Java Language Coding Guidelines


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. It was presented for use on Cay Hortsmann's website and has been slightly edited by your teacher.

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:


Source Files

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:

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 2005-02-15


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:

  1. Static Fields
  2. Instance Fields
  3. Constructors
  4. Static Methods
  5. Instance Methods
  6. Inner classes

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)
. . .

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.

Variables and Constants

Do not define all variables at the beginning of a block: (This does not include instance variables.)

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

When using collections, use type parameters and not  raw  types.

ArrayList<String> names = new ArrayList<String>(); // OK 
ArrayList names = new ArrayList(); // Not OK

A1.6  Control Flow

A1.6.1—The if Statement

Avoid 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

A1.6.2—The for Statement

Use for loops only when a variable runs from somewhere to somewhere with some constant increment/decrement:

for (int i = 0; i < a.length; i++)

Or, even better, use the "for each" loop:

for (int e : a) 

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
{ . . .

A1.6.3—Nonlinear Control Flow

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:

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.

A1.7  Lexical Issues

A1.7.1—Naming Convention

The following rules specify when to use upper- and lowercase letters in identifier names.

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, . . . ). However, it is customary to use single-letter names, such as T or E for generic types.

A1.7.2—Indentation and White Space

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


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)

Some programmers don't line up vertical braces but place the { behind the key word. This is acceptable, but not preferred.

while (i < n) { // NOT PREFERRED.

Doing so makes it hard to check that the braces match.

A1.7.4—Unstable Layout

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. Use a simple layout that is easy to maintain as your programs change.