1 IntroductionThis 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. Show
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 notesIn this document, unless otherwise clarified:
Other "terminology notes" will appear occasionally throughout the document. 1.2 Guide notesExample 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 basics2.1 File nameThe source file name consists of the case-sensitive name of the top-level class it contains (of which there is
exactly one), plus the 2.2 File encoding: UTF-8Source files are encoded in UTF-8. 2.3 Special characters2.3.1 Whitespace charactersAside 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:
2.3.2 Special escape sequencesFor any character that has a special escape
sequence ( 2.3.3 Non-ASCII charactersFor the remaining non-ASCII characters, either the actual Unicode character (e.g. Tip: In the Unicode escape case, and occasionally even when actual Unicode characters are used, an explanatory comment can be very helpful. Examples:
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 structureA source file consists of, in order:
Exactly one blank line separates each section that is present. 3.1 License or copyright information, if presentIf license or copyright information belongs in a file, it belongs here. 3.2 Package statementThe package statement is not line-wrapped. The column limit (Section 4.4, Column limit: 100) does not apply to package statements. 3.3 Import statements3.3.1 No wildcard importsWildcard imports, static or otherwise, are not used. 3.3.2 No line-wrappingImport statements are not line-wrapped. The column limit (Section 4.4, Column limit: 100) does not apply to import statements. 3.3.3 Ordering and spacingImports are ordered as follows:
If there are both static and non-static imports, a single blank line separates the two blocks. There are no other blank lines between import statements. Within each block the imported names appear in ASCII sort order. (Note: this is not the same as the import statements being in ASCII sort order, since '.' sorts before ';'.) 3.3.4 No static import for classesStatic import is not used for static nested classes. They are imported with normal imports. 3.4 Class declaration3.4.1 Exactly one top-level class declarationEach top-level class resides in a source file of its own. 3.4.2 Ordering of class contentsThe order you choose for the members and initializers of your class can have a great effect on learnability. However, there's no single correct recipe for how to do it; different classes may order their contents in different ways. What is important is that each class uses 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 splitMethods of a class that share the same name appear in a single contiguous group with no other members in between. The same applies to multiple
constructors (which always have the same name). This rule applies even when modifiers such as 4 FormattingTerminology Note: block-like construct refers to the body of a class, method or constructor. Note that, by Section 4.8.3.1 on array initializers, any array initializer may optionally be treated as if it were a block-like construct. 4.1 Braces4.1.1 Use of optional bracesBraces are used with Other optional braces, such as those in a lambda expression, remain optional. 4.1.2 Nonempty blocks: K & R styleBraces follow the Kernighan and Ritchie style ("Egyptian brackets") for nonempty blocks and block-like constructs:
Exception: In places where these rules allow a single statement ending with a semicolon ( Examples: return () -> { while (condition()) { method(); } }; return new MyClass() { @Override public void method() { if (condition()) { try { something(); } catch (ProblemException e) { recover(); } } else if (otherCondition()) { somethingElse(); } else { lastThing(); } { int x = foo(); frob(x); } } }; A few exceptions for enum classes are given in Section 4.8.1, Enum classes. 4.1.3 Empty blocks: may be conciseAn empty block or block-like construct may be in K & R style (as described in Section 4.1.2). Alternatively, it may be
closed immediately after it is opened, with no characters or line break in between ( Examples: // This is acceptable void doNothing() {} // This is equally acceptable void doNothingElse() { } // This is not acceptable: No concise empty blocks in a multi-block statement try { doSomething(); } catch (Exception e) {} 4.2 Block indentation: +2 spacesEach 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 lineEach statement is followed by a line break. 4.4 Column limit: 100Java code has a column limit of 100 characters. A "character" means any Unicode code point. Except as noted below, any line that would exceed this limit must be line-wrapped, as explained in Section 4.5, Line-wrapping. Each Unicode code point counts as one character, even if its display width is greater or less. For example, if using fullwidth characters, you may choose to wrap the line earlier than where this rule strictly requires. Exceptions:
4.5 Line-wrappingTerminology Note: When code that might otherwise legally occupy a single line is divided into multiple lines, 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. Note: While the typical reason for line-wrapping is to avoid overflowing the column limit, even code that would in fact fit within the column limit may be line-wrapped at the author's discretion. Tip: Extracting a method or local variable may solve the problem without the need to line-wrap. 4.5.1 Where to breakThe prime directive of line-wrapping is: prefer to break at a higher syntactic level. Also:
Note: The primary goal for line wrapping is to have clear code, not necessarily code that fits in the smallest number of lines. 4.5.2 Indent continuation lines at least +4 spacesWhen 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 Whitespace4.6.1 Vertical WhitespaceA single blank line always appears:
A single blank line may also appear anywhere it improves readability, for example between statements to organize the code into logical subsections. A blank line before the first member or initializer, or after the last member or initializer of the class, is neither encouraged nor discouraged. Multiple consecutive blank lines are permitted, but never required (or encouraged). 4.6.2 Horizontal whitespaceBeyond 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.
