Code Conventions for the Java TM Programming Language

From SUN (Oracle) http://www.oracle.com/technetwork/java/index-135089.html

Code Conventions for the Java ^TM ProgrammingLanguage

Revised April 20, 1999

This Code Conventions for the Java Programming Language documentcontains the standard conventions that we at Sun follow and recommend thatothers follow. It covers filenames, file organization, indentation, comments,declarations, statements, white space, naming conventions, programmingpractices and includes a code example.

• 80% of the lifetime cost of a piece of software goes to maintenance.
• Hardly any software is maintained for its whole life by the original author.
• Code conventions improve the readability of the software, allowing engineers to understand new code more quickly and thoroughly.

The Code Conventions for the Java Programming Language document wasrevised and updated on April 20, 1999.

1 - Introduction

1.1 Why Have Code Conventions

Code conventions are important toprogrammers for a number of reasons:

• 80% of the lifetime cost of a piece of software goes to maintenance.
• Hardly any software is maintained for its whole life by the original author.
• Code conventions improve the readability of the software, allowing engineers to understand new code more quickly and thoroughly.
• If you ship your source code as a product, you need to make sure it is as well packaged and clean as any other product you create.

For theconventions to work, every person writing software must conform to the codeconventions. Everyone.

1.2 Acknowledgments

This document reflects the Java languagecoding standards presented in the Java LanguageSpecification, from Sun Microsystems, Inc. Major contributions are fromPeter King, Patrick Naughton, Mike DeMoney, Jonni Kanerva, Kathy Walrath, andScott Hommel.

This document is maintained by ScottHommel. Comments should be sent to [email protected]

2 - File Names

This section lists commonly used filesuffixes and names.

2.1 File Suffixes

Java Software uses the following filesuffixes:

File Type	Suffix
Java source	.java
Java bytecode	.class

2.2 Common File Names

Frequently used file names include:

File Name	Use
GNUmakefile	The preferred name for makefiles. We use gnumake to build our software.
README	The preferred name for the file that summarizes the contents of a particular directory.

3 - File Organization

A file consists of sections that should beseparated by blank lines and an optional comment identifying each section.

Files longer than 2000 lines arecumbersome and should be avoided.

For an example of a Java program properlyformatted, see "Java Source File Example".

3.1 Java Source Files

Each Java source file contains a singlepublic class or interface. When private classes and interfaces are associatedwith a public class, you can put them in the same source file as the publicclass. The public class should be the first class or interface in the file.

Java source files have the followingordering:

• Beginning comments (see "Beginning Comments")
• Package and Import statements
• Class and interface declarations (see "Class and Interface Declarations")

3.1.1Beginning Comments

All source files should begin with ac-style comment that lists the class name, version information, date, andcopyright notice:

/*
 * Classname
 * 
 * Version information
 *
 * Date
 * 
 * Copyright notice
 */

3.1.2 Package and Import Statements

The first non-comment line of most Javasource files is a package statement. After that, importstatements can follow. For example:

package java.awt;
 
import java.awt.peer.CanvasPeer;

Note: The first component of a uniquepackage name is always written in all-lowercase ASCII letters and should be oneof the top-level domain names, currently com, edu, gov, mil, net, org, or oneof the English two-letter codes identifying countries as specified in ISOStandard 3166, 1981.

3.1.3Class and Interface Declarations

The following table describes the parts ofa class or interface declaration, in the order that they should appear. See "Java Source File Example" for anexample that includes comments.

	Part of Class/Interface Declaration	Notes
1	Class/interface documentation comment (/**...*/)	See "Documentation Comments" on page 9 for information on what should be in this comment.
2	class or interface statement
3	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.
4	Class (static) variables	First the public class variables, then the protected, then package level (no access modifier), and then the private.
5	Instance variables	First public, then protected, then package level (no access modifier), and then private.
6	Constructors
7	Methods	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.

4 - Indentation

Four spaces should be used as the unit ofindentation. The exact construction of the indentation (spaces vs. tabs) isunspecified. Tabs must be set exactly every 8 spaces (not 4).

4.1 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-generallyno more than 70 characters.

4.2 Wrapping Lines

When an expression will not fit on a singleline, 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 methodcalls:

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 indentingmethod 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 ifstatements 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 acceptableways to format ternary expressions:

alpha = (aLongBooleanExpression) ? beta : gamma;  
 
alpha = (aLongBooleanExpression) ? beta
                                 : gamma;  
 
alpha = (aLongBooleanExpression)
        ? beta 
        : gamma; 

5 - Comments

Java programs can have two kinds ofcomments: implementation comments and documentation comments. Implementationcomments are those found in C++, which are delimited by /*...*/, and //.Documentation comments (known as "doc comments") are Java-only, andare delimited by /**...*/. Doc comments can be extracted to HTML files usingthe javadoc tool.

