**This is a simplified form of Google's Java Style Guide: guidelines which do not affect us in this class have been removed.**
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:
@interface).Other "terminology notes" will appear occasionally throughout the document.
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:
A source file consists of:
Exactly one blank line separates each section that is present.
If license or copyright information belongs in a file, it belongs here.
Wildcard imports, static or otherwise, are not used.
Import statements are not line-wrapped. The column limit (Section 4.4, Column limit: 100) does not apply to import statements.
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.
When a class has multiple constructors, or multiple methods with the same name, these appear sequentially, with no other code in between (not even private members).
Terminology 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.
Braces are used with
if,
else,
for,
do and
while statements, even when the
body is empty or contains only a single statement.
Braces follow the Kernighan and Ritchie style ("Egyptian brackets") for nonempty blocks and block-like constructs:
else or a comma.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();
}
}
};
A few exceptions for enum classes are given in Section 4.8.1, Enum classes.
An 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
({}), unless it is part of a
multi-block statement (one that directly contains multiple blocks:
if/else or
try/catch/finally).
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) {}
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.)
Each statement is followed by a line break.
Java code has a column limit of 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:
package and
import statements (see Sections
3.2 Package statement and
3.3 Import statements).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:
.)::)<T extends Foo & Bar>)catch (FooException | BarException e)).for ("foreach") statement.() that follows it.,) stays attached to the token that
precedes it.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.
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.
A 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).
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.
if,
for or
catch, from an open parenthesis
(()
that follows it on that lineelse or
catch, from a closing curly brace
(}) that precedes it on that line{), with two exceptions:
@SomeAnnotation({a, b}) (no space is used)String[][] x = {{"foo"}}; (no space is required
between {{, by item 8 below)<T extends Foo & Bar>catch (FooException | BarException e):) in an enhanced
for ("foreach") statement(String str) -> str.length()::) of a method reference, which
is written like Object::toString.), which is written like
object.toString(),:; or the closing parenthesis
()) of a cast//) that
begins an end-of-line comment. Here, multiple spaces are allowed, but not required.List<String> listnew int[] {5, 6} and
new int[] { 5, 6 } are both valid[] 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 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.
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.
After 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.
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.
default case is presentEach 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.
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
@Nullable
public 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 annotations on parameters, local variables, or types.
This section addresses implementation comments. Javadoc is addressed separately in Section 7, Javadoc.
Any line break may be preceded by arbitrary whitespace followed by an implementation comment. Such a comment renders the line non-blank.
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.
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 strictfp
long-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 are all lowercase, with consecutive words simply concatenated together (no
underscores). 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.
Test classes are named starting with the name of the class they are testing, and ending
with Test. For example,
HashTest or
HashIntegrationTest.
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.
One typical pattern is <methodUnderTest>_<state>,
for example pop_emptyStack. There is no One Correct
Way to name test methods.
Constant names use CONSTANT_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. This includes primitives, Strings, immutable types, and immutable collections of immutable types. 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 ImmutableMap<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 = {};
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 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.
Each type variable is named in one of two styles:
E, T,
X, T2)
T (examples:
RequestT,
FooBarT).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:
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" | YouTubeImporterYoutubeImporter* |
*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.
@Override: always usedA 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.
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) {
}
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
It is extremely rare to override Object.finalize.
Tip: Don't do it. If you absolutely must, first read and understand Effective Java Item 7, "Avoid Finalizers," very carefully, and then don't do it.
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.
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 but the first has <p> immediately before the first word,
with no space after.
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. */.
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.
Javadoc is optional for "simple, obvious" methods 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.
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 7.1.2, 7.1.3, and 7.2, though it is of course recommended.