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Java

Java Programming Style Guidelines

Table of Content

1 Introduction

This document lists Java coding recommendations common in the Java development community.

The recommendations are based on established standards collected from a number of sources, individual experience, local requirements/needs, as well as suggestions given in [1], [2], [3], [4] and [5].

There are several reasons for introducing a new guideline rather than just referring to the ones above. Main reason is that these guides are far too general in their scope and that more specific rules (especially naming rules) need to be established. Also, the present guide has an annotated form that makes it far easier to use during project code reviews than most other existing guidelines. In addition, programming recommendations generally tend to mix style issues with language technical issues in a somewhat confusing manner. The present document does not contain any Java technical recommendations at all, but focuses mainly on programming style.

While a given development environment (IDE) can improve the readability of code by access visibility, color coding, automatic formatting and so on, the programmer should never rely on such features. Source code should always be considered larger than the IDE it is developed within and should be written in a way that maximize its readability independent of any IDE.

1.1 Layout of the Recommendations.

The recommendations are grouped by topic and each recommendation is numbered to make it easier to refer to during reviews.

Layout for the recommendations is as follows:

Guideline short description

Example if applicable

Motivation, background and additional information.

The motivation section is important. Coding standards and guidelines tend to start "religious wars", and it is important to state the background for the recommendation.

1.2 Recommendation Importance

In the guideline sections the terms must, should and can have special meaning. A must requirement must be followed, a should is a strong recommendation, and a can is a general guideline.

2 General Recommendations

1. Any violation to the guide is allowed if it enhances readability.

The main goal of the recommendation is to improve readability and thereby the understanding and the maintainability and general quality of the code. It is impossible to cover all the specific cases in a general guide and the programmer should be flexible.

3 Naming Conventions

3.1 General Naming Conventions

2. Names representing packages should be in all lower case.

mypackage, com.company.application.ui

Package naming convention used by Sun for the Java core packages. The initial package name representing the domain name must be in lower case.

3. Names representing types must be nouns and written in mixed case starting with upper case.

Line, FilePrefix

Common practice in the Java development community and also the type naming convention used by Sun for the Java core packages.

4. Variable names must be in mixed case starting with lower case.

line, filePrefix

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 Line line;

5. Names representing constants (final variables) must be all uppercase using underscore to separate words.

MAX_ITERATIONS, COLOR_RED

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.

6. Names representing methods must be verbs and written in mixed case starting with lower case.

getName(), computeTotalWidth()

Common practice in the Java development community and also the naming convention used by Sun for the Java core packages. This is identical to variable names, but methods in Java are already distinguishable from variables by their specific form.

7. 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.

8. Private class variables should have _ suffix.

class SomeClass { private int length_; ... }

Apart from its name and its type, the scope of a variable is its most important feature. Indicating class scope by using _ 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 _ naming convention is that it nicely resolves the problem of finding reasonable variable names for setter methods:

void setDepth (int depth) { depth_ = depth; }

An issue is whether the _ 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.

9. 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 x) 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;

10. All names should be written in English.

fileName; // NOT: filNavn

English is the preferred language for international development.

11. Variables with a large scope should have long names, variables with a small scope can have short names [1].

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.

12. The name of the object is implicit, and should be avoided in a method name.

line.getLength(); // NOT: line.getLineLength();

The latter seems natural in the class declaration, but proves superfluous in use, as shown in the example.

3.2 Specific Naming Conventions

13. The terms get/set must be used where an attribute is accessed directly.

employee.getName(); matrix.getElement (2, 4); employee.setName (name); matrix.setElement (2, 4, value);

This is the naming convention for accessor methods used by Sun for the Java core packages. When writing Java beans this convention is actually enforced.

14. 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. When writing Java beans this convention is actually enforced for functions.

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.

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;

15. 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.

16. The term find can be used in methods where something is looked up.

vertex.findNearestVertex(); matrix.findMinElement();

Give the reader the immediate clue that this is a simple look up method with a minimum of computations involved. Consistent use of the term enhances readability.

