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?
A few guidelines to help you follow the Principle Meanwhile, back at the Pizza Store Ensuring consistency in your ingredients Families of ingredients Building the ingredient factories Building the New York ingredient factory Reworking the pizzas Reworking the pizzas, continued Revisiting our pizza stores What have we done?
More pizza for Ethan and Joel So now when they order Move to an eagerly created instance rather than a lazily created one.
Meanwhile, back at the Chocolate Factory Free hardware! Taking a look at the vendor classes Cubicle Conversation Meanwhile, back at the Diner Time to write that documentation What are we doing? Time to QA that Undo button! Every remote needs a Party Mode! Being Adaptive: The Adapter and Facade Patterns Adapters all around us Object-oriented adapters If it walks like a duck and quacks like a duck, then it must might be a duck turkey wrapped with a duck adapter Abstracting prepareRecipe What have we done?
What did the Template Method get us? What is compareTo? Can we encapsulate the iteration? What have we done so far? Reviewing our current design Making some improvements Iterator We are almost there Code with C.
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