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CHAPTER 1 INTRODUCING AOP
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experts, and aspects allow you to more easily integrate these functionalities into the rest of the application. No doubt the adoption of AOP will take a long time and require effort, but it can be done gradually when the need is clearly identified.
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The Limitations of OOP
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The importance of OOP for developing complex programs is undeniable. However, we will show that writing clear and elegant programs using only OOP is impossible in at least two cases: when the application contains crosscutting functionalities, and when the application includes code scattering.
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The Case of Crosscutting Functionalities
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Previously, we mentioned that when you analyze how to organize an application into classes, the analysis must be driven by the need for separating and encapsulating the data and its associated processing into coherent entities. Although the classes are programmed independently of one another, they are sometimes behaviorally interdependent. Typically, this is the case when you implement rules of referential integrity. For example, a customer object must not be deleted while an outstanding order remains unpaid; otherwise, the program risks losing the contact details for that customer. To enforce this rule, you could modify the customer-deletion method so that it initially determines whether all the orders have been paid. However, this solution is deficient for several reasons: Determining whether an order has been paid does not belong to customer management but to order management. Therefore, the customer class should not have to manage this functionality. The customer class should not need to be aware of all the data-integrity rules that other classes in the application impose. Modifying the customer class to take these data-integrity rules into account restricts the possibilities of reusing the class in other situations. In other words, once the customer class implements any functionality that is linked to a different class, customer is no longer independently reusable, in many cases. Despite the fact that the customer class is not the ideal place to implement this referential-integrity rule, many object-oriented programs work this way for lack of a better solution. You might be thinking about integrating this functionality into an order class instead, but this solution is no better. No reason exists for the order class to allow the deletion of a customer. Strictly speaking, this rule is linked to neither the customers nor the orders but cuts across these two types of entities. One of the aims of dividing the data into classes is making the classes independent from one another. However, crosscutting functionalities, such as the rules of referential integrity, appear superimposed on the division violating the independence of the classes. In other words, OOP does not allow you to neatly implement crosscutting functionalities. As a compromise, you can resign yourself to implementing them in individual classes, but remain conscious that this solution is not ideal.
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CHAPTER 1 INTRODUCING AOP
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The Case of Code Scattering
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In OOP, the principal way that objects interact is by invoking methods. In other words, an object that needs to carry out an action invokes a method that belongs to another object. (An object can also invoke one of its own methods.) OOP always entail two roles: that of the invoker and that of the invoked. When you write the code to call a method, you do not need to worry about how the service is implemented because the call interacts only with the interface of the invoked object. You need only ensure that the parameters in the call correspond to those of the method s signature. Because methods are implemented within classes, you write each method as a block of code that is clearly delimited. To change a method, you obviously modify the file that contains the class where the method is defined. If you alter just the body of the method, the modification is transparent because the method will still be called in exactly the same way. However, if you change the method s signature (for example, by adding a parameter), further implications arise. You must then modify all the calls to the method, hence you must modify any classes that invoke the method. If these calls exist in several places in the program, making the changes can be extremely time-consuming. The main point is this: Even though the implementation of a method is located in a single class, the calls to that method can be scattered throughout the application. This phenomenon of code scattering slows down maintenance tasks and makes it difficult for object-oriented applications to adapt and evolve. Any change in the way that a service is used requires many other changes a costly process that can also introduce errors.
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