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Style Guide
This is a modified version of the Google Java Style Guide, the original can be found here.
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).
In this document, unless otherwise clarified:
-
The term class is used inclusively to mean an "ordinary" class, enum class, interface or annotation type (@interface).
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The term member (of a class) is used inclusively to mean a nested class, field, method, or constructor; that is, all top-level contents of a class except initializers and comments.
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The term comment always refers to implementation comments. We do not use the phrase "documentation comments", and instead use the common term "Javadoc."
Other "terminology notes" will appear occasionally throughout the document.
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.
The source file name consists of the case-sensitive name of the top-level class it contains (of which there is exactly one), plus the .java extension.
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:
-
All other whitespace characters in string and character literals are escaped.
-
Tab characters are not used for indentation.
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.
A source file consists of, in order:
-
Package statement
-
Import statements
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Exactly one top-level class
Exactly one blank line separates each section that is present.
The package statement is not line-wrapped. The column limit does not apply to package statements.
Import statements are not line-wrapped. The column limit does not apply to import statements.
Imports are ordered as follows:
-
All static imports in a single block.
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All non-static imports in a single block.
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 ';'.)
Static import is not used for static nested classes. They are imported with normal imports.
Each top-level class resides in a source file of its own.
The 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.
Methods 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 static or private differ between the methods.
Terminology Note: block-like construct refers to the body of a class, method or constructor. Note that, by the section on array initializers, any array initializer may optionally be treated as if it were a block-like construct.
Braces are used with if, else, for, do and while statements, even when the body is empty or contains only a single statement.
Other optional braces, such as those in a lambda expression, remain optional.
Braces follow the Kernighan and Ritchie style ("Egyptian brackets") for nonempty blocks and block-like constructs:
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No line break before the opening brace, except as detailed below.
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Line break after the opening brace.
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Line break before the closing brace.
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Line break after the closing brace, only if that brace terminates a statement or terminates 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.
Exception: In places where these rules allow a single statement ending with a semicolon (;), a block of statements can appear, and the opening brace of this block is preceded by a line break. Blocks like these are typically introduced to limit the scope of local variables, for example inside switch statements.
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 the section on enum classes.
An empty block or block-like construct may be in K & R style (as described in the section on nonempty blocks).
Examples:
// This is acceptable
void doNothing() {
}// This is not acceptable
void doNothingElse() {}// This is also not acceptable, the use of empty lines is not necessary here
try {
} catch (Exception e) {
}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 the section on nonempty blocks)
Each statement is followed by a line break.
Java 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 the section on 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:
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Lines where obeying the column limit is not possible (for example, a long URL in Javadoc, or a long JSNI method reference).
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Package and import statements (see the section on the package statement and the section on import statements).
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Command lines in a comment that may be copied-and-pasted into a shell.
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Very long identifiers, on the rare occasions they are called for, are allowed to exceed the column limit. In that case, the valid wrapping for the surrounding code is as produced by google-java-format.
Terminology 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.
The prime directive of line-wrapping is: prefer to break at a higher syntactic level. Also:
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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.)
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This also applies to the following "operator-like" symbols:
- the dot separator (.)
- the two colons of a method reference (::)
- an ampersand in a type bound (<T extends Foo & Bar>)
- a pipe in a catch block (catch (FooException | BarException e))
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When a line is broken at an assignment operator the break comes after the symbol.
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This also applies to the "assignment-operator-like" colon in an enhanced for ("foreach") statement.
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A method or constructor name stays attached to the open parenthesis (() that follows it.
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A comma (,) stays attached to the token that precedes it.
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A line is never broken adjacent to the arrow in a lambda, except that a break may come immediately after the arrow if the body of the lambda consists of a single unbraced expression.
Examples:
MyLambda<String, Long, Object> lambda =
(String label, Long value, Object obj) -> {
...
};Predicate<String> predicate = str ->
longExpressionInvolving(str);Note: The primary goal for line wrapping is to have clear code, not necessarily code that fits in the smallest number of lines.
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.
The section on horizontal alignment addresses the discouraged practice of using a variable number of spaces to align certain tokens with previous lines.
A single blank line always appears:
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Between consecutive members or initializers of a class: fields, constructors, methods, nested classes, static initializers, and instance initializers.
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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.
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Exception: Blank lines between enum constants are covered in the section on enum classes.
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As required by other sections of this document (such as the section on source file structure, and the section on import statements).
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 not allowed.
Multiple consecutive blank lines are not allowed.
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.
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Separating any reserved word, such as if, for or catch, from an open parenthesis (() that follows it on that line
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Separating any reserved word, such as else or catch, from a closing curly brace (}) that precedes it on that line
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Before any open curly brace ({), with two exceptions:
- @SomeAnnotation({a, b}) (no space is used)
- String[][] x = {{"foo"}} (no space is used between the two curly braces ({{))
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On both sides of any binary or ternary operator. This also applies to the following "operator-like" symbols:
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the ampersand in a conjunctive type bound: <T extends Foo & Bar>
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the pipe for a catch block that handles multiple exceptions: catch (FooException | BarException e)
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the colon (:) in an enhanced for ("foreach") statement
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the arrow in a lambda expression: (String str) -> str.length()
but not
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the two colons (::) of a method reference, which is written like Object::toString
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the dot separator (.), which is written like object.toString()
-
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After ,:; or the closing parenthesis ()) of a cast
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Between any content and a double slash (//) which begins a comment.
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Between a double slash (//) which begins a comment and the comment's text.
