Java Style Guidelines

Author: Valentin Karnaukhov

Terminology

• The term class is used inclusively to mean an "ordinary" class, enum class, interface or annotation type ( @interface).
• 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.
• block-like construct refers to the body of a class, method or constructor.
• When code that might otherwise legally occupy a single line is divided into multiple lines, this activity is called _ line-wrapping_.

Source file structure

A source file consists of, in order:

1. License or copyright information, if present
2. Package statement
3. Import statements
4. Exactly one top-level class

Exactly one blank line separates each section that is present.

Package statement

• The package statement is not line-wrapped. The column limit does not apply to package statements.

Import statements

• Wildcard imports, static or otherwise, are not used.
• Import statements are not line-wrapped. The column limit does not apply to import statements.

Imports are ordered as follows:

1. All static imports in a single block.
2. 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.

Static import is not used for static nested classes. They are imported with normal imports.

Class declaration

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

Formatting

Braces

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 for nonempty blocks and block-like constructs:

• No line break before the opening brace, except as detailed below.
• Line break after the opening brace.
• Line break before the closing brace.
• Line break after the closing brace, only if that brace terminates a statement or terminates the body of a method, constructor, or named class.

Empty blocks may be concise. 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).

Block indentation

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.

Column limit

Java code has a column limit of 120 characters. A "character" means any Unicode code point. Except as noted below, any line that would exceed this limit must be line-wrapped.

Exceptions:

• Lines where obeying the column limit is not possible (for example, a long URL in Javadoc, or a long JSNI method reference).
• package and import statements
• Command lines in a comment that may be copied-and-pasted into a shell.
• Very long identifiers, on the rare occasions they are called for, are allowed to exceed the column limit.

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.

info

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.

Breaks

Each statement is followed by a line break.

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

• When a line is broken at a non-assignment operator the break comes before the symbol.
  • 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))
• 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.
• A method or constructor name stays attached to the open parenthesis (() that follows it.
• A comma (,) stays attached to the token that precedes it.
• 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.

info

The primary goal for line wrapping is to have clear code, not necessarily code that fits in the smallest number of lines.

Whitespace

Vertical

A single blank line always appears:

• Between consecutive members or initializers of a class: fields, constructors, methods, nested classes, static initializers, and 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.
  • Exception: Blank lines between enum constants
• As required by other sections of this document (such as source file structure and 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 neither encouraged nor discouraged.

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

Horizontal

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.

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

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

• 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 9 below)

• 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
  • the arrow in a lambda expression: (String str) -> str.length()

  but not

  • the two colons (::) of a method reference, which is written like Object::toString
  • the dot separator (.), which is written like object.toString()

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

• Between any content and a double slash (//) which begins a comment. Multiple spaces are allowed.

• Between a double slash (//) which begins a comment and the comment's text. Multiple spaces are allowed.

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

• Optional just inside both braces of an array initializer

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

• 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.

Grouping parentheses: recommended

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

Variable declarations

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

note

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.

Annotations

Type-use annotations

Type-use annotations appear above annotated type.

@Nullable final String name;

@NotNull(message = "Should be not null")
public Person getPersonByName(String name);

Class annotations

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).

@Deprecated
@CheckReturnValue
public final class Frozzler { ...
}

Method and constructor annotations

The rules for annotations on method and constructor declarations are the same as the previous section.

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

Field annotations

Field annotations appear above annotated type.

@Partial @Mock
DataLoader loader;

Modifiers

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

Numeric Literals

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

Naming

Rules common to all identifiers

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+.

Special prefixes or suffixes are not used. For example: name_, mName, s_name and kName.

Package names

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

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.

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

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 transferMoney_deductsFromSource. There is no One Correct Way to name test methods.

Constant names

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.

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={};

Non-constant field names

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

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

Parameter names

Parameter names are written in lowerCamelCase.

One-character parameter names in public methods should be avoided.

Local variable names

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.

Camel case

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, use 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
"Turn on 2SV"	turnOn2sv	turnOn2Sv