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.

Acknowledgments

This document reflects the Java language coding standards presented in the Java Language Specification, from Sun Microsystems. Major contributions are from Peter King, Patrick Naughton, Mike DeMoney, Jonni Kanerva, Kathy Walrath, and Scott Hommel.

For questions concerning adaptation, modification, or redistribution of this document, please read our copyright notice at http://java.sun.com/docs/codeconv/html/Copyright.doc.html .

Comments on this document should be submitted to our feedback form at http://java.sun.com/docs/forms/sendusmail.html .

File Suffixes

We uses the following file suffixes:

Common File Names

Frequently used file names include:

C++ 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 4)
• Package and Import statements; for example:

  import java.applet.Applet;
  import java.awt.*;
  import java.net.*;

• Class and interface declarations (see “Class and Interface Declarations”)

/*
 * Classname
 *
 * Version info
 *
 * Copyright notice
 */

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

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.

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:

     function(longExpression1, longExpression2, longExpression3,
              longExpression4, longExpression5);
  
     var = function1(longExpression1,
                     function2(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 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.

  //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 (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 acceptable ways to format ternary expressions:

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

Implementation Comment Formats

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

/*
 * Here is a block comment.
 */

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

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

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

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

Documentation Comments

Note : See “Java Source File Example” on page 19 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/j2se/javadoc/writingdoccomments/index.html

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

http://java.sun.com/j2se/javadoc/

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

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

Notice that 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.

Number Per Line

One declaration per line is recommended since it encourages commenting. In other words,

int level; // indentation level
int size; // size of table

is preferred over

int level, size;

In absolutely no case should variables and functions be declared on the same line. Example:

long dbaddr, getDbaddr(); // WRONG!

Do not put different types on the same line. Example:

int foo, fooarray[]; //WRONG!

Note : The examples above use one space between the type and the identifier. Another acceptable alternative is to use tabs, e.g.:

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

Placement

Put declarations only at the beginning of blocks. (A block is any code surrounded by curly braces “” and “ ”.) Don’t wait to declare variables until their first use; it can confuse the unwary programmer and hamper code portability within the scope.

void MyMethod() {
    int int1; // beginning of method block
    
    if (condition) {
        int int2; // beginning of "if" block
        ...
    }
}

The one exception to the rule is indexes of for loops, which in Java can be declared in the for statement:

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

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

int count;
...
func() {
    if (condition) {
        int count; // AVOID!
        ...
    }
    ...
}

Initialization

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

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

Simple Statements

Each line should contain at most one statement. Example:

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

Do not use the comma operator to group multiple statements unless it is for an obvious reason. Example:

if (err) {
    Format.print(System.out, “error”), exit(1); //VERY WRONG!
}

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 singletons, 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.

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);

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 if (condition) {
    statements;
}

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

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

for Statements

A for statement should have the following form:

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

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

for (initialization; condition; update);

When using the comma operator in the initialization or update clause of a for statement, avoid the complexity of using more than three variables. If needed, use separate statements before the for loop (for the initialization clause) or at the end of the loop (for the update clause).

while Statements

A while statement should have the following form:

while (condition) {
    statements;
}

An empty while statement should have the following form:

while (condition);

do-while Statements

A do-while statement should have the following form:

do {
    statements;
} while (condition);

switch Statements

A switch statement should have the 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’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.

try-catch Statements

A try-catch statement should have the following format:

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

Blank Lines

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

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. Examples:

  myMethod((byte) aNum, (Object) x);
  myFunc((int) (cp + 5), ((int) (i + 3))
                               + 1);

Java Source File Example

The following example shows how to format a Java source file containing a single public class. Interfaces are formatted similarly. For more information, see “Class and Interface Declarations” on page 4 and “Documentation Comments” on page 9

/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */
package java.blah;
import java.blah.blahdy.BlahBlah;
/**
 * Class description goes here.
 *
 * @version 1.10 04 Oct 1996
 * @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;
    
    /**
     * ...method 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...
    }
}