1 Introduction 

This document serves as the complete definition of Google's coding standards for source code in the Java™ Programming Language. A Java source file is described as being in Google Style if and only if it adheres to the rules herein.

Like other programming style guides, the issues covered span not only aesthetic issues of formatting, but other types of conventions or coding standards as well. However, this document focuses primarily on the hard-and-fast rules that we follow universally, and avoids giving advice that isn't clearly enforceable (whether by human or tool).

   

1.1 Terminology notes 

In this document, unless otherwise clarified:

1. The term class is used inclusively to mean an "ordinary" class, enum class,  interface or annotation type ([@interface](http://my.oschina.net/u/996807) ).

2. The term comment always refers to implementation comments. We do not  use the phrase "documentation comments", instead using the common term "Javadoc."

Other "terminology notes" will appear occasionally throughout the document.

   

1.2 Guide notes 

Example code in this document is non-normative. That is, while the examples are in Google Style, they may not illustrate the only stylish way to represent the code. Optional formatting choices made in examples should not be enforced as rules.

   

2 Source file basics 

   

2.1 File name 

The source file name consists of the case-sensitive name of the top-level class it contains, plus the .java extension.

   

2.2 File encoding: UTF-8 

Source files are encoded in UTF-8.

   

2.3 Special characters 

   

2.3.1 Whitespace characters 

Aside from the line terminator sequence, the ASCII horizontal space character (0x20) is the only whitespace character that appears anywhere in a source file. This implies that:

1. All other whitespace characters in string and character literals are escaped.

2. Tab characters are not used for indentation.

   

2.3.2 Special escape sequences 

For any character that has a special escape sequence (\b,\t,\n,\f,\r,\",\' and\\), that sequence is used rather than the corresponding octal (e.g. \012) or Unicode (e.g. \u000a) escape.

   

2.3.3 Non-ASCII characters 

For the remaining non-ASCII characters, either the actual Unicode character (e.g. ∞) or the equivalent Unicode escape (e.g. \u221e) is used, depending only on which makes the code easier to read and understand.

Tip: In the Unicode escape case, and occasionally even when actual Unicode characters are used, an explanatory comment can be very helpful.

Examples:

Example

Discussion

String unitAbbrev = "μs";

Best: perfectly clear even without a comment.

String unitAbbrev = "\u03bcs"; // "μs"

Allowed, but there's no reason to do this.

String unitAbbrev = "\u03bcs"; // Greek letter mu, "s"

Allowed, but awkward and prone to mistakes.

String unitAbbrev = "\u03bcs";

Poor: the reader has no idea what this is.

return '\ufeff' + content; // byte order mark

Good: use escapes for non-printable characters, and comment if necessary.

Tip: Never make your code less readable simply out of fear that some programs might not handle non-ASCII characters properly. If that should happen, those programs are broken and they must be fixed.

   

3 Source file structure 

A source file consists of, in order:

1. License or copyright information, if present

2. Package statement

3. Import statements

4. Exactly one top-level class

Exactly one blank line separates each section that is present.

   

3.1 License or copyright information, if present 

If license or copyright information belongs in a file, it belongs here.

   

3.2 Package statement 

The package statement is not line-wrapped. The column limit (Section 4.4,Column limit: 80 or 100) does not apply to package statements.

   

3.3 Import statements 

   

3.3.1 No wildcard imports 

Wildcard imports, static or otherwise, are not used.

   

3.3.2 No line-wrapping 

Import statements are not line-wrapped. The column limit (Section 4.4,Column limit: 80 or 100) does not apply to import statements.

   

3.3.3 Ordering and spacing 

Import statements are divided into the following groups, in this order, with each group separated by a single blank line:

1. All static imports in a single group

2. com.google imports  (only if this source file is in the com.google package  space)

3. Third-party imports, one group per top-level package, in ASCII sort order

• for example: android, com, junit, org,    sun

5. java imports

6. javax imports

Within a group there are no blank lines, and the imported names appear in ASCII sort order. (Note: this is not the same as the import statements being in ASCII sort order; the presence of semicolons warps the result.)

   

3.4 Class declaration 

   

3.4.1 Exactly one top-level class declaration 

Each top-level class resides in a source file of its own.

