JAVA CODING STANDARDS
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Purpose of this document
This document describes a collection of standards, conventions and
guidelines for writing Java code that is easy to understand, to maintain, and
to enhance.
Important features of this document
Existing standards from the industry are used wherever possible
The reason behind each standard is explained so that developers can
understand why they should follow it.
These standards are based on proven software-engineering principles
that lead to improved development productivity, greater maintainability, and
greater scalability.
Target Audience
Professional Software developers who are involved in:
Writing Java code that is easy to maintain and to enhance
Increasing their productivity
Why coding standards are important
Coding standards for Java are important because they lead to greater
consistency within code of all developers. Consistency leads to code that is
easier to understand, which in turn results in a code, which is easier to
develop and maintain. Code that is difficult to understand and maintain runs
the risk of being scrapped and rewritten.
The Prime Directive
A project requirement may vary from the standards mentioned in this
document. When going against the standards, projects should make sure to
document it.
1. Naming Convention
Use full English descriptors that accurately describe the
variable/field/class/interface
For example, use names like firstName, grandTotal, or CorporateCustomer.
Use terminology applicable to the domain
If the users of the system refer to their clients as Customer, then use
the term Customer for the class, not client.
Use mixed case to make names readable
Use abbreviations sparingly, but if you do so then use then
intelligently and document it
For example, to use a short form for the word “number”, choose one of nbr,
no or num.
Avoid long names (<15 characters is a good tradeoff)
Avoid names that are similar or differ only in case
2. Documentation
Comments should add to the clarity of code.
Avoid decoration, i.e., do not use banner-like comments
Document why something is being done, not just what.
Java Comments
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Comment Type
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Usage
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Example
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Documentation
Starts with /** and ends with */
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Used before declarations of interfaces, classes, member functions,
and fields to document them.
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/**
*
Customer – a person or
*
organization
*/
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C style
Starts with /* and ends with */
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Used to document out lines of code that are no longer applicable. It
is helpful in debugging.
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/*
This code was commented out by Ashish Sarin
*/
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Single line
Starts with // and go until the end of the line
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Used internally within member functions to document business logic,
sections of code, and declarations of temporary variables.
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// If the amount is greater
// than 10 multiply by 100
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3. Standards For Member Functions
3. 1 Naming member functions
Member functions should be named using a full English description,
using mixed case with the first letter of any non-initial word capitalized. The
first word of the member function should be a verb.
Examples
openAccount()
printMailingList()
save()
delete()
This results in member functions whose purpose can be determined just
by looking at its name.
3.1.1 Naming Accessor Member Functions
3.1.1.1 Getters: member functions
that return the value of a field / attribute / property of an object.
Use prefix “get” to the name of the field / attribute / property if the
field in not boolean
Use prefix “is” to the name of the field / attribute / property if the
field is Boolean
A viable alternative is to use the prefix ‘has’ or ‘can’ instead of
‘is’ for boolean getters.
Examples
getFirstName()
isPersistent()
3.1.1.2 Setters: member functions
that modify the values of a field.
Use prefix ‘set’ to the name of the field.
Examples
setFirstName()
3.1.1.3 Constructors: member functions
that perform any necessary initialization when an object is created.
Constructors are always given the same name as their class.
Examples
Customer()
SavingsAccount()
3.2 Member Function Visibility
A good design requires minimum coupling between the classes. The
general rule is to be as restrictive as possible when setting the visibility of
a member function. If member function doesn’t have to be public then make it
protected, and if it doesn’t have to be protected than make it private.
3.3 Documenting Member Functions
3.3.1 Member Function Header
Member function documentation should include the following:
What and why the member function does what it does
What member function must be passed as parameters
What a member function returns
Known bugs
Any exception that a member function throws
Visibility decisions (if questionable by other developers)
How a member function changes the object – it is to helps a developer
to understand how a member function invocation will affect the target object.
Include a history of any code changes
Examples of how to invoke the member function if appropriate.