17. The term initialize can be used where an object or a concept is established.

printer.initializeFontSet();

The American initialize should be preferred over the English initialise. Abbreviation init must be avoided.

18. JFC (Java Swing) variables should be suffixed by the element type.

widthScale, nameTextField, leftScrollbar, mainPanel, fileToggle, minLabel, printerDialog

Enhances readability since the name gives the user an immediate clue of the type of the variable and thereby the available resources of the object.

19. List suffix can be used on names representing a list of objects.

vertex (one vertex), vertexList (a list of vertices)

Enhances readability since the name gives the user an immediate clue of the type of the variable and the operations that can be performed on the object.

Simply using the plural form of the base class name for a list (matrixElement (one matrix element), matrixElements(list of matrix elements)) should be avoided since the two only differ in a single character and are thereby difficult to distinguish.

A list in this context is the compound data type that can be traversed backwards, forwards, etc. (typically a Vector). A plain array is simpler. The suffix Array can be used to denote an array of objects.

20. n prefix should be used for variables representing a number of objects.

nPoints, nLines

The notation is taken from mathematics where it is an established convention for indicating a number of objects.

Note that Sun use the term num prefix in the core Java packages for such variables. This is probably meant as an abbreviation of number of, but as it looks more like number it makes the variable name strange and misleading. If "number of" is the preferred statement, numberOf prefix can be used instead of just n. num prefix must not be used.

21. No suffix should be used for variables representing an entity number.

tableNo, employeeNo

The notation is taken from mathematics where it is an established convention for indicating an entity number.

An elegant alternative is to prefix such variables with an i: iTable, iEmployee. This effectively makes them named iterators.

22. Iterator variables should be called i, j, k etc.

while (Iterator i = pointList.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.

23. Complement names must be used for complement entities [1].

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

Reduce complexity by symmetry.

24. Abbreviations in names should be avoided.

computeAverage(); // NOT: compAvg();

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
cp    instead of copy
pt    instead of point
comp instead of compute
init instead of initialize
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.

25. Negated boolean variable names must be avoided.

boolean isError; // NOT: isNotError boolean 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.

26. 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.

27. Exception classes should be suffixed with Exception.

class AccessException { : }

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.

28. 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.

29. 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

30. Java source files should have the extension .java.

Point.java

Enforced by the Java tools.

31. 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.

32. 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.

33. 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.

34. The incompleteness of split lines must be made obvious [1].

totalSum = a + b + c + d + e); function (param1, param2, param3); setText ("Long line split" "into two parts."); for (tableNo = 0; tableNo < maxTable; 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

5.1 Package and Import Statements

35. The package statement must be the first statement of the file. All files should belong to a specific package.

The package statement location is enforced by the Java language. Letting all files belong to an actual (rather than the Java default) package enforces Java language object oriented programming techniques.

36. The import statements must follow the package statement. import statements should be sorted with the most fundamental packages first, and grouped with associated packages together and one blank line between groups.

import java.io.*; import java.net.*; import java.rmi.* import java.rmi.server.*; import javax.swing.*; import javax.swing.event.*; import org.linux.apache.server.*;

The import statement location is enforced by the Java language. The sorting makes it simple to browse the list when there are many imports, and it makes it easy to determine on which packages the present package is designed. The grouping reduce complexity by collapsing related information into a common unit.

5.2 Classes and Interfaces

37. Class and Interface declarations should be organized in the following manner:

Class/Interface documentation.
class or interface statement.
Class (static) variables in the order public, protected, package (no access modifier), private.
Instance variables in the order public, protected, package (no access modifier), private.
Constructors.
Methods (no specific order).

Reduce complexity by making the location of each class element predictable.

5.3 Methods

38. Method modifiers should be given in the following order:
<access> static abstract synchronized <unusual> final native
The <access> modifier (if present) must be the first modifier.

