEVER);
        tipScroller.setVerticalScrollBarPolicy(
            JScrollPane.VERTICAL_SCROLLBAR_AS_NEEDED);
        tipScroller.setBorder(null);
        tipPanel.add(tipScroller);

        BoxPanel rightTip = new BoxPanel();
        rightTip.add(Box.createVerticalStrut(10));
        rightTip.add(didYouKnowPanel);
        rightTip.add(tipPanel);

        JPanel wholeTip = new JPanel(new BorderLayout());
        BoxPanel innerTip = new BoxPanel(BoxPanel.X_AXIS);
        innerTip.add(imagePanel);
        innerTip.add(rightTip);
        innerTip.setBorder(BorderFactory.createLoweredBevelBorder());
        wholeTip.add(Box.createHorizontalStrut(5), BorderLayout.WEST);
        wholeTip.add(Box.createHorizontalStrut(5), BorderLayout.EAST);
        wholeTip.add(Box.createVerticalStrut(5), BorderLayout.NORTH);
        wholeTip.add(Box.createVerticalStrut(5), BorderLayout.SOUTH);
        wholeTip.add(innerTip, BorderLayout.CENTER);

        JPanel startupPanel = new JPanel(new FlowLayout(FlowLayout.LEFT));
        JCheckBox showTips = new JCheckBox(TOTD_STARTUP);
        showTips.setSelected(StartupSettings.SHOW_TOTD.getValue());
        startupPanel.add(showTips);

        JPanel buttonPanel = new JPanel(new FlowLayout(FlowLayout.RIGHT));
        buttonPanel.add(_previous);
        JButton next = new JButton(TOTD_NEXT);
        buttonPanel.add(next);
        JButton close = new JButton(TOTD_CLOSE);
        buttonPanel.add(close);

        JPanel navigation = new JPanel(new BorderLayout());
        navigation.add(startupPanel, BorderLayout.WEST);
        navigation.add(buttonPanel, BorderLayout.EAST);

        showTips.addActionListener(new ShowTipListener());
        next.addActionListener(new NextTipListener());
        close.addActionListener(GUIUtils.getDisposeAction());

        _dialog.setDefaultCloseOperation(WindowConstants.DISPOSE_ON_CLOSE);

        Container pane = _dialog.getContentPane();
        pane.setLayout(new BoxLayout(pane, BoxLayout.Y_AXIS));
        pane.add(wholeTip);
        pane.add(Box.createVerticalStrut(5));
        pane.add(navigation);
        try {
            _dialog.pack();
        } catch(OutOfMemoryError oome) {
            // who knows why it happens, but it's an internal error.
            _canDisplay = false;
        }
    }

    /**
     * A listener for changing the state of the 'Show Tips on Startup'.
     */
    private class ShowTipListener implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            JCheckBox source = (JCheckBox)e.getSource();
            StartupSettings.SHOW_TOTD.setValue(source.isSelected());
        }
    }

    /**
     * A listener for showing the next tip.
     */
    private class NextTipListener implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            setText(getRandomTip());
        }
    }

    /**
     * A listener for showing the previous tip.
     */
    private class PreviousTipListener implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            _currentTip = _currentTip - 2;
            setText(getRandomTip());
        }
    }
}

References

sern.ucalgary.ca

www.dofactory.com

my.execpc.com