Applicable pre conditions and post conditions under which the function
will work properly. These are the assumptions made during writing of the
function.
All concurrency issues should be addressed.
- Explanation of why
keeping a function synchronized must be documented.
When a member function updates a field/attribute/property, of a class
that implements the Runnable
interface, is not synchronized then it should be documented why it is
unsynchronized.
If a member function is overloaded or overridden or synchronization
changed, it should also be documented.
Note: It’s not necessary to document all the factors
described above for each and every member function because not all factors are
applicable to every member function.
3.3.2 Internal Documentation: Comments within
the member functions
Use C style comments to document out lines of unneeded code.
Use single-line comments for business logic.
Internally following should be documented:
Control Structures This includes comparison statements and loops
Why, as well as what, the code does
Local variables
Difficult or complex code
The processing order If there are statements in the code that must be
executed in a defined order
3.3.3 Document the closing braces If there are many
control structures one inside another
4.0 Techniques for Writing Clean Code:
Document the code Already discussed above
Paragraph/Indent the code: Any code between the { and }
should be properly indented
Paragraph and punctuate multi-line statements
Example
Line 1 BankAccount
newPersonalAccount = AccountFactory
Line 2 createBankAccountFor(currentCustomer,
startDate,
Line 3 initialDeposit,
branch)
Lines 2 & 3 have been indented by one unit (horizontal tab)
Use white space
A few blank lines or spaces can help make the code more readable.
Single blank lines to separate logical groups of code, such as control
structures
Two blank lines to separate member function definitions
Specify the order of Operations: Use extra parenthesis to
increase the readability of the code using AND and OR comparisons. This
facilitates in identifying the exact order of operations in the code
Write short, single command lines Code should do one operation
per line So only one statement should be there per line
5.0 Standards for Fields (Attributes / Properties)
5.1 Naming Fields
Use a Full English Descriptor for Field Names
Fields that are collections, such as arrays or vectors, should be given
names that are plural to indicate that they represent multiple values.
Examples
firstName
orderItems
If the name of the field begins with an acronym then the acronym should
be completely in lower case
Example
sqlDatabase
5.2 Naming Components
Use full English descriptor postfixed by the widget type. This makes it
easy for a developer to identify the purpose of the components as well as its
type.
Example
okButton
customerList
fileMenu
newFileMenuItem
5.3 Naming Constants
In Java, constants, values that do not change, are typically
implemented as static final fields of classes. The convention is to use
full English words, all in upper case, with underscores between the words
Example
MINIMUM_BALANCE
MAX_VALUE
DEFAULT_START_DATE[1]
5.4 Field Visibility
Fields should not be declared public for reasons of encapsulation. All
fields should be declared private and accessor methods should be used to access
/ modify the field value. This results in less coupling between classes as the
protected / public / package access of field can result in direct access of the
field from other classes
5.5 Documenting a Field
Document the following:
It’s description
Document all applicable invariants Invariants of a field are
the conditions that are always true about it. By documenting the restrictions
on the values of a field one can understand important business rules, making it
easier to understand how the code works / how the code is supposed to work
Examples For fields that have complex business rules
associated with them one should provide several example values so as to make
them easier to understand
Concurrency issues
Visibility decisions If a field is declared anything but private then it
should be documented why it has not been declared private.
5.6 Usage of Accesors Accessors can be
used for more than just getting and setting the values of instance fields.
Accesors should be used for following purpose also:
Initialize the values of fields Use lazy initialization
where fields are initialized by their getter member functions.
Example
/**
* Answer the branch number,
which is the leftmost four digits of the full account
* number. Account numbers are
in the format BBBBAAAAAA.
*/
protected int getBranchNumber()
{
if(branchNumber ==
0)
{
//
The default branch number is 1000, which is the
//
main branch in downtown Bedrock
setBranchNumber(1000);
}
return
branchNumber;
}
Note:
This approach is advantageous for objects that have fields that aren’t
regularly accessed
Whenever lazy initialization is used in a getter function the
programmer should document what is the type of default value, what the default
value as in the example above.