<access> is one of public, protected or private while <unusual> includes volatile and transient. The most important lesson here is to keep the access modifier as the first modifier. Of the possible modifiers, this is by far the most important, and it must stand out in the method declaration. For the other modifiers, the order is less important, but it make sense to have a fixed convention. The above proposal is taken from one of Charles L. Perkins books on Java.

5.4 Types

39. Type conversions must always be done explicitly. Never rely on implicit type conversion.

floatValue = (float) intValue; // NOT: floatValue = intValue;

By this, the programmer indicates that he is aware of the different types involved and that the mix is intentional.

5.5 Variables

40. 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.

41. Variables must never have dual meaning.

Enhances readability by ensuring all concepts are represented uniquely. Reduce chance of error by side effects.

42. Class variables should never be declared public.

The concept of Java information hiding and encapsulation is violated by public variables. Use private variables and access functions instead. One exception to this rule is when the class is essentially a data structure, with no behavior (equivalent to a C++ struct). In this case it is appropriate to make the class' instance variables public [2].

43. Related variables of the same type can be declared in a common statement.
Unrelated variables should not be declared in the same statement.

float x, y, z; float revenueJanuary, revenueFebrury, revenueMarch;

The common requirement of having declarations on separate lines is not useful in the situations like the ones above. It enhances readability to group variables.

44. Variables should be kept alive for as short a time as possible.

Keeping the operations on a variable within a small scope, it is easier to control the effects and side effects of the variable.

5.6 Loops

45. Only loop control statements must be included in the for() construction.

sum = 0; // NOT: for (i=0, sum=0; i<100; i++) for (i=0; i<100; i++) // sum += value[i]; sum += value[i];

Increase maintainability and readability. Make it crystal clear what controls the loop and what the loop contains.

46. Loop variables should be initialized immediately before the loop.

boolean done = false; // NOT: boolean done = false; while (!done) { // : : // while (!done) { } // : }

47. The use of do .... while loops should be avoided.

There are two reasons for this. First is that the construct is superflous; Any statement that can be written as a do .... while loop can equally well be written as a while lopp or a for loop. Complexity is reduced by minimizing the number of constructs being used.

The other reason is of readability. A loop with the conditional part at the end is more difficult to read than one with the conditional at the top.

48. The use of break and continue in loops should be avoided.

These statements should only be used if they prove to give higher readability than their structured counterparts.

In general break should only be used in case statements and continue should be avoided alltogether.

49. The form for (;;) should be used for empty loops.

for (;;) { // NOT: while (true) { : // : } // }

This form is better than the functionally equivalent while (true) since this implies a test against true, which is neither necessary nor meaningful.

5.7 Conditionals

50. Complex conditional expressions must be avoided. Introduce temporary boolean variables instead [1].

if ((elementNo < 0) || (elementNo > maxElement)|| elementNo == lastElement) { : }

should be replaced by:

boolean isFinished = (elementNo < 0) || (elementNo > maxElement); boolean isRepeatedEntry = elementNo == lastElement; if (isFinished || isRepeatedEntry) { : }

By assigning boolean variables to expressions, the program gets automatic documentation. The construction will be easier to read and to debug.

51. The nominal case should be put in the if-part and the exception in the else-part of an if statement [1].

boolean isError = readFile (fileName); if (!isError) { : } else { : }

Makes sure that the exceptions does not obscure the normal path of execution. This is important for both the readability and performance.

52. The conditional should be put on a separate line.

if (isDone) // NOT: if (isDone) doCleanup(); doCleanup();

This is for debugging purposes. When writing on a single line, it is not apparent whether the test is really true or not.

53. Executable statements in conditionals must be avoided.

file = openFile (fileName, "w"); // NOT: if ((file = openFile (fileName, "w")) != null) { if (file != null) { // : : // } }

Conditionals with executable statements are simply very difficult to read. This is especially true for programmers new to Java.

5.8 Miscellaneous

54. The use of magic numbers in the code should be avoided. Numbers other than 0 and 1 should be considered declared as named constants instead.

If the number does not have an obvious meaning by itself, the readability is enhanced by introducing a named constant instead.

