This coding style guide is a simplified version of one that has been used with good success both in industrial practice and for college courses.
A style guide is a set of mandatory requirements for layout and formatting. Uniform style makes it easier for you to read code from your instructor and classmates. You will really appreciate that if you do a team project. It is also easier for your instructor and your grader to grasp the essence of your programs quickly.
A style guide makes you a more productive programmer because it reduces gratuitous choice. If you don't have to make choices about trivial matters, you can spend your energy on the solution of real problems.
In these guidelines, several constructs are plainly outlawed. That doesn't mean that programmers using them are evil or incompetent. It does mean that the constructs are not essential and can be expressed just as well or even better with other language constructs.
If you already have programming experience, in Java or another language, you may be initially uncomfortable at giving up some fond habits. However, it is a sign of professionalism to set aside personal preferences in minor matters and to compromise for the benefit of your group.
These guidelines are necessarily somewhat dull. They also mention features that you may not yet have seen in class. Here are the most important highlights:
main
and overridden library methods,
must have a comment. continue
or break
is allowed.final
variables must be private. Note to the instructor: Of course, many programmers and organizations have strong feelings about coding style. If this style guide is incompatible with your own preferences or with local custom, please feel free to modify it. For that purpose, this coding style guide is available in electronic form from the author.
Each Java program is a collection of one or more source files. The executable program is obtained by compiling these files. Organize the material in each file as follows:
package
statement, if appropriate import
statements public
class The comment explaining the purpose of this file should be in the format
recognized by the javadoc
utility. Start with a /**
,
and use the @author
and @version
tags:
/** COPYRIGHT (C) 1997 Harry Hacker. All Rights Reserved. Classes to manipulate widgets. Solves CS101 homework assignment #3 @author Harry Hacker @version 1.01 1997/2/15 */ |
Each class should be preceded by a class comment explaining the purpose of the class.
First list all public features, then all private features.
Within the public and private section, use the following order:
Leave a blank line after every method.
All non-final
variables must be private. (However, instance
variables of a private
inner class may be public.) Methods and
final variables can be either public or private, as appropriate.
All features must be tagged public
or private
. Do
not use the default visibility (that is, package visibility) or the
protected
attribute.
Avoid static variables (except final
ones) whenever possible. In
the rare instance that you need static variables, you are permitted one static
variable per class.
Every method (except for main
) starts with a comment in
javadoc
format.
/** Convert calendar date into Julian day. Note: This algorithm is from Press et al., Numerical Recipes in C, 2nd ed., Cambridge University Press, 1992 @param day day of the date to be converted @param month month of the date to be converted @param year year of the date to be converted @return the Julian day number that begins at noon of the given calendar date. */ |
public static int dat2jul(int day, int month, int year) { . . . } |
Parameter names must be explicit, especially if they are integers or Boolean:
public Employee remove(int d, double s) // Huh? public Employee remove(int department, double severancePay) // OK |
Methods must have at most 30 lines of code. The method signature, comments, blank lines, and lines containing only braces are not included in this count. This rule forces you to break up complex computations into separate methods.
Do not define all variables at the beginning of a block:
{ double xold; // Don't double xnew; boolean more; . . . } |
Define each variable just before it is used for the first time:
{ . . . double xold = Integer.parseInt(input); boolean more = false; while (more) { double xnew = (xold + a / xold) / 2; . . . } . . . } |
Do not define two variables on the same line:
int dimes = 0, nickels = 0; // Don't |
Instead, use two separate definitions:
int dimes = 0; // OK int nickels = 0; |
In Java, constants must be defined with the keyword final
. If
the constant is used by multiple methods, declare it as static
final
. It is a good idea to define static final variables as
private
if no other class has an interest in them.
Do not use magic numbers! A magic number is a numeric constant embedded in code, without a constant definition. Any number except -1, 0, 1, and 2 is considered magic:
if (p.getX() < 300) // Don't |
Use final
variables instead:
final double WINDOW_WIDTH = 300; . . . if (p.getX() < WINDOW_WIDTH) // OK |
Even the most reasonable cosmic constant is going to change one day. You think there are 365 days per year? Your customers on Mars are going to be pretty unhappy about your silly prejudice. Make a constant
public static final int DAYS_PER_YEAR = 365; |
so that you can easily produce a Martian version without trying to find all the 365s, 364s, 366s, 367s, and so on, in your code.
When declaring array variables, group the [] with the type, not the variable.
int[] values; // OK int values[]; // Ugh--this is an ugly holdover from C |
if
StatementAvoid the "if
... if
... else
" trap.