   

3.4.2 Class member ordering 

The ordering of the members of a class can have a great effect on learnability, but there is no single correct recipe for how to do it. Different classes may order their members differently.

What is important is that each class order its members in some logical order, which its maintainer could explain if asked. For example, new methods are not just habitually added to the end of the class, as that would yield "chronological by date added" ordering, which is not a logical ordering.

   

3.4.2.1 Overloads: never split 

When a class has multiple constructors, or multiple methods with the same name, these appear sequentially, with no intervening members.

   

4 Formatting 

Terminology Note: block-like construct refers to the body of a class, method or constructor. Note that, by Section 4.8.3.1 onarray initializers, any array initializer_may_ optionally be treated as if it were a block-like construct.

   

4.1 Braces 

   

4.1.1 Braces are used where optional 

Braces are used withif,else,for,do andwhile statements, even when the body is empty or contains only a single statement.

   

4.1.2 Nonempty blocks: K & R style 

Braces follow the Kernighan and Ritchie style ("Egyptian brackets") for nonempty blocks and block-like constructs:

• No line break before the opening brace.

• Line break after the opening brace.

• Line break before the closing brace.

• Line break after the closing brace if that brace terminates a statement or the body  of a method, constructor or named class. For example, there is no line break  after the brace if it is followed by else or a  comma.

Example:

return new MyClass() {
  @Override public void method() {
    if (condition()) {
      try {
        something();
      } catch (ProblemException e) {
        recover();
      }
    }
  }};

A few exceptions for enum classes are given in Section 4.8.1,Enum classes.

   

4.1.3 Empty blocks: may be concise 

An empty block or block-like construct may be closed immediately after it is opened, with no characters or line break in between ({}), unless it is part of a_multi-block statement_ (one that directly contains multiple blocks:if/else-if/else ortry/catch/finally).

Example:

  void doNothing() {}

   

4.2 Block indentation: +2 spaces 

Each time a new block or block-like construct is opened, the indent increases by two spaces. When the block ends, the indent returns to the previous indent level. The indent level applies to both code and comments throughout the block. (See the example in Section 4.1.2,Nonempty blocks: K & R Style.)

   

4.3 One statement per line 

Each statement is followed by a line-break.

   

4.4 Column limit: 80 or 100 

Projects are free to choose a column limit of either 80 or 100 characters. Except as noted below, any line that would exceed this limit must be line-wrapped, as explained in Section 4.5, Line-wrapping.

Exceptions:

1. Lines where obeying the column limit is not possible (for example, a long URL in Javadoc,  or a long JSNI method reference).

2. package and  import statements (see Sections  3.2 Package statement and  3.3 Import statements).

3. Command lines in a comment that may be cut-and-pasted into a shell.

   

4.5 Line-wrapping 

Terminology Note: When code that might otherwise legally occupy a single line is divided into multiple lines, typically to avoid overflowing the column limit, this activity is called_line-wrapping_.

There is no comprehensive, deterministic formula showing exactly how to line-wrap in every situation. Very often there are several valid ways to line-wrap the same piece of code.

Tip: Extracting a method or local variable may solve the problem without the need to line-wrap.

   

4.5.1 Where to break 

The prime directive of line-wrapping is: prefer to break at ahigher syntactic level. Also:

1. When a line is broken at a non-assignment operator the break comes before  the symbol. (Note that this is not the same practice used in Google style for other languages,  such as C++ and JavaScript.)

• This also applies to the following "operator-like" symbols: the dot separator      (.), the ampersand in type bounds      (<T extends Foo & Bar>), and the pipe in      catch blocks      (catch (FooException | BarException e)).

3. When a line is broken at an assignment operator the break typically comes  after the symbol, but either way is acceptable.

• This also applies to the "assignment-operator-like" colon in an enhanced      for ("foreach") statement.

5. A method or constructor name stays attached to the open parenthesis  (() that follows it.

6. A comma (,) stays attached to the token that  precedes it.

   

4.5.2 Indent continuation lines at least +4 spaces 

When line-wrapping, each line after the first (each continuation line) is indented at least +4 from the original line.

When there are multiple continuation lines, indentation may be varied beyond +4 as desired. In general, two continuation lines use the same indentation level if and only if they begin with syntactically parallel elements.