5.6.1 Access constant values Commonly constant
values are declared as static final fields. This approach makes sense
for “constants” that are stable.
If the constants can change because of some changes in the business
rules as the business matures then it is better to use getter member functions
for constants.
By using accesors for constants programmer can decrease the chance of
bugs and at the same time increase the maintainability of the system.
5.6.2 Access Collections The main purpose
of accesors is to encapsulate the access to fields so as to reduce the coupling
within the code. Collections, such as arrays and vectors, being more complex
than single value fields have more than just standard getter and setter member
function implemented for them. Because the business rule may require to add and
remove to and from collections, accessor member functions need to be included
to do so.
Example
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Member function type
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Naming Convention
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Example
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Getter for the collection
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getCollection()
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getOrderItems()
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Setter for the collection
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setCollection()
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setOrderItems()
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Insert an object into the collection
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insertObject()
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insertOrderItems()
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Delete an object from the collection
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deleteObject()
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deleteOrderItems()
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Create and add a new object into the collection
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newObject()
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newOrderItem()
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Note
The advantage of this approach is that the collection is fully
encapsulated, allowing programmer to later replace it with another structure
It is common to that the getter member functions be public and
the setter be protected
Always Initialize Static Fields because one can’t assume
that instances of a class will be created before a static field is accessed
6.0 Standards for Local Variables
6.1 Naming Local Variables
Use full English descriptors with the first letter of any non-initial
word in uppercase.
6.1.1 Naming Streams
When there is a single input and/or output stream being opened, used,
and then closed within a member function the convention is to use in and
out for the names of these streams, respectively.
6.1.2 Naming Loop
Counters
A common way is to use words like loopCounters or simply counter
because it helps facilitate the search for the counters in the program.
i, j, k can also be used as loop counters but the disadvantage is that search
for i ,j and k in the code will result in many hits.
6.1.3 Naming Exception Objects
The use of letter e for a generic exception
6.2 Declaring and Documenting Local Variables
Declare one local variable per line of code
Document local variable with an endline comment
Declare local variables immediately before their use
Use local variable for one operation only. Whenever a local variable is
used for more than one reason, it effectively decreases its cohesion, making it
difficult to understand. It also increases the chances of introducing
bugs into the code from unexpected side effects of previous values of a local
variable from earlier in the code.
Note
Reusing local variables is more efficient because less memory needs to
be allocated, but reusing local variables decreases the maintainability of code
and makes it more fragile
7.0 Standards for Parameters (Arguments) to Member Functions
7.1 Naming Parameters
Parameters should be named following the exact same conventions as for
local variable
Name parameters the same as their corresponding fields (if any)
Example
If Account has an attribute called balance and you needed
to pass a parameter representing a new value for it the parameter would be
called balance The field would be referred to as this.balance in
the code and the parameter would be referred as balance
7.2 Documenting Parameters
Parameters to a member function are documented in the header
documentation for the member function using the javadoc @param tag. It
should describe:
What it should be used for
Any restrictions or preconditions
Examples If it is not completely obvious what a parameter should be,
then it should provide one or more examples in the documentation
Note
Use interface as a parameter to the member function then the object
itself.
Standards for Classes, Interfaces, Packages, and Compilation
Units
8.0 Standards for Classes
8.1 Class Visibility
Use package visibility for classes internal to a component
Use public visibility for the façade of components
8.2 Naming classes
Use full English descriptor starting with the first letter capitalized
using mixed case for the rest of the name
8.3 Documenting a Class
The purpose of the class
Known bugs
The development/maintenance history of the class
Document applicable variants
The concurrency strategy Any class that implements the interface Runnable
should
have its concurrency strategy fully described
8.4 Ordering Member Functions and Fields
The order should be:
Constructors
private fields
public member functions
protected member
functions
private member
functions
finalize()
9.0 Standards for Interfaces
9.1 Naming Interfaces
Name interfaces using mixed case with the first letter of each word
capitalized.