This rule is never interpreted as requiring or forbidding additional space at the start or end of a line; it addresses only interior space. 4.6.3 Horizontal alignment: never requiredTerminology 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 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 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: recommendedOptional 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 constructs4.8.1 Enum classesAfter each comma that follows an enum constant, a line break is optional. Additional blank lines (usually just one) are also allowed. This is one possibility: private enum Answer { YES { @Override public String toString() { return "yes"; } }, NO, MAYBE } 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 on array initializers). private enum Suit { CLUBS, HEARTS, SPADES, DIAMONDS } Since enum classes are classes, all other rules for formatting classes apply. 4.8.2 Variable declarations4.8.2.1 One variable per declarationEvery variable declaration (field or local) declares only one variable: declarations such as Exception: Multiple variable declarations are acceptable in the header of a 4.8.2.2 Declared when neededLocal 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 Arrays4.8.3.1 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 declarationsThe square brackets form a part of the type, not the variable: 4.8.4 Switch statementsTerminology 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 4.8.4.1 IndentationAs with any other block, the contents of a switch block are indented +2. After a switch label, there is a line break, 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: commentedWithin a switch block, each statement group either terminates abruptly (with a switch (input) { case 1: case 2: prepareOneOrTwo(); // fall through case 3: handleOneTwoOrThree(); break; default: handleLargeNumber(input); } Notice that no comment is needed after 4.8.4.3 Presence of the |
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 names checkNonempty
and checkNonEmpty
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 an AssertionError
.)
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 its name is or begins with expected
. The following is a very common idiom for ensuring that the code 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(); // 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 understand Effective Java Item 8, "Avoid finalizers and cleaners" 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 the entirety of the Javadoc block (including comment markers) can fit on a single line. Note that this only applies when there are no block tags such as @return
.
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 block tags if present. Each paragraph except the first has <p>
immediately before the first word, with no space after it. HTML tags for other block-level elements, such as <ul>
or <table>
, are not preceded with <p>
.
7.1.3 Block tags
Any of the standard "block tags" that are used appear in the order @param
, @return
, @throws
, @deprecated
, and these four types never appear with an empty description. When a block tag 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
Each Javadoc block 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 does not begin with A {@code Foo} is a...
, or This 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 every public
class, and every public
or protected
member of such a class, with a few exceptions noted below.
Additional Javadoc content may also be present, as explained in Section 7.3.4, Non-required Javadoc.
7.3.1 Exception: self-explanatory members
Javadoc is optional for "simple, obvious" members like getFoo()
, 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.
7.3.4 Non-required Javadoc
Other classes and members have Javadoc as needed or desired.
Whenever an implementation comment would be used to define the overall purpose or behavior of a class or member, that comment is written as Javadoc instead (using /**
).
Non-required Javadoc is not strictly required to follow the formatting rules of Sections 7.1.1, 7.1.2, 7.1.3, and 7.2, though it is of course recommended.