55. Floating point constants should always be written with decimal point and at least one decimal.

double total = 0.0; // NOT: double total = 0; double speed = 3.0e8; // NOT: double speed = 3e8; double sum; : sum = (a + b) * 10.0;

This empasize the different nature of integer and floating point numbers even if their values might happen to be the same in a specific case.

Also, as in the last example above, it emphasize the type of the assigned variable (sum) at a point in the code where this might not be evident.

56. Floating point constants should always be written with a digit before the decimal point.

double total = 0.5; // NOT: double total = .5;

The number and expression system in Java is borrowed from mathematics and one should adhere to mathematical conventions for syntax wherever possible. Also, 0.5 is a lot more readable than .5; There is no way it can be mixed with the integer 5.

6 Layout and Comments

6.1 Layout

57. Basic indentation should be 2.

for (i = 0; i < nElements; i++) a[i] = 0;

Indentation of 1 is to small to emphasize the logical layout of the code. Indentation larger than 4 makes deeply nested code difficult to read and increase the chance that the lines must be split. Choosing between indentation of 2, 3 and 4, 2 and 4 are the more common, and 2 chosen to reduce the chance of splitting code lines. Note that the Sun recommendation on this point is 4.

58. Block layout should be as illustrated in example 1 below (recommended) or example 2, and must not be as shown in example 3. Class, Interface and method blocks should use the block layout of example 2.
`while (!isDone) { doSomething(); isDone = moreToDo(); }`	`while (!isDone) { doSomething(); isDone = moreToDo(); }`	`while (!isDone) { doSomething(); isDone = moreToDo(); }`
Example 3 introduce an extra indentation level which doesn't emphasize the logical structure of the code as clearly as example 1 and 2.

59. The class or interface declarations should have the following form:

class SomeClass extends AnotherClass implements SomeInterface, AnotherInterface { ... }

This follows from the general block rule above. It is common in the Java developer community to have the opening bracket at the end of the line of the class keyword. Actually, this bracket style is commonly used for all types of blocks. As a matter of personal preference, the C/C++ convention of treating class and method blocks different from other blocks is adopted.

60. The method declarations should have the following form:

public void someMethod () { ... }

See comment on class statements above.

61. The if-else class of statements should have the following form:

if (condition) { statements; } if (condition) { statements; } else { statements; } if (condition) { statements; } else if (condition) { statements; } else { statements; }

This follows partly from the general block rule above. However, it might be discussed if an else clause should be on the same line as the closing bracket of the previous if or else clause:

if (condition) { statements; } else { statements; }This is equivalent to the Sun recommendation. The chosen approach is considered better in the way that each part of the if-else statement is written on separate lines of the file. This should make it easier to manipulate the statement, for instance when moving else clauses around.

62. A for statement should have the following form:

for (initialization; condition; update) { statements; }

This follows from the general block rule above.

63. An empty for statement should have the following form:

for (initialization; condition; update) ;

This emphasize the fact that the for statement is empty and it makes it obvious for the reader that this is intentional.

64. A while statement should have the following form:

while (condition) { statements; }

This follows from the general block rule above.

65. A do-while statement should have the following form:

do { statements; } while (condition);

This follows from the general block rule above.

66. A switch statement should have the following form:

switch (condition) { case ABC : statements; // Fallthrough case DEF : statements; break; case XYZ : statements; break; default : statements; break; }

This differs slightly from the Sun recommendation both in indentation and spacing. In particular, each case keyword is indented relative to the switch statement as a whole. This makes the entire switch statement stand out. Note also the extra space before the : character. The explicit Fallthrough comment should be included whenever there is a case statement without a break statement. Leaving the break out is a common error, and it must be made clear that it is intentional when it is not there.

67. A try-catch statement should have the following form:

try { statements; } catch (Exception exception) { statements; } try { statements; } catch (Exception exception) { statements; } finally { statements; }

This follows partly from the general block rule above. This form differs from the Sun recommendation in the same way as the if-else statement described above.