The code
if ( ... ) if ( ... ) ...; else ...; |
will not do what the indentation level suggests, and it can take hours to
find such a bug. Always use an extra pair of {
... }
when dealing with "if
... if
... else
":
if ( ... ) { if ( ... ) ...; } // {...} are necessary else ...; if ( ... ) { if ( ... ) ...; else ...; } // {...} not necessary, but they keep you out of trouble |
for
StatementUse for
loops only when a variable runs from somewhere to
somewhere with some constant increment/decrement:
for (int i = 0; i < a.length; i++) System.out.println(a[i]); |
Do not use the for
loop for weird constructs such as
for (a = a / 2; count < ITERATIONS; System.out.println(xnew)) // Don't |
Make such a loop into a while
loop. That way, the sequence of
instructions is much clearer.
a = a / 2; while (count < ITERATIONS) // OK { . . . System.out.println(xnew); } |
Avoid the switch
statement, because it is easy to fall through
accidentally to an unwanted case. Use if
/else
instead.
Avoid the break
or continue
statements. Use another
boolean
variable to control the execution flow.
Do not tag a method with an overly general exception specification:
Widget readWidget(Reader in) throws Exception // Bad |
Instead, specifically declare any checked exceptions that your method may throw:
Widget readWidget(Reader in) throws IOException, MalformedWidgetException // Good |
Do not "squelch" exceptions:
try { double price = in.readDouble(); } catch (Exception e) {} // Bad |
Beginners often make this mistake "to keep the compiler happy". If the
current method is not appropriate for handling the exception, simply use a
throws
specification and let one of its callers handle it.
The following rules specify when to use upper- and lowercase letters in identifier names.
firstPlayer
. CLOCK_RADIUS
.BankTeller
. Names must be reasonably long and descriptive. Use firstPlayer
instead of fp
. No drppng f vwls. Local variables that are fairly
routine can be short (ch
, i
) as long as they are
really just boring holders for an input character, a loop counter, and so on.
Also, do not use ctr
, c
, cntr
,
cnt
, c2
for variables in your method. Surely these
variables all have specific purposes and can be named to remind the reader of
them (for example, current
, next
,
previous
, result
, . . . ).
Use tab stops every three columns. That means you will need to change the tab stop setting in your editor!
Use blank lines freely to separate parts of a method that are logically distinct.
Use a blank space around every binary operator:
x1 = (-b - Math.sqrt(b * b - 4 * a * c)) / (2 * a); // Good x1=(-b-Math.sqrt(b*b-4*a*c))/(2*a);//Bad |
Leave a blank space after (and not before) each comma or semicolon. Do not leave a space before or after a parenthesis or bracket in an expression. Leave spaces around the ( . . . ) part of an if, while, for, or catch statement.
if (x == 0) y = 0; f(a, b[i]); |
Every line must fit on 80 columns. If you must break a statement, add an indentation level for the continuation:
a[n] = .................................................. + .................; |
Start the indented line with an operator (if possible).
If the condition in an if
or while
statement must be broken, be sure to brace the body in, even if it consists
of only one statement:
if ( ......................................................... && .................. || .......... ) { . . . } |
If it weren't for the braces, it would be hard to separate the continuation of the condition visually from the statement to be executed.
Opening and closing braces must line up, either horizontally or vertically:
while (i < n) { System.out.println(a[i]); i++; } while (i < n) { System.out.println(a[i]); i++; } |
Some programmers don't line up vertical braces but place the {
behind the key word:
while (i < n) { // DON'T System.out.println(a[i]); i++; } |
Doing so makes it hard to check that the braces match.
Some programmers take great pride in lining up certain columns in their code:
firstRecord = other.firstRecord; lastRecord = other.lastRecord; cutoff = other.cutoff; |
This is undeniably neat, but the layout is not stable under change. A new variable name that is longer than the preallotted number of columns requires that you move all entries around:
firstRecord = other.firstRecord; lastRecord = other.lastRecord; cutoff = other.cutoff; marginalFudgeFactor = other.marginalFudgeFactor; |
This is just the kind of trap that makes you decide to use a short variable
name like mff
instead.
Do not use //
comments for comments that extend for more than
two lines. You don't want to have to move the //
around when you
edit the comment.
// comment ù don't do this // more comment // more comment |
Use /*
... */
comments instead. When using
/*
... */
comments, don't "beautify" them with
additional asterisks:
/* commentùdon't do this * more comment * more comment */ |
It looks neat, but it is a major disincentive to update the comment. Some people have text editors that lay out comments. But even if you do, you don't know whether the next person who maintains your code has such an editor.
Instead, format long comments like this:
/* comment more comment more comment */ |
or this:
/* comment more comment more comment */ |
These comments are easier to maintain as your program changes. If you have to choose between pretty but unmaintained comments and ugly comments that are up to date, truth wins over beauty.