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2 - File Names
Java Code Conventions
1 - Introduction
1.1 Why Have Code Conventions
Code conventions are important to programmers 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 the conventions to work, every person writing software must conform to the code
conventions. Everyone.
1.2 Acknowledgments
This document reflects the Java language coding standards presented in the Java Language
Specification, from Sun Microsystems, Inc. Major contributions are from Peter King, Patrick
Naughton, Mike DeMoney, Jonni Kanerva, Kathy Walrath, and Scott Hommel.
This document is maintained by Scott Hommel. Comments should be sent to
2 - File Names
This section lists commonly used file suffixes and names.
2.1 File Suffixes
Java Software uses the following file suffixes:
File Type Suffix
Java source .java
Java bytecode .class
3 - File Organization
2.2 Common File Names
Frequently used file names include:
3 - File Organization
A file consists of sections that should be separated by blank lines and an optional comment
identifying each section.
Files longer than 2000 lines are cumbersome and should be avoided.
For an example of a Java program properly formatted, see “Java Source File Example” on page
3.1 Java Source Files
Each Java source file contains a single public class or interface. When private classes and
interfaces are associated with a public class, you can put them in the same source file as the
public class. The public class should be the first class or interface in the file.
Java source files have the following ordering:
- Beginning comments (see “Beginning Comments” on page 2)
- Package and Import statements
- Class and interface declarations (see “Class and Interface Declarations” on page 3)
3.1.1 Beginning Comments
All source files should begin with a c-style comment that lists the class name, version
information, date, and copyright notice:
- Classname
- Version information
- Date
- Copyright notice */
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.
4 - Indentation
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).
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—generally no
more than 70 characters.
4.2 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)
longName1 = longName2 * (longName3 + longName
- 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.
5 - Comments
//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 (
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;
5 - Comments
Java programs can have two kinds of comments: implementation comments and
documentation comments. Implementation comments are those found in C++, which are
delimited by /.../, and //. Documentation comments (known as “doc comments”) are
Java-only, and are delimited by /*.../. Doc comments can be extracted to HTML files
using the javadoc tool.
5 - Comments
Note: If you don’t use indent (1), you don’t have to use /*- in your code or make any other
concessions to the possibility that someone else might run indent (1) on your code.
See also “Documentation Comments” on page 8.
5.1.2 Single-Line Comments
Short comments can appear on a single line indented to the level of the code that follows. If a
comment can’t be written in a single line, it should follow the block comment format (see
section 5.1.1). A single-line comment should be preceded by a blank line. Here’s an example
of a single-line comment in Java code:
if (condition) {
/* Handle the condition. */ ... }
5.1.3 Trailing Comments
Very short comments can appear on the same line as the code they describe, but should be
shifted far enough to separate them from the statements. If more than one short comment
appears in a chunk of code, 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 in
consecutive 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. }
5 - Comments
//if (bar > 1) { // // // Do a triple-flip. // ... //} //else{ // return false; //}
5.2 Documentation Comments
Note: See “Java Source File Example” on page 18 for examples of the comment formats
described here.
For further details, see “How to Write Doc Comments for Javadoc” which includes
information on the doc comment tags (@return, @param, @see):
http://java.sun.com/products/jdk/javadoc/writingdoccomments.html
For further details about doc comments and 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 inside the 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 and interfaces are not indented, while their members are. The first
line of doc comment (/**) for classes and interfaces is not indented; subsequent doc comment
lines each have 1 space of indentation (to vertically align the asterisks). Members, including
constructors, have 4 spaces for the first doc comment line and 5 spaces thereafter.
If you need to give information about a class, interface, variable, or method that isn’t
appropriate for documentation, use an implementation block comment (see section 5.1.1) or
single-line (see section 5.1.2) comment immediately after the declaration. For example, details
about the implementation of a class should go in in such an implementation block comment
following the class statement, not in the class doc comment.
Doc comments should not be positioned inside a method or constructor definition block,
because Java associates documentation comments with the first declaration after the comment.
7 - Statements
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; // 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 one statement. Example:
argv++; // Correct argc++; // Correct argv++; argc--; // AVOID!
7 - Statements
7.2 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.
7.3 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);
7.4 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;
8 - White Space
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.
7.9 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; }
8 - White Space
8.1 Blank Lines
Blank lines improve readability by setting off sections of code that are logically related.
8 - White Space
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 (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 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++; } prints("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))
10 - Programming Practices
10 - Programming Practices
10.1 Providing Access to Instance and Class Variables
Don’t make any instance or class variable public without good reason. Often, instance
variables don’t need to be explicitly set or gotten—often that happens as a side effect of
method calls.
One example of appropriate public instance variables is the case where the class is essentially a
data structure, with no behavior. In other words, if you would have used a struct instead of a
class (if Java supported struct), then it’s appropriate to make the class’s instance variables
public.
10.2 Referring to Class Variables and Methods
Avoid using an object to access a class (static) variable or method. Use a class name instead.
For example:
classMethod(); //OK AClass.classMethod(); //OK
Variables Except for variables, all instance, class, and
class constants are in mixed case with a lower-
case first letter. Internal words start with capi-
tal letters. Variable names should not start with
underscore _ or dollar sign $ characters, even
though both are allowed.
Variable names should be short yet meaning-
ful. The choice of a variable name should be
mnemonic— that is, designed to indicate to the
casual observer the intent of its use. One-char-
acter variable names should be avoided except
for temporary “throwaway” variables. Com-
mon 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 con-
stants and of ANSI constants should be all
uppercase with words separated by under-
scores (“_”). (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;
Identifier Type Rules for Naming Examples
10 - Programming Practices
anObject.classMethod(); //AVOID!
10.3 Constants
Numerical constants (literals) should not be coded directly, except for -1, 0, and 1, which can
appear in a for loop as counter values.
10.4 Variable Assignments
Avoid assigning several variables to the same value in a single statement. It is hard to read.
Example:
fooBar.fChar = barFoo.lchar = 'c'; // AVOID!
Do not use the assignment operator in a place 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 an attempt 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 use parentheses liberally in expressions involving mixed operators
to avoid operator precedence problems. Even if the operator precedence seems clear to you, it
might not be to others—you shouldn’t assume that other programmers know precedence as
well as you do.
if (a == b && c == d) // AVOID!
if ((a == b) && (c == d)) // USE
11 - Code Examples
- @(#)Blah.java 1.82 99/03/
- 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... }
11 - Code Examples
- ...method doSomethingElsedocumentation comment...
- @param someParamdescription */ public void doSomethingElse(Object someParam) { // ...implementation goes here... } }