Prefix the letter “I” or “Ifc” to the interface name
9.2 Documenting Interfaces
The Purpose
How it should and shouldn’t be used
10.0 Standards for Packages
Local packages
names begin with an identifier that is not all upper case
Global package names begin with the reversed Internet domain name for
the organization
Package names should be singular
10.1 Documenting a Package
The rationale for
the package
The classes in the
packages
11.0 Standards for Compilation Unit (Source code file)
11.1 Naming a Compilation Unit
A compilation unit should be given the name of the primary class or
interface that is declared within it. Use the same name of the class for the
file name, using the same case.
11.2 Beginning Comments
/**
* Classname
*
* Version information
*
* Date
*
* Copyright notice
*/
11.3 Declaration
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Class/interface documentation comment (
/**...*/) |
See Documentation standard for class / interfaces
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Class or interface statement |
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Class/interface implementation comment (
/*...*/), if necessary |
This comment should contain any class-wide or interface-wide
information that wasn't appropriate for the class/interface documentation
comment.
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Class (
static) variables |
First the
public class
variables, then the protected, then package
level (no access modifier), and then the private. |
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Instance variables
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First
public, then protected, then package level (no
access modifier), and then private. |
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Methods
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These methods should be grouped by functionality rather than by scope
or accessibility. For example, a private class method can be in between two
public instance methods. The goal is to make reading and understanding the
code easier.
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11.4 Indentation
Four spaces should be used as the unit of indentation. The exact
construction of the indentation (spaces vs. tabs) is unspecified. Tabs must be
set exactly every 8 spaces (not 4).
11.5 Line Length
Avoid lines longer than 80 characters, since they're not handled well
by many terminals and tools.
Note: Examples for use in documentation should have a
shorter line length-generally no more than 70 characters.
11.5 Wrapping Lines
When an expression will not fit on a single line, break it according to
these general principles:
Break after a comma.
Break before an operator.
Prefer higher-level breaks to lower-level breaks.
Align the new line with the beginning of the expression at the same
level on the previous line.
If the above rules lead to confusing code or to code that's squished up
against the right margin, just indent 8 spaces instead.
Here are some examples of
breaking method calls:
someMethod(longExpression1, longExpression2, longExpression3,
longExpression4, longExpression5);
var
= someMethod1(longExpression1,
someMethod2(longExpression2,
longExpression3));
Following are two examples of breaking an arithmetic expression. The
first is preferred, since the break occurs outside the parenthesized expression,
which is at a higher level.
longName1 = longName2 * (longName3 + longName4 - longName5)
+ 4 * longname6; // PREFER
longName1 = longName2 * (longName3 + longName4
-
longName5) + 4 * longname6; // AVOID
Following are two examples of indenting method declarations. The first
is the conventional case. The second would shift the second and third lines to
the far right if it used conventional indentation, so instead it indents only 8
spaces.
//CONVENTIONAL INDENTATION
someMethod(int anArg, Object anotherArg, String yetAnotherArg,
Object
andStillAnother) {
...
}
//INDENT 8 SPACES TO AVOID VERY DEEP INDENTS
private static synchronized horkingLongMethodName(int anArg,
Object anotherArg, String
yetAnotherArg,
Object andStillAnother) {
...
}
Line wrapping for
if statements
should generally use the 8-space rule, since conventional (4 space) indentation
makes seeing the body difficult. For example:
//DON'T USE THIS INDENTATION
if ((condition1 && condition2)
|| (condition3 &&
condition4)
||!(condition5 &&
condition6)) { //BAD WRAPS
doSomethingAboutIt(); //MAKE THIS LINE EASY TO MISS
}
//USE THIS INDENTATION INSTEAD
if ((condition1 && condition2)
|| (condition3 &&
condition4)
||!(condition5 &&
condition6)) {
doSomethingAboutIt();
}
//OR USE THIS
if ((condition1 && condition2) || (condition3 &&
condition4)
||!(condition5 &&
condition6)) {
doSomethingAboutIt();
}
Here are three acceptable ways to format ternary expressions:
alpha = (aLongBooleanExpression) ? beta : gamma;
alpha = (aLongBooleanExpression) ? beta
: gamma;
alpha = (aLongBooleanExpression)
? beta
: gamma;
11.6 Declaration
One declaration per line is recommended since it encourages commenting.
