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objects will invoke that data access code to retrieve and update the objects data. This chapter will not focus on the data access mechanism; instead, it will leave those details for 18.
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Responsibility-Driven Design
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Object-oriented design has been around for many years. Unfortunately, there is no unified approach to doing OO design, and this often leads to confusion. Just because someone says he s doing OO design doesn t necessarily mean he s following a similar process to another person who also says he s doing OO design. Many people want to use objects primarily so they can use dot notation to get at their database. They want to write code like this: int qty = Customer[9].Order[4].LineItem[11].Product.Inventory.Quantity; Each object here is actually some table or data entity that is hopefully loaded automatically and efficiently when it is used. The dot-notation motivation for using objects is not a bad one, but it is not what most people consider object-oriented.
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Use-Case or Story-Based Analysis
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Ideally, your objects exist to serve the needs of a specific use case, or user story, or user scenario. In many cases, this means that the objects exist to provide the required business logic and data for each form the user will use to complete a task. That user task is the use case. Following this approach first means that you need to identify the use cases for your application. This is an analysis process where you interview the business stakeholders and end users of the application to identify the specific tasks that the application must accomplish for the users. Each task is a use case. Of course, some tasks are big and some are small. If a task is big and complex, you might need to subdivide it into a set of smaller tasks, each of which is a use case. This means that some use cases may rely on, or interact with, other use cases. Your ultimate goal, however, is for each low-level use case to be a stand-alone task a scenario you can clearly describe. What I m describing here is the basic concept of decomposition, where complex problems are decomposed into simpler subproblems. Eventually, you get to the point that the individual subproblems (low-level use cases) are simple enough that you can design and implement them without getting lost in the overall complexity.
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Once you have a low-level use case, you can start to identify the objects that are required to implement that use case. One important thing to remember is that you should not focus on data at this time. Don t worry about the data in each object. Instead, focus on what the object does. What is its job or role within the use case Some objects are responsible for collecting data from the user. Others may be responsible for validating data or calculating values. Notice how I m phrasing this: each object is responsible for doing something. Objects are not passive; they are actors that have a job to do within the use case. In fact, objects should have a single responsibility. This is called single-responsibility design and is the goal for this object design technique. Each object should have exactly one job one responsibility within the use case. An object may have numerous behaviors (properties, methods, and so on), but all those behaviors should exist so the object can meet its responsibility.
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Objects Exist for Use Cases
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An important side effect of what I m talking about here is that objects are designed for use cases not for an enterprise, not even for an application. They are designed for a specific use case. This might appear to be a problem, because it could reduce reuse. I suggest that this is not a problem. Reuse is not a goal for software; it is one tool we use to achieve the real goals of maintainability and lower cost of development. In fact, reuse is often counterproductive. Most reuse causes a side effect called coupling, where code becomes overly dependent on other code. Coupling is perhaps the ultimate enemy of maintainability, and it s a primary driver for increasing the cost of software development. Figure 3-1 illustrates how reuse causes code to become interdependent.
Figure 3-1. Object A reusing code in object B Let s imagine that objects A and B from Figure 3-1 are in different use cases within the same application. Then let s imagine that object B has some useful behavior that object A also needs. It is natural to have object A reuse the behavior provided by object B. Unfortunately, there is a very real risk that object B might change at some point in the future, probably due to a change in the use case to which it belongs. And changing object B will very likely break object A, often in a way that you may catch only if you have extensive unit and integration testing in place. Coupling leads to unintended consequences and fragile code.
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