Implementation comments are mean forcommenting out code or for comments about the particular implementation. Doccomments are meant to describe the specification of the code, from animplementation-free perspective. to be read by developers who might notnecessarily have the source code at hand.

Comments should be used to give overviewsof code and provide additional information that is not readily available in thecode itself. Comments should contain only information that is relevant toreading and understanding the program. For example, information about how thecorresponding package is built or in what directory it resides should not beincluded as a comment.

Discussion of nontrivial or nonobviousdesign decisions is appropriate, but avoid duplicating information that ispresent in (and clear from) the code. It is too easy for redundant comments toget out of date. In general, avoid any comments that are likely to get out ofdate as the code evolves.

Note:Thefrequency of comments sometimes reflects poor quality of code. When you feelcompelled to add a comment, consider rewriting the code to make it clearer.

Comments should not be enclosed in largeboxes drawn with asterisks or other characters.
Comments should never include special characters such as form-feed andbackspace.

5.1 Implementation Comment Formats

Programs can have four styles ofimplementation comments: block, single-line, trailing, and end-of-line.

5.1.1 Block Comments

Block comments are used to providedescriptions of files, methods, data structures and algorithms. Block commentsmay be used at the beginning of each file and before each method. They can alsobe used in other places, such as within methods. Block comments inside afunction or method should be indented to the same level as the code theydescribe.

A block comment should be preceded by ablank line to set it apart from the rest of the code.

/*
 * Here is a block comment.
 */

Block comments can startwith /*-, which is recognized by indent(1) asthe beginning of a block comment that should not be reformatted. Example:

/*-
 * Here is a block comment with some very special
 * formatting that I want indent(1) to ignore.
 *
 *    one
 *        two
 *            three
 */

Note: If youdon't use indent(1), you don't have touse /*- in your code or make any other concessions to the possibility thatsomeone else might run indent(1) onyour code.

See also "DocumentationComments".

5.1.2Single-Line Comments

Short comments can appear on a single lineindented to the level of the code that follows. If a comment can't be writtenin a single line, it should follow the block comment format (see section 5.1.1). A single-line commentshould be preceded by a blank line. Here's an example of a single-line commentin Java code (also see "Documentation Comments"):

if (condition) {
 
    /* Handle the condition. */
    ...
}

5.1.3 Trailing Comments

Very short comments can appear on the sameline as the code they describe, but should be shifted far enough to separatethem from the statements. If more than one short comment appears in a chunk ofcode, they should all be indented to the same tab setting.

Here's an example of a trailing comment in Java code:

if (a == 2) {
    return TRUE;            /* special case */
} else {
    return isPrime(a);      /* works only for odd a */
}

5.1.4 End-Of-Line Comments

The // comment delimiter can comment out a complete line or only a partial line. It shouldn't be used on consecutive multiple lines for text comments; however, it can be used inconsecutive multiple lines for commenting out sections of code. Examples of all three styles follow:

if (foo > 1) {
 
    // Do a double-flip.
    ...
}
else {
    return false;          // Explain why here.
}
//if (bar > 1) {
//
//    // Do a triple-flip.
//    ...
//}
//else {
//    return false;
//}

5.2 Documentation Comments

Note: See "Java Source File Example" for examples of thecomment formats described here.

For further details, see "How toWrite Doc Comments for Javadoc" which includes information on the doc commenttags (@return, @param, @see):

http://java.sun.com/products/jdk/javadoc/writingdoccomments.html

For further details about doc commentsand javadoc, see the javadoc home page at:

http://java.sun.com/products/jdk/javadoc/

Doc comments describe Java classes,interfaces, constructors, methods, and fields. Each doc comment is set insidethe comment delimiters /**...*/, with one comment per class,interface, or member. This comment should appear just before the declaration:

/**
 * The Example class provides ...
 */
public class Example { ...

Notice that top-level classes andinterfaces are not indented, while their members are. The first line of doccomment (/**) for classes and interfaces is not indented; subsequent doccomment lines each have 1 space of indentation (to vertically align the asterisks).Members, including constructors, have 4 spaces for the first doc comment lineand 5 spaces thereafter.