Section 4.6.3 on Horizontal alignment addresses the discouraged practice of using a variable number of spaces to align certain tokens with previous lines.

   

4.6 Whitespace 

   

4.6.1 Vertical Whitespace 

A single blank line appears:

1. Between consecutive members (or initializers) of a class: fields, constructors,  methods, nested classes, static initializers, instance initializers.

• Exception: A blank line between two consecutive    fields (having no other code between them) is optional. Such blank lines are used as needed to    create logical groupings of fields.

3. Within method bodies, as needed to create logical groupings of statements.

4. Optionally before the first member or after the last member of the class (neither  encouraged nor discouraged).

5. As required by other sections of this document (such as Section 3.3,  Import statements).

Multiple consecutive blank lines are permitted, but never required (or encouraged).

   

4.6.2 Horizontal whitespace 

Beyond where required by the language or other style rules, and apart from literals, comments and Javadoc, a single ASCII space also appears in the following places only.

1. Separating any reserved word, such as  if,  for or  catch, from an open parenthesis  (()  that follows it on that line

2. Separating any reserved word, such as  else or  catch, from a closing curly brace  (}) that precedes it on that line

3. Before any open curly brace  ({), with two exceptions:

• @SomeAnnotation({a, b}) (no space is used)

• String[][] x = {{"foo"}}; (no space is required    between {{, by item 8 below)

5. On both sides of any binary or ternary operator. This also applies to the following  "operator-like" symbols:

• the ampersand in a conjunctive type bound:    <T extends Foo & Bar>

• the pipe for a catch block that handles multiple exceptions:    catch (FooException | BarException e)

• the colon (:) in an enhanced    for ("foreach") statement

7. After ,:; or the closing parenthesis  ()) of a cast

8. On both sides of the double slash (//) that  begins an end-of-line comment. Here, multiple spaces are allowed, but not required.

9. Between the type and variable of a declaration:  List<String> list

10. Optional just inside both braces of an array initializer

• new int[] {5, 6} and    new int[] { 5, 6 } are both valid

Note: This rule never requires or forbids additional space at the start or end of a line, only interior space.

   

4.6.3 Horizontal alignment: never required 

Terminology Note: Horizontal alignment is the practice of adding a variable number of additional spaces in your code with the goal of making certain tokens appear directly below certain other tokens on previous lines.

This practice is permitted, but is never required by Google Style. It is not even required to maintain horizontal alignment in places where it was already used.

Here is an example without alignment, then using alignment:

private int x; // this is fineprivate Color color; // this tooprivate int   x;      // permitted, but future editsprivate Color color;  // may leave it unaligned

Tip: Alignment can aid readability, but it creates problems for future maintenance.  Consider a future change that needs to touch just one line. This change may leave the formerly-pleasing formatting mangled, and that is allowed. More often it prompts the coder (perhaps you) to adjust whitespace on nearby lines as well, possibly triggering a cascading series of reformattings. That one-line change now has a "blast radius." This can at worst result in pointless busywork, but at best it still corrupts version history information, slows down reviewers and exacerbates merge conflicts.

   

4.7 Grouping parentheses: recommended 

Optional grouping parentheses are omitted only when author and reviewer agree that there is no reasonable chance the code will be misinterpreted without them, nor would they have made the code easier to read. It is not reasonable to assume that every reader has the entire Java operator precedence table memorized.

   

4.8 Specific constructs 

   

4.8.1 Enum classes 

After each comma that follows an enum constant, a line-break is optional.

An enum class with no methods and no documentation on its constants may optionally be formatted as if it were an array initializer (see Section 4.8.3.1 onarray initializers).

private enum Suit { CLUBS, HEARTS, SPADES, DIAMONDS }

Since enum classes are classes, all other rules for formatting classes apply.

   

4.8.2 Variable declarations 

   

4.8.2.1 One variable per declaration 

Every variable declaration (field or local) declares only one variable: declarations such asint a, b; are not used.

   

4.8.2.2 Declared when needed, initialized as soon as possible 

Local variables are not habitually declared at the start of their containing block or block-like construct. Instead, local variables are declared close to the point they are first used (within reason), to minimize their scope. Local variable declarations typically have initializers, or are initialized immediately after declaration.

