Mediator

1. Mediator

2. Law of Demeter • “… the methods of a class should not depend in any way on the structure of any class, except the immediate (top-level) structure of their own class. Further, each method should send messages to objects belonging to a very limited set of classes only.” • OOAD, page 116

3. Why Mediator? • Common problem: Multiple objects of same or different classes must communicate/interact. Multiple dependencies complicate objects, lead to “spaghetti code.” • Solution: Define an object that encapsulates how a set of objects interact and interconnect. Make that object the hub of communications, responsible for controlling and coordinating the interactions of clients – the colleague objects.

4. Analogies/Metaphors • Air traffic control • Stock market • eBay

5. Advantages/Uses • Partition system into smaller objects. Simplifies classes/objects by placing inter-object communications in mediator. • Promotes loose coupling. • Clarify complex relationships. • Limit subclasses. • Improve object reusability. • Simplify object protocols. • Facilitates refinement by containing changes in mediator or single colleagues rather than requiring updates in all classes.

6. Dangers • Mediator can become monolithic, violating proscription against “God” or manager classes and making it hard to maintain.

7. Sample Code 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.setEnable(false); btnView.setEnabled(true) btnSearch.setEnabled(true); show.setText(“booking…”);

8. continued • 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);

9. continued 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

10. UML

11. UML

12. Related Patterns • Façade – provides a unified interface to a set of interfaces to make a subsystem easier to use, but Façade does not add functionality and is not known by the subsystem classes. • Observer – defines a one-to-many dependency between objects so that when one object changes state all dependents are notified and updated automatically. Observer distributes communication by introducing “observer” and “subject” classes; mediator encapsulates communication between other objects.

13. Sources • http://www.javacamp.org/designPattern/mediator.html • http://www.fluffycat.com//javasource/javapatterns/DvdLowercaseTitle.txt • http://www.vincehuston.org/dp/mediator.html • http://www.patterndepot.com/put/8/mediator.pdf • Design Pattern, GoF • OOAD, Booch et al