If you need to give information about aclass, interface, variable, or method that isn't appropriate for documentation,use an implementation block comment (see section 5.1.1) orsingle-line (see section 5.1.2)comment immediately afterthe declaration. For example, details about the implementation of a classshould go in in such an implementation block comment following the classstatement, not in the class doc comment.

Doc comments should not be positioned insidea method or constructor definition block, because Java associates documentationcomments with the firstdeclaration afterthe comment.

6 - Declarations

6.1 Number Per Line

One declaration per line is recommendedsince 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 sameline. Example:

                     int foo,  fooarray[]; //WRONG!

Note: Theexamples above use one space between the type and the identifier. Anotheracceptable alternative is to use tabs, e.g.:

int     level;          // indentation level
int     size;            // size of table
Object  currentEntry;   // currently selected table entry

6.2 Initialization

Try to initialize local variables wherethey're declared. The only reason not to initialize a variable where it'sdeclared is if the initial value depends on some computation occurring first.

6.3 Placement

Put declarations only at the beginning ofblocks. (A block is any code surrounded by curly braces "{" and"}".) Don't wait to declare variables until their first use; it canconfuse 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 offor loops, which in Java can be declared in the forstatement:

for (int i = 0; i < maxLoops; i++) { ... }

Avoid local declarations that hide declarations at higher levels. For example, do not declare the same variablename in an inner block:

int count;
...
myMethod() {
    if (condition) {
        int count = 0;     // AVOID!
        ...
    }
    ...
}

6.4 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() {}
 
    ...
}

• Methods are separated by a blank line

7 - Statements

7.1 Simple Statements

Each line should contain at most onestatement. Example:

argv++;       // Correct
argc--;       // Correct  
argv++; argc--;       // AVOID!

7.2 Compound Statements

Compound statements are statements thatcontain 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.

7.3 return Statements

A return statement with avalue should not use parentheses unless they make the return value more obviousin some way. Example:

return;
 
return myDisk.size();
 
return (size ? size : defaultSize);

7.4 if, if-else, if else-if elseStatements

The if-else class ofstatements should have the following form:

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

Note: ifstatements always use braces {}. Avoid the following error-prone form:

if (condition) //AVOID! THIS OMITS THE BRACES {}!
    statement;

7.5 for Statements

A for statement should havethe following form:

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

An empty for statement (one inwhich all the work is done in the initialization, condition, and updateclauses) should have the following form:

for (initialization; condition; update);

When using the comma operator in theinitialization or update clause of a for statement, avoid thecomplexity of using more than three variables. If needed, use separatestatements before the for loop (for the initialization clause) orat the end of the loop (for the update clause).

7.6 while Statements

A while statement should havethe following form:

while (condition) {
    statements;
}

An empty while statementshould have the following form:

while (condition);

7.7 do-while Statements

A do-while statement shouldhave the following form:

do {
    statements;
} while (condition);

7.8 switch Statements

A switch statement should havethe 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'tinclude a break statement), add a comment where the breakstatement would normally be. This is shown in the preceding code example withthe /* falls through */ comment.

Every switch statement shouldinclude a default case. The break in the default case isredundant, but it prevents a fall-through error if later another caseis added.

7.9 try-catch Statements

A try-catch statement shouldhave the following format:

try {
    statements;
} catch (ExceptionClass e) {
    statements;
}

A try-catch statement mayalso be followed by finally, which executes regardless of whetheror not the try block has completed successfully.

try {
    statements;
} catch (ExceptionClass e) {
    statements;
} finally {
    statements;
}

8 - White Space

8.1 Blank Lines

Blank lines improve readability by settingoff sections of code that are logically related.

Two blank lines should always be used inthe following circumstances:

• Between sections of a source file
• Between class and interface definitions

One blank line should always be used in thefollowing circumstances:

• Between methods
• Between the local variables in a method and its first statement
• Before a block (see section 5.1.1) or single-line (see section 5.1.2) comment
• Between logical sections inside a method to improve readability

8.2 Blank Spaces

Blank spaces should be used in thefollowing circumstances:

• A keyword followed by a parenthesis should be separated by a space. Example:

       while (true) {
           ...
       }

Note that a blank space shouldnot be used between a method name and its opening parenthesis. This helps todistinguish 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);

9 - Naming Conventions

Naming conventions make programs moreunderstandable by making them easier to read. They can also give informationabout the function of the identifier-for example, whether it's a constant,package, or class-which can be helpful in understanding the code.