   

4.8.3 Arrays 

   

4.8.3.1 Array initializers: can be "block-like" 

Any array initializer may optionally be formatted as if it were a "block-like construct." For example, the following are all valid (not an exhaustive list):

new int[] {           new int[] {
  0, 1, 2, 3            0,}                       1,
                        2,new int[] {             3,
  0, 1,               }
  2, 3}                     new int[]
                          {0, 1, 2, 3}

   

4.8.3.2 No C-style array declarations 

The square brackets form a part of the type, not the variable:String[] args, notString args[].

   

4.8.4 Switch statements 

Terminology Note: Inside the braces of a_switch block_ are one or more statement groups. Each statement group consists of one or more switch labels (either case FOO: ordefault:), followed by one or more statements.

   

4.8.4.1 Indentation 

As with any other block, the contents of a switch block are indented +2.

After a switch label, a newline appears, and the indentation level is increased +2, exactly as if a block were being opened. The following switch label returns to the previous indentation level, as if a block had been closed.

   

4.8.4.2 Fall-through: commented 

Within a switch block, each statement group either terminates abruptly (with abreak,continue,return or thrown exception), or is marked with a comment to indicate that execution will or might continue into the next statement group. Any comment that communicates the idea of fall-through is sufficient (typically// fall through). This special comment is not required in the last statement group of the switch block. Example:

switch (input) {
  case 1:
  case 2:
    prepareOneOrTwo();
    // fall through
  case 3:
    handleOneTwoOrThree();
    break;
  default:
    handleLargeNumber(input);}

   

4.8.4.3 The default case is present 

Each switch statement includes a default statement group, even if it contains no code.

   

4.8.5 Annotations 

Annotations applying to a class, method or constructor appear immediately after the documentation block, and each annotation is listed on a line of its own (that is, one annotation per line). These line breaks do not constitute line-wrapping (Section 4.5, Line-wrapping), so the indentation level is not increased. Example:

@Override@Nullablepublic String getNameIfPresent() { ... }

Exception: A single parameterless annotation_may_ instead appear together with the first line of the signature, for example:

@Override public int hashCode() { ... }

Annotations applying to a field also appear immediately after the documentation block, but in this case, multiple annotations (possibly parameterized) may be listed on the same line; for example:

@Partial @Mock DataLoader loader;

There are no specific rules for formatting parameter and local variable annotations.

   

4.8.6 Comments 

   

4.8.6.1 Block comment style 

Block comments are indented at the same level as the surrounding code. They may be in/* ... */ style or// ... style. For multi-line/* ... */ comments, subsequent lines must start with* aligned with the * on the previous line.

/*
 * This is          // And so           /* Or you can
 * okay.            // is this.          * even do this. */
 */

Comments are not enclosed in boxes drawn with asterisks or other characters.

Tip: When writing multi-line comments, use the/* ... */ style if you want automatic code formatters to re-wrap the lines when necessary (paragraph-style). Most formatters don't re-wrap lines in// ... style comment blocks.

   

4.8.7 Modifiers 

Class and member modifiers, when present, appear in the order recommended by the Java Language Specification:

public protected private abstract static final transient volatile synchronized native strictfp

   

4.8.8 Numeric Literals 

long-valued integer literals use an uppercase L suffix, never lowercase (to avoid confusion with the digit 1). For example, 3000000000Lrather than 3000000000l.

   

5 Naming 

   

5.1 Rules common to all identifiers 

Identifiers use only ASCII letters and digits, and in two cases noted below, underscores. Thus each valid identifier name is matched by the regular expression \w+ .

In Google Style special prefixes or suffixes, like those seen in the examples name_,mName, s_name andkName, are not used.

   

5.2 Rules by identifier type 

   

5.2.1 Package names 

Package names are all lowercase, with consecutive words simply concatenated together (no underscores). For example, com.example.deepspace, notcom.example.deepSpace orcom.example.deep_space.

   

5.2.2 Class names 

Class names are written in UpperCamelCase.

Class names are typically nouns or noun phrases. For example,Character orImmutableList. Interface names may also be nouns or noun phrases (for example, List), but may sometimes be adjectives or adjective phrases instead (for example,Readable).

