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Remember the Decorator Pattern? We wrapped objects to give them new responsibilities. Why would we do that? So we can adapt a design expecting one interface to a class that implements a different interface. Te a Boil some wat 1 2 Ste ep er C o f fe e 1 the water the tea bag in 3 Pou r tea in a 4 Add lemon 2 3 cup 4 Bo il some Brew th e Po ur co ff Add suga r wate r co ffee gr ee in a cu inds p and mi lk Caffeine Beverage generalize 1 2 Brew relies on 3 Pour beverage in a cup 4 Add condiments subclass for ss Tea subcla some steps 2 Steep the teabag in the 4 Add lemon xviii water Boil some water knows Caffeine Beveragesteps of and controls theperforms the recipe, and f, but itsel 3 steps 1 and fee relies on Tea orandCof4.

Put them in an Array, a Stack, a List, a Map, take your pick. Each has its own advantages and tradeoffs. But when your client wants to iterate over your objects, are you going to show him your implementation? We certainly hope not! Iterator What does this get us? As you know, the Strategy Pattern went on to create a wildly successful business around interchangeable algorithms.

State, however, took the perhaps more noble path of helping objects learn to control their behavior by changing their internal state. Believe it or not, some of the most powerful OO designs use several patterns together. The beat is set at BPM and you would like to increase it to You click on the increase beat button. View Which results in the controller being invoked. View The controller asks the model to update its BPM by one.

It calls getBP the model state. Ho s Gang of Four eriou myst the of tify when the true iden patterns user b Be there e table books any coffe the — s with the neighbor b Keep up must own. Richa Your Objectville guide Design Pattern defined Looking more closely at the Design Pattern definition May the force be with you Pattern catalogs How to create patterns So you wanna be a Design Patterns writer?

A lot has changed in the last 10 years. But these patterns are awesome in their own right, and if your situation calls for them, you should apply them with your head held high. Our goal in this appendix is to give you a high level idea of what these patterns are all about.

The Client asks the Visitor to get information from the Composite structure New methods can be added to the Visitor without affecting the Composite.

The getDescription method returns the description. Beverage description The cost method is abstract; subclassses need to define their own implementation. DarkRoast HouseBlend cost cost Decaf cost Espresso cost Each subclass implements cost to return the cost of the beverage. Starbuzz charges a bit for each of these, so they really need to get them built into their order system. What happens when the price of milk goes up? What do they do when they add a new caramel topping?

This is stupid; why do we need all these classes? Subclasses will still override cost , but they will also invoke the super version so that they can calculate the total cost of the basic beverage plus the costs of the added condiments. Sharpen your pencil What requirements or other factors might change that will impact this design? Price changes for condiments will force us to alter existing code. New condiments will force us to add new methods and alter the cost method in the superclass.

We may have new beverages. For some of these beverages iced tea? What if a customer wants a double mocha? Cha ery bad av the decorator pattern Master and Student Master: Grasshopper, it has been some time since our last meeting. Have you been deep in meditation on inheritance? Student: Yes, Master. Master: Ah yes, you have made some progress. So, tell me my student, how then will you achieve reuse if not through inheritance? Master: Please, go on Student: When I inherit behavior by subclassing, that behavior is set statically at compile time.

In addition, all subclasses must inherit the same behavior. Master: Very good, Grasshopper, you are beginning to see the power of composition. Student: Yes, it is possible for me to add multiple new responsibilities to objects through this technique, including responsibilities that were not even thought of by the designer of the superclass. Master: What have you learned about the effect of composition on maintaining your code? Student: Well, that is what I was getting at.

By dynamically composing objects, I can add new functionality by writing new code rather than altering existing code. Master: Very good. Enough for today, Grasshopper. I would like for you to go and meditate further on this topic Feel free to extend our classes with any new behavior you like. If your needs or requirements change and we know they will , just go ahead and make your own extensions.

Our goal is to allow classes to be easily extended to incorporate new behavior without modifying existing code. What do we get if we accomplish this? That sounds very contradictory. How can a design be both? It certainly sounds contradictory at first. After all, the less modifiable something is, the harder it is to extend, right? Think about the Observer Pattern in Chapter Making OO design flexible and open to extension without the modification of existing code takes time and effort.

Following the Open-Closed Principle usually introduces new levels of abstraction, which adds complexity to our code. You want to concentrate on those areas that are most likely to change in your designs and apply the principles there. Q: How do I know which areas of change are more important? A: That is partly a matter of experience in designing OO systems and also a matter of the knowing the domain you are working in.

Looking at other examples will help you learn to identify areas of change in your own designs. A: While it may seem like a contradiction, there are techniques for allowing code to be extended without direct modification. Many of the patterns give us time tested designs that protect your code from being modified by supplying a means of extension.

How can I make every part of my design follow the Open-Closed Principle? Remember us? Starbuzz Coffee? Do you think you could use some of those design principles to actually help us? Its ject is a decorisatdecorating, The Mocha obth object it type mirrors Beeverage.

So, a DarkRoast wrapped in Mocha and Whip is still a Beverage and we can do anything with it we can do with a DarkRoast, including call its cost method. We do this by calling cost on the outermost decorator, Whip, and Whip is going to delegate computing the cost to the objects it decorates.

Once it gets a cost, it will add on the cost of the Whip. Each component can be used on its own, or wrapped by a decorator. It extends Component. Sue: What do you mean? Mary: Look at the class diagram. The CondimentDecorator is extending the Beverage class. Sue: True.

But where does the behavior come in? Sue: When we compose a decorator with a component, we are adding new behavior. We are acquiring new behavior not by inheriting it from a superclass, but by composing objects together. Page 14 Encapsulation Page 15 Inheritance Page 18 Polymorphism Page 19 Relations Between Objects Page 30 Features of Good Design Page 31 Design Principles Page 35 Encapsulate What Varies Page 36 Program to an Interface, not an Implementation Page 40 Favor Composition Over Inheritance Page 49 Single Responsibility Principle Page 52 Liskov Substitution Principle Page 55 Interface Segregation Principle Page 62 Dependency Inversion Principle Page 69 Creational Design Patterns Page Structural Design Patterns