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Between the type and variable of a declaration: List list
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Between a type annotation and [] or ....
This rule is never interpreted as requiring or forbidding additional space at the start or end of a line; it addresses only interior space.
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 not allowed
Here is an example without alignment, then using alignment:
private int x; // this is fine
private Color color; // this too
private int x; // not allowed, because future
private Color color; // edits may leave it unalignedOptional 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.
After each comma that follows an enum constant, a line break is optional. An additional blank line is also allowed. This is one possibility:
private enum Answer {
YES {
@Override public String toString() {
return "yes";
}
},
NO,
MAYBE
}Since enum classes are classes, all other rules for formatting classes apply.
Every variable declaration (field or local) declares only one variable: declarations such as int a, b; are not used.
Exception: Multiple variable declarations are acceptable in the header of a for loop.
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.
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}The square brackets form a part of the type, not the variable: String[] args, not String args[].
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: or default:), followed by one or more statements (or, for the last statement group, zero or more statements).
As 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.
Within a switch block, each statement group either terminates abruptly (with a break, 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);
}Notice that no comment is needed after case 1:, only at the end of the statement group.
Each switch statement includes a default statement group, even if it contains no code.
Exception: A switch statement for an enum type may omit the default statement group, if it includes explicit cases covering all possible values of that type. This enables IDEs or other static analysis tools to issue a warning if any cases were missed.
Type-use annotations appear immediately before the annotated type. An annotation is a type-use annotation if it is meta-annotated with @Target(ElementType.TYPE_USE). Example:
final @Nullable String name;
public @Nullable Person getPersonByName(String name);Annotations applying to a class 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, so the indentation level is not increased. Example:
@Deprecated
@CheckReturnValue
public final class Frozzler { ... }The rules for annotations on method and constructor declarations are the same as the previous section. Example:
@Deprecated
@Override
public String getNameIfPresent() { ... }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 annotations on parameters or local variables (except, of course, when the annotation is a type-use annotation).
Class and member modifiers, when present, appear in the order recommended by the Java Language Specification:
public protected private abstract default static final transient volatile synchronized native strictfplong-valued integer literals use an uppercase L suffix, never lowercase (to avoid confusion with the digit 1). For example, 3000000000L rather than 3000000000l.
Identifiers use only ASCII letters and digits, and, in a small number of cases noted below, underscores. Thus each valid identifier name is matched by the regular expression \w+ .
In Google Style, special prefixes or suffixes are not used. For example, these names are not Google Style: name_, mName, s_name and kName.
Package names use only lowercase letters and digits (no underscores). Consecutive words are simply concatenated together. For example, com.example.deepspace, not com.example.deepSpace or com.example.deep_space.
Class names are written in UpperCamelCase.
Class names are typically nouns or noun phrases. For example, Character or ImmutableList. 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 apart from the fact that they should be written in UpperCamelCase.
A test class has a name that ends with Test, for example, HashIntegrationTest. If it covers a single class, its name is the name of that class plus Test, for example HashImplTest.
Method names are written in lowerCamelCase.
Method names are typically verbs or verb phrases. For example, sendMessage or stop.
Underscores may appear in JUnit test method names to separate logical components of the name, with each component written in lowerCamelCase, for example method_scenario_expectedOutcome.
Constant names use UPPER_SNAKE_CASE: all uppercase letters, with each word separated from the next by a single underscore. But what is a constant, exactly?
Constants are static final fields whose contents are deeply immutable and whose methods have no detectable side effects. Examples include primitives, strings, immutable value classes, and anything set to null. If any of the instance's observable state can change, it is not a constant. Merely intending to never mutate the object is not enough. Examples:
// Constants
static final int NUMBER = 5;
static final ImmutableList<String> NAMES = ImmutableList.of("Ed", "Ann");
static final Map<String, Integer> AGES = ImmutableMap.of("Ed", 35, "Ann", 32);
static final Joiner COMMA_JOINER = Joiner.on(','); // because Joiner is immutable
static final SomeMutableType[] EMPTY_ARRAY = {};// 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 ImmutableMap<String, SomeMutableType> mutableValues =
ImmutableMap.of("Ed", mutableInstance, "Ann", mutableInstance2);
static final Logger logger = Logger.getLogger(MyClass.getName());
static final String[] nonEmptyArray = {"these", "can", "change"};These names are typically nouns or noun phrases.
Non-constant field names (static or otherwise) are written in lowerCamelCase.
These names are typically nouns or noun phrases. For example, computedValues or index.
Parameter names are written in lowerCamelCase.
One-character parameter names in public methods should be avoided.
Local variable names are written in lowerCamelCase.
Even when final and immutable, local variables are not considered to be constants, and should not be styled as constants.
The name of each type variable starts with the capital letter T, followed by a name in the form used for classes (see the section on class names) (examples: TRequest, TFooBar).
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:
-
Convert the phrase to plain ASCII and remove any apostrophes. For example, "Müller's algorithm" might become "Muellers algorithm".
-
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.
-
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
-
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 names checkNonempty and checkNonEmpty are likewise both correct.
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.
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);Note that the code above is incorrect as it asks for forgiveness as opposed to permission.
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) {
}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 badIt 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.
The 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) { ... }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>.
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 @.
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. */.
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 the section on non-required Javadoc.
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!
Javadoc is not always present on a method that overrides a supertype method.
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 the sections on general form, paragraphs, block tags, and the summary fragment, though it is of course recommended.