In other words,
int level; // indentation level
int size; // size of table
is preferred over
int level, size;
Do not put different types on the same line. Example:
int foo, fooarray[]; //WRONG!
Note: The examples above use one space between the type
and the identifier. Another acceptable alternative is to use tabs, e.g.:
int level;
// indentation level
int size;
// size of table
Object currentEntry; // currently selected table entry
11.7 Initialization
Try to initialize local variables where they're declared. The only
reason not to initialize a variable where it's declared is if the initial value
depends on some computation occurring first.
11.8 Placement
Put declarations only at the beginning of blocks. (A block is any code
surrounded by curly braces "{" and "}".) Don't wait to
declare variables until their first use; it can confuse the unwary programmer
and hamper code portability within the scope.
void myMethod() {
int int1 = 0; //
beginning of method block
if (condition) {
int int2 = 0; // beginning of "if" block
...
}
}
The one exception to the rule is indexes of
for loops, which in Java can be
declared in the for statement:
for (int i = 0; i < maxLoops; i++) { ... }
Avoid local declarations that hide declarations at higher levels. For
example, do not declare the same variable name in an inner block:
int count;
...
myMethod() {
if (condition) {
int count = 0; // AVOID!
...
}
...
}
11.9 Class and Interface Declarations
When coding Java classes and interfaces, the following formatting rules
should be followed:
No space between a method name and the parenthesis "("
starting its parameter list
Open brace "{" appears at the end of the same line as the
declaration statement
Closing brace "}" starts a line by itself indented to match
its corresponding opening statement, except when it is a null statement the
"}" should appear immediately after the "{"
class Sample extends Object {
int ivar1;
int ivar2;
Sample(int i, int j) {
ivar1 = i;
ivar2 = j;
}
int emptyMethod() {}
...
}
A blank line separates methods
11.10 Statements
Simple Statements
Each line should contain at most one statement.
Example:
argv++; // Correct
argc--; // Correct
argv++; argc--; // AVOID!
Compound Statements
Compound statements are statements that contain lists of statements
enclosed in braces "
{
statements }". See the following sections for examples.
The enclosed statements should be indented one more level than the
compound statement.
The opening brace should be at the end of the line that begins the
compound statement; the closing brace should begin a line and be indented to
the beginning of the compound statement.
Braces are used around all statements, even single statements, when
they are part of a control structure, such as a
if-else or for statement. This makes it
easier to add statements without accidentally introducing bugs due to
forgetting to add braces.
return Statements
A
return statement with a
value should not use parentheses unless they make the return value more obvious
in some way.
Example:
return;
return myDisk.size();
return (size ? size : defaultSize);
if, if-else, if else-if else Statements
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;
}
Note:
if statements always use braces {}. Avoid the following error-prone form:
if (condition) //AVOID! THIS OMITS THE BRACES {}!
statement;
for Statements
A
for statement should
have the following form:
for (initialization; condition; update) {
statements;
}
An empty
for statement (one
in which all the work is done in the initialization, condition, and update
clauses) should have the following form:
for (initialization; condition; update);
When using the comma operator in the initialization or update clause of
a
for statement, avoid
the complexity of using more than three variables. If needed, use separate
statements before the for loop (for the
initialization clause) or at the end of the loop (for the update clause).
while Statements
A
while statement should
have the following form:
while (condition) {
statements;
}
An empty
while statement should
have the following form:
while (condition);
do-while Statements
A
do-while statement should
have the following form:
do {
statements;
} while (condition);
switch Statements
A
switch statement should
have the following form:
switch (condition) {
case ABC:
statements;
/* falls through */
case DEF:
statements;
break;
case XYZ:
statements;
break;
default:
statements;
break;
}
Every time a case falls through (doesn't include a
break statement), add a comment
where the break statement would
normally be. This is shown in the preceding code example with the /* falls through */ comment.
