Friday, September 17, 2010

Common naming convention : Coding Style

Variable names must be in mixed case starting with lower case. 
Common practice in the Java development community and also the naming convention for variables used by Sun for the Java core packages. Makes variables easy to distinguish from types, and effectively resolves potential naming collision as in the declaration
int state;

Names representing constants (final variables) must be all uppercase using underscore to separate words.
Common practice in the Java development community and also the naming convention used by Sun for the Java core packages.

In general, the use of such constants should be minimized. In many cases implementing the value as a method is a better choice:

int getMaxIterations() // NOT: MAX_ITERATIONS = 25
return 25;

This form is both easier to read, and it ensures a uniform interface towards class values.

Names representing methods must be verbs and written in mixed case starting with lower case. getName(), computeTotalWidth() 

Abbreviations and acronyms should not be uppercase when used as name.
exportHtmlSource(); // NOT: exportHTMLSource();
openDvdPlayer(); // NOT: openDVDPlayer();

Using all uppercase for the base name will give conflicts with the naming conventions given above. A variable of this type whould have to be named dVD, hTML etc. which obviously is not very readable. Another problem is illustrated in the examples above; When the name is connected to another, the readability is seriously reduced; The word following the acronym does not stand out as it should.

Private class variables should have underscore suffix.
class Person { 
               private String name_; 
... }
Apart from its name and its type, the scope of a variable is its most important feature. Indicating class scope by using underscore makes it easy to distinguish class variables from local scratch variables. This is important because class variables are considered to have higher significance than method variables, and should be treated with special care by the programmer.

A side effect of the underscore naming convention is that it nicely resolves the problem of finding reasonable variable names for setter methods:

void setName(String name)
name_ = name;

An issue is whether the underscore should be added as a prefix or as a suffix. Both practices are commonly used, but the latter is recommended because it seem to best preserve the readability of the name.

It should be noted that scope identification in variables have been a controversial issue for quite some time. It seems, though, that this practice now is gaining acceptance and that it is becoming more and more common as a convention in the professional development community.

Generic variables should have the same name as their type.
void setTopic(Topic topic) // NOT: void setTopic(Topic value) 
                                        // NOT: void setTopic(Topic aTopic) 
                                         // NOT: void setTopic(Topic t) 
void connect(Database database) // NOT: void connect(Database db) 
                                                   // NOT: void connect(Database oracleDB)
Reduce complexity by reducing the number of terms and names used. Also makes it easy to deduce the type given a variable name only.

If for some reason this convention doesn't seem to fit it is a strong indication that the type name is badly chosen.

Non-generic variables have a role. These variables can often be named by combining role and type:

Point startingPoint, centerPoint;
Name loginName;
All names should be written in English.English is the preferred language for international development.

Variables with a large scope should have long names, variables with a small scope can have short names
Scratch variables used for temporary storage or indices are best kept short. A programmer reading such variables should be able to assume that its value is not used outside a few lines of code. Common scratch variables for integers are i, j, k, m, n and for characters c and d.

The name of the object is implicit, and should be avoided in a method name.
line.getLength(); // NOT: line.getLineLength();
The latter might seem natural in the class declaration, but proves superfluous in use, as shown in the example.

The terms get/set must be used where an attribute is accessed directly.
matrix.getElement(2, 4);
matrix.setElement(2, 4, value);

is prefix should be used for boolean variables and methods.
isSet, isVisible, isFinished, isFound, isOpen
This is the naming convention for boolean methods and variables used by Sun for the Java core packages.

Using the is prefix solves a common problem of choosing bad boolean names like status or flag. isStatus or isFlag simply doesn't fit, and the programmer is forced to chose more meaningful names.

Setter methods for boolean variables must have set prefix as in:

void setFound(boolean isFound);

There are a few alternatives to the is prefix that fits better in some situations. These are has, can and should prefixes:

boolean hasLicense();
boolean canEvaluate();
boolean shouldAbort = false;

The term compute can be used in methods where something is computed.
valueSet.computeAverage(); matrix.computeInverse()
Give the reader the immediate clue that this is a potential time consuming operation, and if used repeatedly, he might consider caching the result. Consistent use of the term enhances readability.

 Iterator variables should be called i, j, k etc.
for (Iterator i = points.iterator(); i.hasNext(); ) { : } for (int i = 0; i < nTables; i++) { : } 

The notation is taken from mathematics where it is an established convention for indicating iterators. Variables named j, k etc. should be used for nested loops only. 

