Mediator pattern

The mediator pattern defines an object that encapsulates how a set of objects interact. This pattern is considered to be a behavioral pattern due to the way it can alter the program's running behavior.

Usually a program is made up of a large number of classes. So the logic and computation is distributed among these classes. However, as more classes are developed in a program, especially during maintenance and/or refactoring, the problem of communication between these classes may become more complex. This makes the program harder to read and maintain. Furthermore, it can become difficult to change the program, since any change may affect code in several other classes.

With the mediator pattern, communication between objects is encapsulated with a mediator object. Objects no longer communicate directly with each other, but instead communicate through the mediator. This reduces the dependencies between communicating objects, thereby lowering the coupling.

Definition

The essence of the Mediator Pattern is to "Define an object that encapsulates how a set of objects interact. Mediator promotes loose coupling by keeping objects from referring to each other explicitly, and it lets you vary their interaction independently.

Participants

Mediator - defines the interface for communication between Colleague objects

ConcreteMediator - implements the Mediator interface and coordinates communication between Colleague objects. It is aware of all the Colleagues and their purpose with regards to inter communication.

ConcreteColleague - communicates with other Colleagues through its Mediator

Example

In the following example a mediator object controls the status of three collaborating buttons: for this it contains three methods (book(),view() and search()) that set the status of the buttons. The methods are called by each button upon activation (via the execute() method in each of them).

Hence here the collaboration pattern is that each participant (here the buttons) communicates to the mediator its activity and the mediator dispatches the expected behavior to the other participants.

import java.awt.Font;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
 
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
 
//Colleague interface
interface Command {
    void execute();
}
 
//Abstract Mediator
interface IMediator {
    public void book();
    public void view();
    public void search();
    public void registerView(BtnView v);
    public void registerSearch(BtnSearch s);
    public void registerBook(BtnBook b);
    public void registerDisplay(LblDisplay d);
}
 
//Concrete mediator
class Mediator implements IMediator {
 
    BtnView btnView;
    BtnSearch btnSearch;
    BtnBook btnBook;
    LblDisplay show;
 
    //....
    public void registerView(BtnView v) {
        btnView = v;
    }
 
    public void registerSearch(BtnSearch s) {
        btnSearch = s;
    }
 
    public void registerBook(BtnBook b) {
        btnBook = b;
    }
 
    public void registerDisplay(LblDisplay d) {
        show = d;
    }
 
    public void book() {
        btnBook.setEnabled(false);
        btnView.setEnabled(true);
        btnSearch.setEnabled(true);
        show.setText("booking...");
    }
 
    public void view() {
        btnView.setEnabled(false);
        btnSearch.setEnabled(true);
        btnBook.setEnabled(true);
        show.setText("viewing...");
    }
 
    public void search() {
        btnSearch.setEnabled(false);
        btnView.setEnabled(true);
        btnBook.setEnabled(true);
        show.setText("searching...");
    }
 
}
 
//A concrete colleague
class BtnView extends JButton implements Command {
 
    IMediator med;
 
    BtnView(ActionListener al, IMediator m) {
        super("View");
        addActionListener(al);
        med = m;
        med.registerView(this);
    }
 
    public void execute() {
        med.view();
    }
 
}
 
//A concrete colleague
class BtnSearch extends JButton implements Command {
 
    IMediator med;
 
    BtnSearch(ActionListener al, IMediator m) {
        super("Search");
        addActionListener(al);
        med = m;
        med.registerSearch(this);
    }
 
    public void execute() {
        med.search();
    }
 
}
 
//A concrete colleague
class BtnBook extends JButton implements Command {
 
    IMediator med;
 
    BtnBook(ActionListener al, IMediator m) {
        super("Book");
        addActionListener(al);
        med = m;
        med.registerBook(this);
    }
 
    public void execute() {
        med.book();
    }
 
}
 
class LblDisplay extends JLabel {
 
    IMediator med;
 
    LblDisplay(IMediator m) {
        super("Just start...");
        med = m;
        med.registerDisplay(this);
        setFont(new Font("Arial", Font.BOLD, 24));
    }
 
}
 
class MediatorDemo extends JFrame implements ActionListener {
 
    IMediator 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();
    }
 
}

See also

• Data mediation
• Design Patterns, the book which gave rise to the study of design patterns in computer science
• Design pattern (computer science), a standard solution to common problems in software design

External links

• Mediator Design Pattern