... / ...
CommitLineData
1\section{Code style}
2\label{code}
3TODO: Conventions, ex.: iterators
4As the final system hopefully will have a long living cycle TODO
5and will be used and refined by many people, high code quality was an important aim.
6Beyond architectural issues this also involves cleanness on the lower level,
7like the design of classes and the implementation of methods.
8Common software development principles were followed TODO and
9the unfamiliar reader was constantly taken into account
10to yield clean, readable and extensible code.
11
14Comments were used at places ambiguities or misinterpretations could arise,
15yet care was taken to face such problems at their roots and solve them
17This approach is further explained in section \fullref{speaking} and
18rendered many uses of comments unnecessary.
19
21consciously minimized, to enforce speaking code and avoid redundancy.
22An exception from this was the highlighting of subdivisions.
23In class and method implementations, comments like
24\codepar{//********************** Constructors **********************\textbackslash\textbackslash}
25
26were deliberately used to ease navigation inside source files,
27but also to enhance readability: parts of method
28implementations, for example, were optically separated this way.
29Another alternative would have been to use separate methods for these code
30pieces, and thereby sticking strictly to the so-called
31Composed Method Pattern'' \cite{composed},
32as was done in other cases.
33However, sticking to this pattern too rigidly would have introduced additional
34artifacts with either long or non-speaking names,
35would have interrupted the reading flow and also would have increased complexity,
36because these methods would have been callable at least from everywhere
37in the source file.
38Consequently, having longer methods at some places that are optically separated
39into smaller units that are in fact independent from each other was considered
40an elegant solution, although, surprisingly, this technique does not seem to be
41proposed that often in the literature.
42
44plain comments, for example to specify parameters, return types, exceptions
45and links to other parts of the documentation.
46This proved even more useful due to the fact that \name{Doxygen} supports all
48
49\subsection{Speaking code''}
50\label{speaking}
51As mentioned in section \fullref{comments}, the code was tried to be designed to
52speak for itself'' as much as possible instead of making its readers depend on
54In doing so, besides reducing code size due to the missing comments,
55clean code amenable to unfamiliar readers and unpredictable changes was enforced.
56This is especially important since, as described in section \fullref{arch},
57\myprog{} was designed to not only be a standalone program but also offer
58components suitable for reusability.
59%TODO: understandability <- code size
60
61The following topics were identified to be addressed to get what can be
62conceived as speaking code'':
63\begin{itemize}
64 \item Meaningful typing
65 \item Method names
66 \item Variable names
67 \item Intuitive control flow
68 \item Limited nesting
69 \item Compact code units
70 \item Usage of well-known structures
71\end{itemize}
72
73~\\The rest of this section describes these topics in some detail.
74Besides, an intuitive architecture and suitable, well-designed libraries
75also contribute to the clarity of the code.
76
77\subsubsection{Meaningful typing}
78Meaningful typing includes the direct mapping of entities of the modeled world
79to code entities \cite{str4} as well as an expressive naming scheme
80for the obtained types.
81Furthermore, inheritance should be used to express commonalities, to avoid
82code duplication and to separate implementations from interfaces \cite{str4}.
83
84All real-world artifacts to be modeled like database schemata, tables, table schemata.
85columns, keys and OBDA Specifications with their certain map types were directly
86translated into classes having simple predicting names like \code{Table},
87\code{TableSchema} and \code{Key}.
88Package affiliation provided the correct context to unambiguously understand these names.
89
90\subsubsection{Method names}
91Assigning expressive names to methods is a substantially important part of
92producing speaking code, since methods encapsulate operation and as such
93are important building blocks'' for other methods \cite{str4} and ultimately
94the whole program.
95Furthermore, method names often occur in interfaces and therefore are not limited
96to a local scope, and neither are easily changeable without affecting callers
97\cite{java}.
98
99Care was taken that method names reflect all important aspects of the respective
100method's behavior and
101
102Consider the following method in \file{CLIDatabaseInteraction.java}:
103\codepar{public static void promptAbortRetrieveDBSchemaAndWait\\
104 \ind(final FutureTask<DBSchema> retriever) throws SQLException}
105
106It could have been called \code{promptAbortRetrieveDBSchema} only, with the
107waiting mentioned in a comment.
108However, the waiting is such an important part of its behavior, that this
109wouldn't have been enough, so the waiting was included in the function name.
110Since the method is called at one place only, the lengthening of the method
111name by 7 characters or about 26 \% is really not a problem.
112
113\subsubsection{Variable names}
114To keep implementation code readable, care was taken to name variables
115meaningful yet concise. If this was not possible, expressiveness was preferred
116over conciseness.
117
118For example, in the implementation of the database schema retrieval,
119variables containing data directly obtained from querying the database
120and thus being subject to further processing was consequently prefixed
121with \code{recvd}'', although in most cases this technically would not have
122been necessary.