Complement names must be used for complement entities 
get/set, add/remove, create/destroy, start/stop, insert/delete, increment/decrement, old/new, begin/end, first/last, up/down, min/max, next/previous, old/new, open/close, show/hide, suspend/resume, etc. 
Reduce complexity by symmetry. 

Abbreviations in names should be avoided
computeAverage(); // NOT: compAvg(); 
ActionEvent event; // NOT: ActionEvent e; 
catch (Exception exception) { // NOT: catch (Exception e) { 
There are two types of words to consider. First are the common words listed in a language dictionary. These must never be abbreviated.  
Never write
cmd instead of command 
comp instead of compute cp
instead of copy 
e instead of exception 
init instead of initialize
pt instead of point etc. 
Then there are domain specific phrases that are more naturally known through their acronym or abbreviations. These phrases should be kept abbreviated. Never write: HypertextMarkupLanguage instead of html CentralProcessingUnit instead of cpu PriceEarningRatio instead of pe etc. 

Negated boolean variable names must be avoided. 
bool isError; // NOT: isNoError 
bool isFound; // NOT: isNotFound 
The problem arise when the logical not operator is used and double negative arises. It is not immediately apparent what !isNotError means.

Associated constants (final variables) should be prefixed by a common type name.

final int COLOR_RED = 1; 
final int COLOR_GREEN = 2; 
final int COLOR_BLUE = 3; 
This indicates that the constants belong together, and what concept the constants represents. 
An alternative to this approach is to put the constants inside an interface effectively prefixing their names with the name of the interface: interface Color { final int RED = 1; final int GREEN = 2; final int BLUE = 3; }

Exception classes should be suffixed with Exception. 

class AccessException extends Exception { : } 
Exception classes are really not part of the main design of the program, and naming them like this makes them stand out relative to the other classes. This standard is followed by Sun in the basic Java library.

Default interface implementations can be prefixed by Default. 

class DefaultTableCellRenderer implements TableCellRenderer { : } 
It is not uncommon to create a simplistic class implementation of an interface providing default behaviour to the interface methods. The convention of prefixing these classes by Default has been adopted by Sun for the Java library.

Singleton classes should return their sole instance through method getInstance

class UnitManager { private final static UnitManager instance_ = new UnitManager(); private UnitManager() { ... } public static UnitManager getInstance() // NOT: get() or instance() or unitManager() etc. { return instance_; } } Common practice in the Java community though not consistently followed by Sun in the JDK. The above layout is the preferred pattern.

Classes that creates instances on behalf of others (factories) can do so through method new[ClassName]

class PointFactory { public Point newPoint(...) { ... } } 
Indicates that the instance is created by new inside the factory method and that the construct is a controlled replacement of new Point().

 Functions (methods returning an object) should be named after what they return and procedures (void methods) after what they do. Increase readability. Makes it clear what the unit should do and especially all the things it is not supposed to do. This again makes it easier to keep the code clean of side effects. 4 Files

Classes should be declared in individual files with the file name matching the class name. 

Secondary private classes can be declared as inner classes and reside in the file of the class they belong to. Enforced by the Java tools.

 File content must be kept within 80 columns. 80 columns is the common dimension for editors, terminal emulators, printers and debuggers, and files that are shared between several developers should keep within these constraints. It improves readability when unintentional line breaks are avoided when passing a file between programmers.

Special characters like TAB and page break must be avoided. 

 These characters are bound to cause problem for editors, printers, terminal emulators or debuggers when used in a multi-programmer, multi-platform environment.

The incompleteness of split lines must be made obvious 
totalSum = a + b + c + 
                   d + e; 
method(param1, param2, 
setText ("Long line split" +
                "into two parts."); 
 for (int tableNo = 0; tableNo < nTables; 
           tableNo += tableStep) { ... } 

Split lines occurs when a statement exceed the 80 column limit given above. It is difficult to give rigid rules for how lines should be split, but the examples above should give a general hint. In general: 
Break after a comma. 
Break after an operator. 
Align the new line with the beginning of the expression on the previous line. 5 Statements

Type conversions must always be done explicitly. Never rely on implicit type conversion.
floatValue = (int) intValue; // NOT: floatValue = intValue;
By this, the programmer indicates that he is aware of the different types involved and that the mix is intentional.

Variables should be initialized where they are declared and they should be declared in the smallest scope possible.
This ensures that variables are valid at any time. Sometimes it is impossible to initialize a variable to a valid value where it is declared. In these cases it should be left uninitialized rather than initialized to some phony value.

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