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Behavioral Pattern
Mediator Pattern
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Mediator Pattern
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Define
mediator pattern
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Define an object that
encapsulates details and other objects interact with such object. The
relationships are loosely decoupled.
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Where
to use & benefits
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Partition a system into pieces
or small objects.
Centralize control to manipulate participating objects(a.k.a
colleagues)
Clarify the complex relationship by providing a board committee.
Limit subclasses.
Improve objects reusabilities.
Simplify object protocols.
The relationship between the control class and other participating
classes is multidirectional.
Related patterns include
- Facade, which abstracts a subsystem to provide a more
convenient interface, and its protocol is unidirectional, whereas a
mediator enables cooperative behavior and its protocol is
multidirectional.
- Command, which is used to coordinate functionality.
- Observer, which is used in mediator pattern to enhance
communication.
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Example
of mediator pattern
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If you have a complex GUI,
whenever a button has been clicked, the related actions should be
disabled or enabled. You may design a Mediator class to include all
related classes:
interface Command {
void execute();
}
class Mediator {
BtnView btnView;
BtnSearch btnSearch;
BtnBook btnBook;
LblDisplay show;;
//....
void registerView(BtnView v) {
btnView = v;
}
void registerSearch(BtnSearch s) {
btnSearch = s;
}
void registerBook(BtnBook b) {
btnBook = b;
}
void registerDisplay(LblDisplay d) {
show = d;
}
void book() {
btnBook.setEnabled(false);
btnView.setEnabled(true);
btnSearch.setEnabled(true);
show.setText("booking...");
}
void view() {
btnView.setEnabled(false);
btnSearch.setEnabled(true);
btnBook.setEnabled(true);
show.setText("viewing...");
}
void search() {
btnSearch.setEnabled(false);
btnView.setEnabled(true);
btnBook.setEnabled(true);
show.setText("searching...");
}
}
Then, you may define classes which should be controlled by the Mediator
class.
class BtnView extends JButton implements Command {
Mediator med;
BtnView(ActionListener al, Mediator m) {
super("View");
addActionListener(al);
med = m;
med.registerView(this);
}
public void execute() {
med.view();
}
}
class BtnSearch extends JButton implements Command {
Mediator med;
BtnSearch(ActionListener al, Mediator m) {
super("Search");
addActionListener(al);
med = m;
med.registerSearch(this);
}
public void execute() {
med.search();
}
}
class BtnBook extends JButton implements Command {
Mediator med;
BtnBook (ActionListener al, Mediator m) {
super("Book");
addActionListener(al);
med = m;
med.registerBook(this);
}
public void execute() {
med.book();
}
}
class LblDisplay extends JLabel{
Mediator med;
LblDisplay (Mediator m) {
super("Just start...");
med = m;
med.registerDisplay(this);
setFont(new
Font("Arial",Font.BOLD,24));
}
}
From the above design, you can see that the relationships among the
classes, which also known as collegues or participating classes, are
multidirectional. Mediator class contains all the information about
these classes and knows what these classes are going to do. The
participating classes have to register themselves to the Mediator
class.
The MediatorDemo class will show the cooperation among the classes.
class MediatorDemo extends JFrame implements ActionListener {
Mediator med = new Mediator();
MediatorDemo() {
JPanel p = new JPanel();
p.add(new BtnView(this,med));
p.add(new BtnBook(this,med));
p.add(new BtnSearch(this, med));
getContentPane().add(new
LblDisplay(med), "North");
getContentPane().add(p, "South");
setSize(400,200);
setVisible(true);
}
public void actionPerformed(ActionEvent ae) {
Command comd =
(Command)ae.getSource();
comd.execute();
}
public static void main(String[] args) {
new MediatorDemo();
}
}
The following is a complete code for the above program.
interface Command {
void execute();
}
class Mediator {
BtnView btnView;
BtnSearch btnSearch;
BtnBook btnBook;
LblDisplay show;;
//....
void registerView(BtnView v) {
btnView = v;
}
void registerSearch(BtnSearch s) {
btnSearch = s;
}
void registerBook(BtnBook b) {
btnBook = b;
}
void registerDisplay(LblDisplay d) {
show = d;
}
void book() {
btnBook.setEnabled(false);
btnView.setEnabled(true);
btnSearch.setEnabled(true);
show.setText("booking...");
}
void view() {
btnView.setEnabled(false);
btnSearch.setEnabled(true);
btnBook.setEnabled(true);
show.setText("viewing...");
}
void search() {
btnSearch.setEnabled(false);
btnView.setEnabled(true);
btnBook.setEnabled(true);
show.setText("searching...");
}
}
class BtnView extends JButton implements Command {
Mediator med;
BtnView(ActionListener al, Mediator m) {
super("View");
addActionListener(al);
med = m;
med.registerView(this);
}
public void execute() {
med.view();
}
}
class BtnSearch extends JButton implements Command {
Mediator med;
BtnSearch(ActionListener al, Mediator m) {
super("Search");
addActionListener(al);
med = m;
med.registerSearch(this);
}
public void execute() {
med.search();
}
}
class BtnBook extends JButton implements Command {
Mediator med;
BtnBook (ActionListener al, Mediator m) {
super("Book");
addActionListener(al);
med = m;
med.registerBook(this);
}
public void execute() {
med.book();
}
}
class LblDisplay extends JLabel{
Mediator med;
LblDisplay (Mediator m) {
super("Just start...");
med = m;
med.registerDisplay(this);
setFont(new
Font("Arial",Font.BOLD,24));
}
}
class MediatorDemo extends JFrame implements ActionListener {
Mediator med = new Mediator();
MediatorDemo() {
JPanel p = new JPanel();
p.add(new BtnView(this,med));
p.add(new BtnBook(this,med));
p.add(new BtnSearch(this, med));
getContentPane().add(new
LblDisplay(med), "North");
getContentPane().add(p, "South");
setSize(400,200);
setVisible(true);
}
public void actionPerformed(ActionEvent ae) {
Command comd =
(Command)ae.getSource();
comd.execute();
}
public static void main(String[] args) {
new MediatorDemo();
}
}
java MediatorDemo
A window will pop up. Try the features.
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