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Java Code Conventions

Please adhere as far as possible to Sun's Code Conventions. While these conventions are no longer updated, they are a good basis for writing comprehensible code.

The code conventions will be enforced using checkstyle.

A checkstyle file for the important rules below is available in the repository under key/scripts/tools/checkstyle/key_checks.xml.

The rule identifiers in parentheses refer to rules in this checkstyle file.

Here are adapted excerpts of the most important guidelines:

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.


Use JavaDoc to annotate at least: * every class, * every field (even if private), * and every public method that is not a setter or getter.

When you create a new class, add yourself as the author of the class (using @author). If you make a noteworthy contribution to a class, add yourself as a further author.

(JavadocMethod), (JavadocType), (JavadocVariable)


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).

(FileTabCharacter), (Indentation)


Avoid lines longer than 100 characters.

(The conventions require that all lines are no longer than 80 characters. For KeY, we have relaxed this rule.)

Avoid trailing spaces in lines.


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.

(LeftCurly), (RightCurly), (NeedBraces)

if, if-else, if else-if else Statements

The if-else class of statements should have the following form:

 if (condition) {

 if (condition) {
 } else {

 if (condition) {
 } else if (condition) {
 } else {

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

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

try-catch Statements

A try-catch statement should have the following format:

 try {
 } catch (ExceptionClass e) {

Naming Conventions

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


Package names are always written in all-lowercase ASCII letters [...].

Examples: * com.sun.eng * * edu.cmu.cs.bovik.cheese


Classes / Interfaces / Enums

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).

Examples * class Raster * class ImageSprite



Methods should be verbs, in mixed case with the first letter lowercase, with the first letter of each internal word capitalized.

Examples: * run(); * runFast(); * getBackground();



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.

Examples: * int i; * char c; * float myWidth;

(LocalFinalVariableName), (LocalVariableName), (ParameterName)


The names of variables declared class constants (static final) should be all uppercase with words separated by underscores ("_").

Examples * static final int MIN_WIDTH = 4; * static final int MAX_WIDTH = 999; * static final int GET_THE_CPU = 1;


No prefixes

Be aware that names are not to be prefixed by the type of entity they are.

  • Do not name a private boolean member field m_updateFlag, but just updateFlag.
  • Do not name an interface IJob and the implementation Job. Rather name them Job and JobImpl if need be.
  • Exception: If you work in KeY projects related to eclipse Eclipse, interface names may begin with capital I as this is common practice for Eclipse projects.


  • One declaration per line is recommended since it encourages commenting.
  • Put declarations only at the beginning of blocks. Don't wait to declare variables until their first use. (The one exception to the rule is indexes of for loops.)
  • In a class declaration keep the following order of declarations:
  • First the public static fields, then the protected and then the private.
  • Then the public instance (non-static) fields, then the protected and then the private.
  • Constructors
  • Methods

(DeclarationOrder), (MultipleVariableDeclarations)


  • Do not use a++ or ++a inside expressions, only as individual statements.
  • Avoid long methods with more than 60 lines of code.

(MethodLength), (NoIncrementInExpr)