There are no specific rules or even well-established conventions for naming annotation types.

Test classes are named starting with the name of the class they are testing, and ending with Test. For example,HashTest orHashIntegrationTest.

   

5.2.3 Method names 

Method names are written in lowerCamelCase.

Method names are typically verbs or verb phrases. For example,sendMessage orstop.

Underscores may appear in JUnit test method names to separate logical components of the name. One typical pattern is test_<MethodUnderTest>___<state>_, for example testPop_emptyStack. There is no One Correct Way to name test methods.

   

5.2.4 Constant names 

Constant names use CONSTANT_CASE: all uppercase letters, with words separated by underscores. But what is a constant, exactly?

Every constant is a static final field, but not all static final fields are constants. Before choosing constant case, consider whether the field really feels like a constant. For example, if any of that instance's observable state can change, it is almost certainly not a constant. Merely intending to never mutate the object is generally not enough. Examples:

// Constantsstatic final int NUMBER = 5;static final ImmutableList<String> NAMES = ImmutableList.of("Ed", "Ann");static final Joiner COMMA_JOINER = Joiner.on(',');  // because Joiner is immutablestatic final SomeMutableType[] EMPTY_ARRAY = {};enum SomeEnum { ENUM_CONSTANT }// Not constantsstatic String nonFinal = "non-final";final String nonStatic = "non-static";static final Set<String> mutableCollection = new HashSet<String>();static final ImmutableSet<SomeMutableType> mutableElements = ImmutableSet.of(mutable);static final Logger logger = Logger.getLogger(MyClass.getName());static final String[] nonEmptyArray = {"these", "can", "change"};

These names are typically nouns or noun phrases.

   

5.2.5 Non-constant field names 

Non-constant field names (static or otherwise) are written in lowerCamelCase.

These names are typically nouns or noun phrases.  For example,computedValues orindex.

   

5.2.6 Parameter names 

Parameter names are written in lowerCamelCase.

One-character parameter names should be avoided.

   

5.2.7 Local variable names 

Local variable names are written in lowerCamelCase, and can be abbreviated more liberally than other types of names.

However, one-character names should be avoided, except for temporary and looping variables.

Even when final and immutable, local variables are not considered to be constants, and should not be styled as constants.

   

5.2.8 Type variable names 

Each type variable is named in one of two styles:

• A single capital letter, optionally followed by a single numeral (such as  E, T,  X, T2)

• A name in the form used for classes (see Section 5.2.2,  Class names), followed by the capital letter  T (examples:  RequestT,  FooBarT).

   

5.3 Camel case: defined 

Sometimes there is more than one reasonable way to convert an English phrase into camel case, such as when acronyms or unusual constructs like "IPv6" or "iOS" are present. To improve predictability, Google Style specifies the following (nearly) deterministic scheme.

Beginning with the prose form of the name:

1. Convert the phrase to plain ASCII and remove any apostrophes. For example, "Müller's  algorithm" might become "Muellers algorithm".

2. Divide this result into words, splitting on spaces and any remaining punctuation (typically  hyphens).

• Recommended: if any word already has a conventional camel-case appearance in common    usage, split this into its constituent parts (e.g., "AdWords" becomes "ad words"). Note    that a word such as "iOS" is not really in camel case per se; it defies any    convention, so this recommendation does not apply.

4. Now lowercase everything (including acronyms), then uppercase only the first  character of:

• ... each word, to yield upper camel case, or

• ... each word except the first, to yield lower camel case

6. Finally, join all the words into a single identifier.

Note that the casing of the original words is almost entirely disregarded. Examples:

Prose form

Correct

Incorrect

"XML HTTP request"

XmlHttpRequest

XMLHTTPRequest

"new customer ID"

newCustomerId

newCustomerID

"inner stopwatch"

innerStopwatch

innerStopWatch

"supports IPv6 on iOS?"

supportsIpv6OnIos

supportsIPv6OnIOS

"YouTube importer"

YouTubeImporter
YoutubeImporter*

*Acceptable, but not recommended.

Note: Some words are ambiguously hyphenated in the English language: for example "nonempty" and "non-empty" are both correct, so the method namescheckNonempty andcheckNonEmpty are likewise both correct.