68. Single statement if-else, for or while statements can be written without brackets.

if (condition) statement; while (condition) statement; for (initialization; condition; update) statement;

It is a common recommendation (Sun Java recommendation included) that brackets should always be used in all these cases. However, brackets are in general a language construct that groups several statements. Brackets are per definition superfluous on a single statement.

6.2 White Space

69.
- Conventional operators should be surrounded by a space character.
- Java reserved words should be followed by a white space.
- Commas should be followed by a white space.
- Colons should be surrounded by white space.
- Semicolons in for statements should be followed by a space character.

a = (b + c) * d; // NOT: a=(b+c)*d while (true) { // NOT: while(true) ... doSomething (a, b, c, d); // NOT: doSomething (a,b,c,d); case 100 : // NOT: case 100: for (i = 0; i < 10; i++) { // NOT: for (i=0;i<10;i++){

Makes the individual components of the statements stand out. Enhances readability. It is difficult to give a complete list of the suggested use of whitespace in Java code. The examples above however should give a general idea of the intentions.

70. Function names should be followed by a white space when it is followed by another name.

doSomething (currentFile); // NOT: doSomething(currentFile);

Makes the individual names stand out. Enhances readability. When no name follows, the space can be omitted (doSomething()) since there is no doubt about the name in this case. An alternative to this approach is to require a space after the opening parenthesis. Those that adhere to this standard usually also leave a space before the closing parentheses: doSomething( currentFile );. This do make the individual names stand out as is the intention, but the space before the closing parenthesis is rather artificial, and without this space the statement looks rather asymmetrical (doSomething( currentFile);).

71. Logical units within a block should be separated by one blank line.

Enhances readability by introducing white space between logical units of a block.

72. Methods should be separated by 3-5 blank lines.

By making the space larger than space within a method, the methods will stand out within the class.

73. Variables in declarations should be left aligned.

AsciiFile file; int nPoints; float x, y;

Enhances readability. The variables are easier to spot from the types by alignment.

74. Statements should be aligned wherever this enhances readability.

if (a == lowValue) compueSomething(); else if (a == mediumValue) computeSomethingElse(); else if (a == highValue) computeSomethingElseYet(); value = (potential * oilDensity) / constant1 + (depth * waterDensity) / constant2 + (zCoordinateValue * gasDensity) / constant3; minPosition = computeDistance (min, x, y, z); averagePosition = computeDistance (average, x, y, z); switch (value) { case PHASE_OIL : phaseString = "Oil"; break; case PHASE_WATER : phaseString = "Water"; break; case PHASE_GAS : phaseString = "Gas"; break; }

There are a number of places in the code where white space can be included to enhance readability even if this violates common guidelines. Many of these cases have to do with code alignment. General guidelines on code alignment are difficult to give, but the examples above should give some general hints. In short, any construction that enhances readability is allowed.

6.3 Comments

75. Tricky code should not be commented but rewritten. [1]

In general, the use of comments should be minimized by making the code self-documenting by appropriate name choices and an explicit logical structure.

76. All comments should be written in English.

In an international environment English is the preferred language.

77. Use // for all non-JavaDoc comments, including multi-line comments.

// Comment spanning // more than one line

Since multilevel Java commenting is not supported, using // comments ensure that it is always possible to comment out entire sections of a file using /* */ for debugging purposes etc.

78. Comments should be indented relative to their position in the code. [1]

while (true) { // NOT: while (true) { // Do something // // Do something something(); // something(); } // }

This is to avoid that the comments break the logical structure of the program.

79. The declaration of collection variables should be followed by a comment stating the common type of the elements of the collection.

private Vector pointList_; // Vector of Point private Set shapeSet_; // Set of Shape

Without the extra comment it can be hard to figure out what the collection consist of, and thereby how to treat the elements of the collection. In methods taking collection variables as input, the common type of the elements should be given in the associated JavaDoc comment.

80. All public classes and public and protected functions within public classes should be documented using the Java documentation (javadoc) conventions.

This makes it easy to keep up-to-date online code documentation.