Mediator Kensho Tsuchihashi. Mediator Page 2 Table of Contents 1.What is Mediator? 2.What problem...

Mediator

Kensho Tsuchihashi

Page 2Mediator

Table of Contents

1. What is Mediator?

2. What problem does Mediator solve?

3. Advantage and Disadvantage

4. Additional information

5. Structure

6. Example

7. Code

Page 3Mediator


Mediator is one of the 23 Design Patterns which were selected by the GoF (Gang of Four).

Purpose

BehaviorStructureCreation

InterpreterTemplate

Scope

Factory Method

Chain of ResponsibilityCommandIteratorMediatorMementoObserverStateStrategyVisitor

InterpreterTemplate

Class

Objects

AdapterBridgeCompositeDecoratorFaçadeFlyweightProxy

Abstract FactoryBuilderPrototypeSingleton

Page 4Mediator


Mediator

- Somebody who tries to settle a dispute between other parties.

In this context..

Mediator

- Pattern that defines simplified communication between classes.

Page 5Mediator


More formally..

- Mediator defines an object that encapsulates how a set of objects interact. Mediator promotes loose coupling by keeping objects from reffering to each other explicitly, and it lets you vary their interaction indepently.

Page 6Mediator


What if every objects communicate to each other one by one?

Page 7Mediator


Existence of Mediator makes things much simpler.

Page 8Mediator

Object-Oriented Design

Motivation

Encourages the distribution of behavior among objects. As a result, an object structure tends to be many connections between objects; in the worst

case, every object ends up knowing each other. (Spaghetti code)

Mediator

Is responsible for controlling and coordinating the interactions for a group from referring to each other explicitly.


Page 9Mediator

Use Mediator when

Applicability


A set of objects communicate in well-defined but complex ways. The resulting interdependencies are unstructured and difficult to understand.

Reuse an object is difficult because it refers to and communicates with many other objects.

A behavior that’s distributed between several classes should be customizable without a lot of subclassing.

Page 10Mediator

3-1. Advantage

Mediator helps clarify how objects interact in a system.

3-2. Disadvantage

Mediator pattern trades complexity of interaction for complexity in the mediator.

Mediator can become more complex than any individual colleague, which makes mediator itself hard to maintain.

3. Advantage and Disadvantage

Page 11Mediator

Conductor Meeting scheduler Airport control tower Traffic controller Stock Market

4. Additional information

Analogies

Page 12Mediator

5. Structure

In UML

ConcreteColleague2ConcreteColleague2ConcreteMediator

ColleagueMediator+Mediator

Page 13Mediator

6. Example

Login Dialog

Select “Guest“ or “Login“.

When user login, input “username“ and “password“.

Press “OK“ or “Cancel“.

.. Looks so simple

Page 14Mediator

6. Example

But what if with these requirements?

When “Guest“ is selected, “username“and “password“ should be disabled.

When “Login“ is selected, “username“ should be enabled.

When there is no character in “username“, “password“ should be disabled.

When user inputs any characters in “username“, “password“ should be enabled.

When there is any character both in “username“ and “password“, “OK“ should be enabled. If not, “OK“ should be disabled. (When “Guest“ is selected, “OK“ should be enabled.)

.. pretty complicated!

Page 15Mediator

See real program.

Page 16Mediator

7. Code

7-1. List of classes

No Name Description

1 Mediator Interface that defines API of mediator

2 Colleague Interface that defines API of member

3 ColleagueButton Class that represents button and implements Colleague Interface

4 ColleagueTextField Class that represents text field and implements Colleague Interface

5 ColleagueCheckbox Class that represents checkbox and implements Colleague Interface

6 LoginFrame Class that represents login dialog and implements Mediator Interface

7 Main Class that has main

Page 17Mediator

7. Code

7-2. Class Diagram<<interface>>

Colleague

setMediatorsetColleagueEnabled

ColleagueTextField

mediatorsetMediatorsetColleagueEnabledtextValueChanged

TextField

Frame

CheckboxColleagueCheckbox

mediatorsetMediatorsetColleagueEnableditemStateChanged

ColleagueButton

mediatorsetMediatorsetColleagueEnabled

<<interface>>Mediator

createColleaguecolleagueChanged

LoginFrame

checkGuestcheckLogintextUsertextPassbuttonOkbuttonCancel

createColleaguecolleagueChangeduserpassChangedactionPerformed

Button

Page 18Mediator

7. Code

7-3. Sequence Diagram

: LoginFrame : ColleagueCheckbox : ColleagueTextField : ColleagueButton

Page 19Mediator

7. Code

7-4. (1) Mediator

public interface Mediator {

public abstract void createColleagues();

public abstract void colleagueChanged();

}

Page 20Mediator

7. Code

7-4. (2) Colleague

public interface Colleague {

public abstract void setMediator(Mediator mediator);

public abstract void setColleagueEnabled(boolean enabled);

}

Page 21Mediator

7. Code

7-4. (3) ColleagueButton

import java.awt.Button;

public class ColleagueButton extends Button implements Colleague {

private Mediator mediator;

public ColleagueButton(String caption) {

super(caption);