123
124\subsubsection{Intuitive control flow}
125
126\subsubsection{Limited nesting}
127
128\subsubsection{Compact code units}
129
130\subsubsection{Usage of well-known structures}
131
132\subsection{Robustness against incorrect use}
133Care was taken to produce code that is robust to incorrect use, making it
135potentially even unpracticed programmers, who besides have their emphasis
136on the concepts of bootstrapping rather than details of the present code anyway.
137In fact, carefully avoiding the introduction of technical artifacts to mind,
138preventing programmers from focusing on the actual program logic,
139is an important principle of writing clean code \cite{str4}.
140
141In modern object-oriented programming languages, of course the main instruments
142for achieving this are the type system and exceptions.
143In particular, static type information should be used to reflect data
144abstraction and the kind'' of data, an object reflects,
145while dynamic type information should only be used implicitly,
146through dynamically dispatching method invocations \cite{str3}.
147Exceptions on the other hand should be used at any place related to errors
148and error handling, separating error handling noticeably from other code and
149enforcing the treatment of errors \cite{str4}, preventing the programmer from using
150corrupted information in many cases.
151
152An example of both mechanisms, static type information and exceptions, acting
153in combination, while cleanly fitting into the context of dynamic dispatching,
154are the following methods from \file{Column.java}:
155\codepar{public Boolean isNonNull()\\public Boolean isUnique()}
156
157There return type is the \name{Java} class \code{Boolean}, not the plain type
158\code{boolean}, because the information they return is not always known.
159In an early stage of the program, they returned \code{boolean} and were
160accompanied by two methods
161\code{public boolean knownIsNonNull()} and \code{public boolean knownIsUnique()},
162telling the caller whether the respective information was known and thus the
163value returned by \code{isNonNull()} or \code{isUnique()}, respectively,
164was reliable.
165
166They were then changed to return the \name{Java} class \code{Boolean} and to return
167null pointers in case the respective information is not known.
168This eliminated any possibility of using unreliable data in favor of generating
169exceptions instead, in this case a \code{NullPointerException}, which is thrown
170automatically by the \name{Java Runtime Environment} if the programmer forgets the
171null check and tries to get a definite value from one of these methods
172when the correct value currently is not known.
173
174Comparing two unknown values -- thus, two null pointers --
175also yields the desired result, \code{true}, since the change,
176even when the programmer forgets that he deals with objects.
177However, when comparing two return values of one of the methods in general
178-- as opposed to comparing one such return value against a constant --,
179errors could occur if the programmer mistakenly writes \code{col1.isUnique() == col2.isUnique()}
181In this case, since the two \code{Boolean} objects are compared for identity \cite{java},
182the former comparison can return \code{false}, even when the two boolean values are in fact
183the same.
184However, since this case was considered much less common than cases in which the other
185solution could make programmers making mistakes produce undetected errors, it was preferred.
186
187TODO: summary
188
189\subsection{Classes}
190\label{code_classes}
191Following the object-oriented programming paradigm, classes were heavily used
192to abstract from implementation details and to yield intuitively usable objects with
193a set of useful operations \cite{obj}.
194
195\subsubsection{Identification of classes}
196To identify potential classes, entities from the problem domain were -- if reasonable --
197directly represented as \name{Java} classes.
198The approach of choosing the program that most directly models the aspects of the
199real world that we are interested in'' to yield clean code,
200as described and recommended by Stroustrup \cite{str3}, proved to be extremely useful
201and effective.
202As a consequence, the code declares classes like \code{Column}, \code{ColumnSet},
203\code{ForeignKey}, \code{Table}, \code{TableSchema} and \code{SQLType}.
204As described in section \fullref{speaking}, class names were chosen to be concise
205but nevertheless expressive TODO.
206\name{Java} packages were used to help attain this aim,
207which is why the previously mentioned class names are unambiguous.
208For details about package use, see section \fullref{code_packages}.
209
210Care was taken not to introduce unnecessary classes, thereby complicating
211code structure and increasing the number of source files and program entities.
212Especially artificial classes, having little or no reference to real-world
213objects, could most often be avoided.
214On the other hand of course, it usually is not the cleanest solution
215to avoid such artificial classes entirely.
216
217Section \fullref{hierarchies} describes how the classes of \myprog{} are organized
218into class hierarchies.
219
220\subsubsection{Const correctness}
221\label{const}
222Specifying in the code which objects may be altered and which shall remain constant,
223thus allowing for additional static checks preventing undesired modifications,
224is commonly referred to as const correctness'' TODO.
225TODO: powerful, preventing errors, clarity
226
227Unfortunately, \name{Java} lacks a keyword like \name{C++}'s \code{const},
228making it harder to achieve const correctness \cite{final}.
229It only specifies the similar keyword \code{final}, which is much less expressive and
230doesn't allow for a similarly effective error prevention \cite{final}.
