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CHAPTER 1 ESSENTIALS OF OBJECT-ORIENTED PROGRAMMING
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Logical errors are more problematic. A logical error is when the type changes its details without causing a syntax error. These types of errors are especially challenging because the compiler won t report the error. Defensive code (code that has built-in error mechanisms) can catch the error when the program is running. Logical errors are more commonly called bugs, or in slang terms, features. Logical errors are often due to bad code design, poor implementation, or lack of testing. Logical errors arise because of a misunderstanding between the consumer and the module. It could be argued that a logical error isn t an error, but the result of a necessary change. However, this brings back what my Engineering Statics professor said: Either the bridge stands or it falls; there is no middle ground. It doesn t matter what a logical error is called; the code still doesn t work, and it needs to be fixed. Regardless of why the logical error occurred, the costs of a logical error can be disproportionately high in financial or political impression terms. A logical error can occur because the code contains uncontrolled or unmanageable referencing of other types. In the simple consumer code example you saw previously, only two types were referenced. Imagine four simple consumer code examples that only reference two or three types. If any of the consumer code implementations made a method call at the wrong moment, or made certain assumptions, the number of logical errors once you factor in combinations and permutations can be staggering. The logical-error scenario gets worse when a fix causes other logical errors to occur, a symptom known as feature interaction that s a particularly deadly form of logical error. Feature-interaction errors are difficult and tedious to battle, and more often than not result in code that needs to be rewritten. Feature-interaction errors should be avoided; patterns help with this problem. Test-driven development keeps feature-interaction errors in check because any changes that cause other problems to occur are immediately recognized. An immediate recognition causes an immediate fix, ensuring that a feature-interaction error is squashed while it s still small and tamable. Consider the following: imagine a room of people. Each person is a type, and like a party where each person can communicate with each other person, types can reference one another without regard. If the communication is organized, then there will be only a slight murmur of voices when several people are communicating at one time. If things were to get out of control, the noise would reach extremely loud levels, spiraling into cacophony. This is because for somebody to communicate with another person, he has to speak louder than the noise level of the room. The next person does the same thing, and shortly, you ll find everybody shouting. If the sound can be kept to a lower level, then everything is still controllable. Relating this back to types and referencing, if types reference one another without any order, the code will quickly become hard to understand because too many types reference other types.
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Modular vs. Reusable Code
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Modular code is code that has been split into blocks of functionality. Reusable code is code that s usable in different situations. It s important to understand that modular code isn t necessarily reusable code, and reusable code isn t necessarily modular code. Reusable code that has been reused often tends not to be modular code. These two statements might confuse, because we tend to think of reusability and modularity as the same thing.
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CHAPTER 1 ESSENTIALS OF OBJECT-ORIENTED PROGRAMMING
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One of the best explanations of modularity is given in the book Software Engineering: A Practitioner s Approach.1 In the book, modularity is explained as a four-decade-old concept that can be best summarized as software that has been split into distinct separately named and addressed blocks of source code, which are integrated into a program to solve a problem. A modular program contains unique pieces of code that interface with each other to solve a problem. Modularity makes it possible to use a divide-and-conquer strategy when implementing the solution. A module is defined by its contract, which specifies the module s functionality. When developers know the contract, they only have to write source code to meet the requirements of the contract. At a later point in time, the individual modules are combined, and if the contracts are correctly defined and implemented, the resulting program works. Metaphorically speaking, this process is comparable to building a bridge by starting on both sides. If the contracts are properly defined and implemented, then the two bridge segments will meet at the right place in the middle. The following source code is an example of a modular application: class User1 { } class User2 { } class User3 { } class MainApp { static void testMethod() { User1 cls1 = new User1(); // Do something with cls1 User2 cls2 = new User2(); // Do something with cls2 User3 cls3 = new User3(); // Do something with cls3 } } User1, User2, and User3 are three unique and distinct types that have no interaction with one another. The method testMethod instantiates the three types and uses them to execute some logic. The consumer of the three types uses the types independently of one another. In a real-world scenario there might be some interaction, but the objective of modularizing an application is to separate the code pieces so that they don t interact.
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Note Reusability has a different aim than modularizing source code, and is best defined as behavior that
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