1 Introduction 

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

Like other programming style guides, the issues covered span not only aesthetic issues offormatting, but other types of conventions or coding standards as well. However, this documentfocuses primarily on the hard-and-fast rules that we follow universally, andavoids 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).
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 examplesare in Google Style, they may not illustrate the only stylish way to represent thecode. 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 spacecharacter (0x20) is the only whitespace character that appearsanywhere 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 sequenceis 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 whichmakes the code easier to read and understand.

Tip: In the Unicode escape case, and occasionally even when actualUnicode 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 thatsome programs might not handle non-ASCII characters properly. If that should happen, thoseprograms are broken and they must be fixed.

3 Source file structure 

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 importstatements.

3.3.3 Ordering and spacing 

Import statements are divided into the following groups, in this order, with each groupseparated 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
4. java imports
5. javax imports

Within a group there are no blank lines, and the imported names appear in ASCII sortorder. (Note: this is not the same as the import statements being inASCII 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 isno single correct recipe for how to do it. Different classes may order their membersdifferently.

What is important is that each class order its members in some logicalorder, which its maintainer could explain if asked. For example, new methods are notjust habitually added to the end of the class, as that would yield "chronological by dateadded" 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 appearsequentially, with no intervening members.

4 Formatting 

Terminology Note: block-like construct refers tothe body of a class, method or constructor. Note that, by Section 4.8.3.1 onarray initializers, any array initializermay 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 thebody 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 isopened, with no characters or line break in between({}), unless it is part of amulti-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 twospaces. When the block ends, the indent returns to the previous indent level. The indent levelapplies 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 inSection 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 legallyoccupy a single line is divided into multiple lines, typically to avoid overflowing the columnlimit, this activity is calledline-wrapping.

There is no comprehensive, deterministic formula showing exactly how to line-wrap inevery 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 problemwithout 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)).
2. 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.
3. A method or constructor name stays attached to the open parenthesis (() that follows it.
4. 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 indentedat least +4 from the original line.

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

Section 4.6.3 on Horizontal alignment addressesthe discouraged practice of using a variable number of spaces to align certain tokens withprevious 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.
2. Within method bodies, as needed to create logical groupings of statements.
3. Optionally before the first member or after the last member of the class (neither encouraged nor discouraged).
4. 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 andJavadoc, 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)
4. 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
5. After ,:; or the closing parenthesis ()) of a cast
6. On both sides of the double slash (//) that begins an end-of-line comment. Here, multiple spaces are allowed, but not required.
7. Between the type and variable of a declaration: List<String> list
8. 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 thestart or end of a line, only interior space.

4.6.3 Horizontal alignment: never required 

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

This practice is permitted, but is never required by Google Style. It is noteven 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 fine
private Color color; // this too

private int   x;      // permitted, but future edits
private Color color;  // may leave it unaligned

Tip: Alignment can aid readability, but it creates problems forfuture maintenance. Consider a future change that needs to touch just one line. This change mayleave the formerly-pleasing formatting mangled, and that is allowed. More oftenit prompts the coder (perhaps you) to adjust whitespace on nearby lines as well, possiblytriggering 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 historyinformation, 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 noreasonable chance the code will be misinterpreted without them, nor would they have made the codeeasier to read. It is not reasonable to assume that every reader has the entire Javaoperator 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 formattedas 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 aspossible 

Local variables are not habitually declared at the start of their containingblock or block-like construct. Instead, local variables are declared close to the point they arefirst used (within reason), to minimize their scope. Local variable declarations typically haveinitializers, 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-likeconstruct." For example, the following are all valid (not an exhaustivelist):

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 aswitch block are one or more statement groups. Each statement group consists ofone 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 asif a block were being opened. The following switch label returns to the previous indentationlevel, 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 commentto indicate that execution will or might continue into the next statement group. Anycomment that communicates the idea of fall-through is sufficient (typically// fall through). This special comment is not required inthe 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 statementgroup, even if it contains no code.

4.8.5 Annotations 

Annotations applying to a class, method or constructor appear immediately after thedocumentation block, and each annotation is listed on a line of its own (that is, one annotationper line). These line breaks do not constitute line-wrapping (Section4.5, Line-wrapping), so the indentation level is notincreased. Example:

@Override
@Nullable
public String getNameIfPresent() { ... }

Exception: A single parameterless annotationmay 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 inthis 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 tore-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 orderrecommended 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, neverlowercase (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. Thuseach valid identifier name is matched by the regular expression \w+ .

In Google Style special prefixes orsuffixes, 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 (nounderscores). 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 ornoun phrases (for example, List), but may sometimes beadjectives 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 endingwith 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 thename. One typical pattern is test<MethodUnderTest>_<state>,for example testPop_emptyStack. There is no One CorrectWay to name test methods.

5.2.4 Constant names 

Constant names use CONSTANT_CASE: all uppercaseletters, 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. Beforechoosing constant case, consider whether the field really feels like a constant. Forexample, if any of that instance‘s observable state can change, it is almost certainly not aconstant. Merely intending to never mutate the object is generally notenough. Examples:

// Constants
static final int NUMBER = 5;
static final ImmutableList<String> NAMES = ImmutableList.of("Ed", "Ann");
static final Joiner COMMA_JOINER = Joiner.on(‘,‘);  // because Joiner is immutable
static final SomeMutableType[] EMPTY_ARRAY = {};
enum SomeEnum { ENUM_CONSTANT }

// Not constants
static 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 writtenin 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 beabbreviated 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 notbe 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 improvepredictability, 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.
3. 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
4. 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 Englishlanguage: 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 annotationwhenever it is legal. This includes a class method overriding a superclass method, a class methodimplementing an interface method, and an interface method respecifying a superinterfacemethod.

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 caughtexception. (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 isjustified 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 ignoredwithout comment if it is named expected. Thefollowing is a very common idiom for ensuring that the method under test does throw anexception 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‘sname, not with a reference or expression of that class‘s type.

Foo aFoo = ...;
Foo.aStaticMethod(); // good
aFoo.aStaticMethod(); // bad
somethingThatYieldsAFoo().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 noat-clauses present, and the entirety of the Javadoc block (including comment markers) can fit on asingle 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" ifpresent. 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 neverappear with an empty description. When an at-clause doesn‘t fit on a single line, continuation linesare 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. Thisfragment is very important: it is the only part of the text that appears in certain contexts such asclass 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 sentencelike Save the record.. However, the fragment is capitalized andpunctuated 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 bechanged 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 exceptionsnoted below.

Other classes and members still have Javadoc as needed. Whenever an implementationcomment would be used to define the overall purpose or behavior of a class, method or field, thatcomment 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 isnothing else worthwhile to say but "Returns the foo".

Important: it is not appropriate to cite this exception to justifyomitting relevant information that a typical reader might need to know. For example, for a methodnamed 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 ideawhat the term "canonical name" means!

7.3.2 Exception: overrides 

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