   

6 Programming Practices 

   

6.1 @Override: always used 

A method is marked with the @Override annotation whenever it is legal.  This includes a class method overriding a superclass method, a class method implementing an interface method, and an interface method respecifying a superinterface method.

Exception:@Override may be omitted when the parent method is@Deprecated.

   

6.2 Caught exceptions: not ignored 

Except as noted below, it is very rarely correct to do nothing in response to a caught exception. (Typical responses are to log it, or if it is considered "impossible", rethrow it as anAssertionError.)

When it truly is appropriate to take no action whatsoever in a catch block, the reason this is justified is explained in a comment.

try {
  int i = Integer.parseInt(response);
  return handleNumericResponse(i);} catch (NumberFormatException ok) {
  // it's not numeric; that's fine, just continue}return handleTextResponse(response);

Exception: In tests, a caught exception may be ignored without comment if it is named expected. The following is a very common idiom for ensuring that the method under test does throw an exception of the expected type, so a comment is unnecessary here.

try {
  emptyStack.pop();
  fail();} catch (NoSuchElementException expected) {}

   

6.3 Static members: qualified using class 

When a reference to a static class member must be qualified, it is qualified with that class's name, not with a reference or expression of that class's type.

Foo aFoo = ...;Foo.aStaticMethod(); // goodaFoo.aStaticMethod(); // badsomethingThatYieldsAFoo().aStaticMethod(); // very bad

   

6.4 Finalizers: not used 

It is extremely rare to override Object.finalize.

Tip: Don't do it. If you absolutely must, first read and understandEffective JavaItem 7, "Avoid Finalizers," very carefully, and then don't do it.

   

7 Javadoc 

   

7.1 Formatting 

   

7.1.1 General form 

The basic formatting of Javadoc blocks is as seen in this example:

/**
 * Multiple lines of Javadoc text are written here,
 * wrapped normally...
 */public int method(String p1) { ... }

... or in this single-line example:

/** An especially short bit of Javadoc. */

The basic form is always acceptable. The single-line form may be substituted when there are no at-clauses present, and the entirety of the Javadoc block (including comment markers) can fit on a single line.

   

7.1.2 Paragraphs 

One blank line—that is, a line containing only the aligned leading asterisk (*)—appears between paragraphs, and before the group of "at-clauses" if present. Each paragraph but the first has <p> immediately before the first word, with no space after.

   

7.1.3 At-clauses 

Any of the standard "at-clauses" that are used appear in the order @param,@return, @throws, @deprecated, and these four types never appear with an empty description. When an at-clause doesn't fit on a single line, continuation lines are indented four (or more) spaces from the position of the @.

   

7.2 The summary fragment 

The Javadoc for each class and member begins with a brief summary fragment. This fragment is very important: it is the only part of the text that appears in certain contexts such as class and method indexes.

This is a fragment—a noun phrase or verb phrase, not a complete sentence. It doesnot begin with A {@code Foo} is a..., orThis method returns..., nor does it form a complete imperative sentence like Save the record.. However, the fragment is capitalized and punctuated as if it were a complete sentence.

Tip: A common mistake is to write simple Javadoc in the form/** @return the customer ID */. This is incorrect, and should be changed to /** Returns the customer ID. */.

   

7.3 Where Javadoc is used 

At the minimum, Javadoc is present for everypublic class, and everypublic orprotected member of such a class, with a few exceptions noted below.

Other classes and members still have Javadoc as needed.  Whenever an implementation comment would be used to define the overall purpose or behavior of a class, method or field, that comment is written as Javadoc instead. (It's more uniform, and more tool-friendly.)

   

7.3.1 Exception: self-explanatory methods 

Javadoc is optional for "simple, obvious" methods likegetFoo, in cases where there really and truly is nothing else worthwhile to say but "Returns the foo".

Important: it is not appropriate to cite this exception to justify omitting relevant information that a typical reader might need to know. For example, for a method named getCanonicalName, don't omit its documentation (with the rationale that it would say only/** Returns the canonical name. */) if a typical reader may have no idea what the term "canonical name" means!

   

7.3.2 Exception: overrides 

Javadoc is not always present on a method that overrides a supertype method.

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Last changed: March 21, 2014