231In particular, because \code{final} is not part of an object's type information,
232it is not possible to declare methods that return read-only objects TODO --
233placing a \code{final} before the method's return type would declare the
234method \code{final}. Similarly, there is no way to express that a method must not change
235the state of its object parameters. A method like \code{public f(final Object obj)}
236is only liable to not assigning a new value to its parameter object \code{obj} \cite{java}
237(which, if allowed, wouldn't affect the caller anyway \cite{java}).
238Methods changing its state, however, are allowed to be called on \code{obj} without
239restrictions \cite{java}.
240
241Several possibilities were considered to address this problem:
242\begin{itemize}
243 \item Not implementing const correctness, but stating the access rules in
245 \item Giving the methods which modify object states special names
246 like\\\code{setName\textendash\textendash USE\_WITH\_CARE}
247 \item Delegating changes of objects to special editor'' objects to be
248 obtained when an object shall be altered TODO
249 \item Deriving classes offering the modifying methods from the read-only
250 classes
251\end{itemize}
252
253Not implementing const correctness at all of course would have been the simplest
254possibility, producing the shortest and most readable code, but since
255incautious manipulation of objects would possibly have introduced subtle,
256hard-to-spot errors which in many cases would have occurred under additional
257conditions only and at other places, for example when inserting a \code{Column}
258into a \code{ColumnSet}, this method was not seriously considered.
259
260Using intentionally angular, conspicuous names also was not considered seriously,
261since it would have cluttered the code for the only sake of hopefully warning
262programmers of possible errors -- and not attempting to avoid them technically.
263
264So the introduction of new classes was considered the most effective and cleanest
265solution, either in the form of editor'' classes or derived classes offering the
266modifying methods directly. Again -- as in the identification of classes --,
267the most direct solution was considered the best, so the latter form of introducing
270functionality and usually occur in interfaces.
271Their counterparts including modifying methods also were derived from them and the
272implications of modifications were explained in their documentation, while the
273issue and the approach as such were also mentioned in the documentation of the
275The \code{Readable...} classes can be converted to their fully-functional
276counterparts via downcasting (only), thereby giving a strong hint to
277programmers that the resulting objects are to be used with care.
278
279\subsubsection{Java interfaces}
280\label{code_interfaces}
281In \name{Java} programming, it is quiet common and often recommended, that every
282class has at least one \code{interface} it \code{implements},
283specifying the operations the class provides. TODO
284If no obvious \code{interface} exists for a class or the desired
285interface name is already given to some other entity,
286the interface is often given names like \code{ITableSchema}
287or \code{TableSchemaInterface}.
288
289However, for a special purpose program with a relatively fixed set of classes
290mostly representing real-world artifacts from the problem domain,
291this approach was considered overly cluttering, introducing artificial
292code entities for no benefit.
293In particular, as explained in section TODO, all program classes either are
294standing alone TODO or belong to a class hierarchy derived from at least one
295interface.
296So, except from the standalone classes, an interface existed anyway, either
297naturally'' (as in the case of \code{Key}, for example) or because of
298the chosen way to implement const correctness.
299In some cases, these were interfaces declared in the program code, while
300in some cases, \name{Java} interfaces like \code{Set} were implemented
301(an obvious choice, of course, for \code{ColumnSet}).
302Introducing artificial interfaces for the standalone classes was considered
303unnecessary at least, if not messy.
304
305\subsection{Packages}
306\label{code_packages}
307As mentioned in section \fullref{code_classes}, class names were chosen to be
308concise but nevertheless expressive.
309This only was possible through the use of \name{Java} \code{package}s,
310which also helped structure the program.
311
312For the current, relatively limited, extent of the program which currently
313comprises $45$ (\code{public}) classes, a flat package structure was
314considered ideal, because it is simple and doesn't stash source files deep
315in subdirectories (in \name{Java}, the directory structure of the source tree
316is required to reflect the package structure TODO).
317Because also every class belongs to a package,
318each source file is to be found exactly one directory below the root
319program source directory, which in many cases eases their handling.
320
321The following $11$ packages exist in the program
322(their purpose and more details about the package structure are
323described in section \fullref{coarse}):
324\begin{multicols}{3}\begin{itemize}
325 \item \code{boostrapping}
326 \item \code{cli}
327 \item \code{database}
328 \item \code{helpers}
329 \item \code{log}
330 \item \code{main}
331 \item \code{osl}
332 \item \code{output}
333 \item \code{settings}
334 \item \code{specification}
335 \item \code{test}
336\end{itemize}\end{multicols}
337
338For the description of the packages, their interaction and considerations on
339their structuring, see section \fullref{coarse}.
340For a detailed package description, refer to Appendix TODO.
341
342Each package is documented in the source code also, namely in a file
343\file{package-info.java} residing in the respective package directory.
344This is a common scheme supported by the \name{Eclipse} IDE as well as the
345documentation generation systems \name{javadoc} and \name{doxygen} TODO
346(all of which were used in the creation of the program,
347as described in section \fullref{tools}).