}

public void setMediator(Mediator mediator) {

this.mediator = mediator;

}

public void setColleagueEnabled(boolean enabled) { // Mediator will tell you enabled/disabled

setEnabled(enabled);

}

}

Page 22Mediator

7. Code

7-4. (4) ColleagueTextField

import java.awt.*;

public class ColleagueTextField extends TextField implements TextListener, Colleague {


public ColleagueTextField(String text, int columns) {

super(text, columns);

}



}



setBackground(enabled ? Color.white : Color.lightGray);

}

public void textValueChanged(TextEvent e) { // If texfield changed, report to Mediator

mediator.colleagueChanged();

}

}

Page 23Mediator

7. Code

7-4. (5) ColleagueCheckbox

import java.awt.*;

public class ColleagueCheckbox extends Checkbox implements ItemListener, Colleague {


public ColleagueCheckbox(String caption, CheckboxGroup group, boolean state) {

super(caption, group, state);

}



}



}

public void itemStateChanged(ItemEvent e) { // If checkbox state changed, report to Mediator mediator.colleagueChanged();

}

}

Page 24Mediator

7. Code

7-4. (6) LoginFrame (1/5)

import java.awt.Frame;

import java.awt.Label;

import java.awt.Color;

import java.awt.CheckboxGroup;

import java.awt.GridLayout;

import java.awt.event.ActionListener;

import java.awt.event.ActionEvent;

public class LoginFrame extends Frame implements ActionListener, Mediator {

private ColleagueCheckbox checkGuest;

private ColleagueCheckbox checkLogin;

private ColleagueTextField textUser;

private ColleagueTextField textPass;

private ColleagueButton buttonOk;

private ColleagueButton buttonCancel;

Page 25Mediator

7. Code


public LoginFrame(String title) {

super(title);

setBackground(Color.lightGray);

setLayout(new GridLayout(4, 2)); // Creating grid 4 x 2 using layout manager

createColleagues(); // Creating Colleagues

add(checkGuest); // Layout from here

add(checkLogin);

add(new Label("Username:"));

add(textUser);

add(new Label("Password:"));

add(textPass);

add(buttonOk);

add(buttonCancel);

colleagueChanged(); // Initial setting for enabled/disabled

pack();

show();

}

Page 26Mediator

7. Code


public void createColleagues() {

CheckboxGroup g = new CheckboxGroup();

checkGuest = new ColleagueCheckbox("Guest", g, true);

checkLogin = new ColleagueCheckbox("Login", g, false);

textUser = new ColleagueTextField("", 10);

textPass = new ColleagueTextField("", 10);

textPass.setEchoChar('*');

buttonOk = new ColleagueButton("OK");

buttonCancel = new ColleagueButton("Cancel");

// Setting Mediator here

checkGuest.setMediator(this);

checkLogin.setMediator(this);

textUser.setMediator(this);

textPass.setMediator(this);

buttonOk.setMediator(this);

buttonCancel.setMediator(this);

Page 27Mediator

7. Code


checkGuest.addItemListener(checkGuest);

checkLogin.addItemListener(checkLogin);

textUser.addTextListener(textUser);

textPass.addTextListener(textPass);

buttonOk.addActionListener(this);

buttonCancel.addActionListener(this);

}

// When Colleage notices their change, Mediator judges if each colleague should be enabled/disabled

public void colleagueChanged() {

if (checkGuest.getState()) { // Guest mode

textUser.setColleagueEnabled(false);

textPass.setColleagueEnabled(false);

buttonOk.setColleagueEnabled(true);

} else { // Login mode

textUser.setColleagueEnabled(true);

userpassChanged();

}

}

Page 28Mediator

7. Code


// textUser or textPass has been changed. Mediator judges if each colleague should be enabled/disabled

private void userpassChanged() {

if (textUser.getText().length() > 0) {

textPass.setColleagueEnabled(true);

if (textPass.getText().length() > 0) {

buttonOk.setColleagueEnabled(true);

} else {

buttonOk.setColleagueEnabled(false);

}

} else {

textPass.setColleagueEnabled(false);

buttonOk.setColleagueEnabled(false);

}

}

public void actionPerformed(ActionEvent e) {

System.out.println(e.toString());

System.exit(0);

}

}

Page 29Mediator

7. Code

7-4. (7) Main

import java.awt.*;

import java.awt.event.*;

public class Main {

static public void main(String args[]) {

new LoginFrame("Mediator Sample");

}

}

Page 30Mediator

Sources

http://www.dofactory.com/Patterns/PatternMediator.aspx http://sourcemaking.com/design_patterns/mediator “Introduction to Design Pattern in Java” by Hiroshi Yuki “Design Pattern” by GoF

Date post:	14-Jan-2016
Category:	Documents
Upload:	wilfred-thomas-gordon
View:	224 times
Download:	2 times

Mediator Kensho Tsuchihashi. Mediator Page 2 Table of Contents 1.What is Mediator? 2.What problem...

Documents