Every
switch statement should
include a default case. The break in the default case is redundant, but it prevents a fall-through error
if later another case is added.
try-catch Statements
A
try-catch statement should
have the following format:
try {
statements;
} catch (ExceptionClass e) {
statements;
}
A
try-catch statement may
also be followed by finally, which executes
regardless of whether or not the try block has completed successfully.
try {
statements;
} catch (ExceptionClass e) {
statements;
} finally {
statements;
}
Blank Lines
Blank lines improve readability by setting off sections of code that
are logically related.
Two blank lines should always be used in the following circumstances:
Between sections of a source file
Between class and interface definitions
One blank line should always be used in the following circumstances:
Between methods
Between the local variables in a method and its first statement
Before a block or single-line comment
Between logical sections inside a method to improve readability
Blank Spaces
Blank spaces should be used in the following circumstances:
A keyword followed by a parenthesis should be separated by a space.
Example:
while (true) {
...
}
Note that a blank space should not be used between a method name and
its opening parenthesis. This helps to distinguish keywords from method calls.
A blank space should appear after commas in argument lists.
All binary operators except
. should be separated from their operands by spaces.
Blank spaces should never separate unary operators such as unary minus,
increment ("++"), and decrement ("--") from their operands.
Example:
a += c + d;
a = (a + b) / (c * d);
while (d++ = s++) {
n++;
}
printSize("size is
" + foo + "\n");
The expressions in a
for statement should be separated by blank spaces. Example:
for (expr1; expr2; expr3)
Casts should be followed by a blank space. Examples:
myMethod((byte) aNum,
(Object) x);
myMethod((int) (cp + 5),
((int) (i + 3))
+ 1);
Naming Conventions Summary
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Identifier Type
|
Rules for Naming
|
Examples
|
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Packages
|
The prefix of a unique package name is always written in
all-lowercase ASCII letters and should be one of the top-level domain names,
currently com, edu, gov, mil, net, org, or one of the English two-letter
codes identifying countries as specified in ISO Standard 3166, 1981.
Subsequent components of the package name vary according to an
organization's own internal naming conventions. Such conventions might
specify that certain directory name components be division, department,
project, machine, or login names.
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com.sun.eng
com.apple.quicktime.v2
edu.cmu.cs.bovik.cheese
|
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Classes
|
Class names should be nouns, in mixed case with the first letter of
each internal word capitalized. Try to keep your class names simple and
descriptive. Use whole words-avoid acronyms and abbreviations (unless the
abbreviation is much more widely used than the long form, such as URL or
HTML).
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class Raster;
class ImageSprite; |
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Interfaces
|
Interface names should be capitalized like class names.
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interface RasterDelegate;
interface Storing; |
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Methods
|
Methods should be verbs, in mixed case with the first letter
lowercase, with the first letter of each internal word capitalized.
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run();
runFast(); getBackground(); |
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Variables
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Except for variables, all instance, class, and class constants are in
mixed case with a lowercase first letter. Internal words start with capital
letters. Variable names should not start with underscore _ or dollar sign $
characters, even though [2]both
are allowed.
Variable names should be short yet meaningful. The choice of a
variable name should be mnemonic- that is, designed to indicate to the casual
observer the intent of its use. One-character variable names should be
avoided except for temporary "throwaway" variables. Common names
for temporary variables are
i, j, k, m, and n for integers; c, d, and e for characters. |
int i;
char c;
float myWidth;
|
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Constants
|
The names of variables declared class constants and of ANSI constants
should be all uppercase with words separated by underscores ("_").
(ANSI constants should be avoided, for ease of debugging.)
|
static final int MIN_WIDTH = 4;
static final int MAX_WIDTH = 999;
static final int GET_THE_CPU = 1;
|
References:
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