Identifier Type	Rules for Naming	Examples
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.	com.sun.eng com.apple.quicktime.v2 edu.cmu.cs.bovik.cheese
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).	class Raster; class ImageSprite;
Interfaces	Interface names should be capitalized like class names.	interface RasterDelegate; interface Storing;
Methods	Methods should be verbs, in mixed case with the first letter lowercase, with the first letter of each internal word capitalized.	run(); runFast(); getBackground();
Variables	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 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;
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;

10 - ProgrammingPractices

10.1 Providing Access to Instance andClass Variables

Don't make any instance or class variablepublic without good reason. Often, instance variables don't need to beexplicitly set or gotten-often that happens as a side effect of method calls.

One example of appropriate public instancevariables is the case where the class is essentially a data structure, with nobehavior. In other words, if you would have used a struct insteadof a class (if Java supported struct), then it's appropriate tomake the class's instance variables public.

10.2 Referring to Class Variables andMethods

Avoid using an object to access a class(static) variable or method. Use a class name instead. For example:

classMethod();             //OK
AClass.classMethod();      //OK
anObject.classMethod();    //AVOID!

10.3 Constants

Numerical constants (literals) should notbe coded directly, except for -1, 0, and 1, which can appear in a forloop as counter values.

10.4 Variable Assignments

Avoid assigning several variables to thesame value in a single statement. It is hard to read. Example:

fooBar.fChar = barFoo.lchar = 'c'; // AVOID!

Do not use the assignment operator in aplace where it can be easily confused with the equality operator. Example:

if (c++ = d++) {        // AVOID! (Java disallows)
    ...
}

should be written as

if ((c++ = d++) != 0) {
    ...
}

Do not use embedded assignments in anattempt to improve run-time performance. This is the job of the compiler.Example:

d = (a = b + c) + r;        // AVOID!

should be written as

a = b + c;
d = a + r;

10.5 Miscellaneous Practices

10.5.1 Parentheses

It is generally a good idea to useparentheses liberally in expressions involving mixed operators to avoidoperator precedence problems. Even if the operator precedence seems clear toyou, it might not be to others-you shouldn't assume that other programmers knowprecedence as well as you do.

if (a == b && c == d)     // AVOID!
if ((a == b) && (c == d)) // RIGHT

10.5.2 Returning Values

Try to make the structure of your programmatch the intent. Example:

if (booleanExpression) {
    return true;
} else {
    return false;
}

should instead be written as

return booleanExpression;

Similarly,

if (condition) {
    return x;
}
return y;

should be written as

return (condition ? x : y);

10.5.3 Expressions before `?' in theConditional Operator

If an expression containing a binaryoperator appears before the ? in the ternary ?: operator,it should be parenthesized. Example:

(x >= 0) ? x : -x;

10.5.4 Special Comments

Use XXX in a comment to flagsomething that is bogus but works. Use FIXME to flag somethingthat is bogus and broken.

11 - Code Examples

11.1 JavaSource File Example

The following example shows how to format aJava source file containing a single public class. Interfaces are formattedsimilarly. For more information, see "Class andInterface Declarations" and "DocumentationComments"

/*
 * @(#)Blah.java        1.82 99/03/18
 *
 * Copyright (c) 1994-1999 Sun Microsystems, Inc.
 * 901 San Antonio Road, Palo Alto, California, 94303, U.S.A.
 * All rights reserved.
 *
 * This software is the confidential and proprietary information of Sun
 * Microsystems, Inc. ("Confidential Information").  You shall not
 * disclose such Confidential Information and shall use it only in
 * accordance with the terms of the license agreement you entered into
 * with Sun.
 */
 
 
package java.blah;
 
import java.blah.blahdy.BlahBlah;
 
/**
 * Class description goes here.
 *
 * @version    1.82 18 Mar 1999
 * @author     Firstname Lastname
 */
public class Blah extends SomeClass {
    /* A class implementation comment can go here. */
 
    /** classVar1 documentation comment */
    public static int classVar1;
 
    /** 
     * classVar2 documentation comment that happens to be
     * more than one line long
     */
    private static Object classVar2;
 
    /** instanceVar1 documentation comment */
    public Object instanceVar1;
 
    /** instanceVar2 documentation comment */
    protected int instanceVar2;
 
    /** instanceVar3 documentation comment */
    private Object[] instanceVar3;
 
    /** 
     * ...constructor Blah documentation comment...
     */
    public Blah() {
        // ...implementation goes here...
    }
 
    /**
     * ...method doSomething documentation comment...
     */
    public void doSomething() {
        // ...implementation goes here... 
    }
 
    /**
     * ...method doSomethingElse documentation comment...
     * @param someParam description
     */
    public void doSomethingElse(Object someParam) {
        // ...implementation goes here... 
    }
}