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Chapter 10: Classes
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Following the standard Java convention, a class should begin with a list of variables. Public static constants, if any, should come first. Then private static variables, followed by private instance variables. There is seldom a good reason to have a public variable.
Public functions should follow the list of variables. We like to put the private utilities called by a public function right after the public function itself. This follows the stepdown rule and helps the program read like a newspaper article.
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The first rule of classes is that they should be small. The second rule of classes is that they should be smaller than that.
With functions we measured size by counting physical lines. With classes we use a different measure. We count responsibilities.
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The name of a class should describe what responsibilities it fulfills. In fact, naming is probably the first way of helping determine class size. If we cannot derive a concise name for a class, then it’s likely too large. The more ambiguous the class name, the more likely it has too many responsibilities. For example, class names including weasel words like Processor or Manager or Super often hint at unfortunate aggregation of responsibilities.
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The Single Responsibility Principle (SRP) states that a class or module should have one, and only one, reason to change. This principle gives us both a definition of responsibility, and a guidelines for class size. Classes should have one responsibility—one reason to change.
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SRP is one of the more important concept in OO design. It’s also one of the simpler concepts to understand and adhere to. Yet oddly, SRP is often the most abused class design principle. We regularly encounter classes that do far too many things. Why?
Getting software to work and making software clean are two very different activities. Most of us have limited room in our heads, so we focus on getting our code to work more than organization and cleanliness. This is wholly appropriate. Maintaining a separation of concerns is just as important in our programming activities as it is in our programs.
The problem is that too many of us think that we are done once the program works. We fail to switch to the other concern of organization and cleanliness. We move on to the next problem rather than going back and breaking the overstuffed classes into decoupled units with single responsibilities.
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Classes should have a small number of instance variables. Each of the methods of a class should manipulate one or more of those variables. In general the more variables a method manipulates the more cohesive that method is to its class. A class in which each variable is used by each method is maximally cohesive.
In general it is neither advisable nor possible to create such maximally cohesive classes; on the other hand, we would like cohesion to be high. When cohesion is high, it means that the methods and variables of the class are co-dependent and hang together as a logical whole.
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So breaking a large function into many smaller functions often gives us the opportunity to split several smaller classes out as well. This gives our program a much better organization and a more transparent structure.
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Our restructured Sql logic represents the best of all worlds. It supports the SRP. It also supports another key OO class design principle known as the Open-Closed Principle, or OCP: Classes should be open for extension but closed for modification. Our restructured Sql class is open to allow new functionality via subclassing, but we can make this change while keeping every other class closed. We simply drop our UpdateSql class in place.
We want to structure our systems so that we muck with as little as possible when we update them with new or changed features. In an ideal system, we incorporate new features by extending the system, not by making modifications to existing code.
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Needs will change, therefore code will change. We learned in OO 101 that there are concrete classes, which contain implementation details (code), and abstract classes, which represent concepts only. A client class depending upon concrete details is at risk when those details change. We can introduce interfaces and abstract classes to help isolate the impact of those details.
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By minimizing coupling in this way, our classes adhere to another class design principle known as the Dependency Inversion Principle (DIP). In essence, the DIP says that our classes should depend upon abstractions, not on concrete details.