Whitespace characters

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:

1. All other whitespace characters in string and character literals are escaped.
2. Tab characters are not used for indentation.

Special escape sequences

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.

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 which makes the code easier to read and understand.

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.

No wildcard imports in the main tree

Wildcard imports, static or otherwise, are not used.

Static wildcard imports in the test tree

Wildcard static imports are encouraged for test imports like JUnit, EasyMock, and Hamcrest.

No line-wrapping

Import statements are not line-wrapped. The column limit (Column limit: 120) does not apply to import statements.

Ordering and spacing

Import statements are divided into the following groups, in this order, with each group separated by a single blank line:

1. java
2. javax
3. org
4. com
5. All static imports in a single group

Within a group there are no blank lines, and the imported names appear in ASCII sort order. (Note: this is not the same as the import statements being in ASCII sort order; the presence of semicolons warps the result.)

IDE settings for ordering imports automatically can be found in the source distributions under src/ide. For example:

• Eclipse: src/ide/eclipse/4.3.2/organize-imports.importorder
• IntelliJ: src/ide/Intellij/13/IntellijSettings.jar

Exactly one top-level class declaration

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

Class member ordering

Class members should be grouped in the following order>.

1. static variables grouped in the order shown below. Within a group variables may appear in any order.
2. 1. public
   2. protected
   3. package
   4. private
3. instance variables grouped in the order shown below. Within a group variables may appear in any order
4. 1. public
   2. protected
   3. package
   4. private
5. constructors
6. methods may be specified in the following order but may appear in another order if it improves the clarity of the program.
7. 1. public
   2. protected
   3. package
   4. private

Braces are used where optional

Braces are used with if, else, for, do and while statements, even when the body is empty or contains only a single statement.

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.

Empty blocks: may be concise

An empty block or block-like construct 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-if/else or try/catch/finally).

Example:

      void doNothing() {}
    

Where to break

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

Indent continuation lines at least +8 spaces

When line-wrapping, each line after the first (each continuation line) is indented at least +8 from the original line.

When there are multiple continuation lines, indentation may be varied beyond +8 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.

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 Import statements).

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

Horizontal whitespace

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.

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:
   • 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 the start or end of a line, only interior space.

Horizontal alignment: never required

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.

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 formatted as if it were an array initializer (see array initializers).

      private enum Suit { CLUBS, HEARTS, SPADES, DIAMONDS }
    

Since enum classes are classes, all other rules for formatting classes apply.

Variable declarations

Arrays

      new int[] {           new int[] {
        0, 1, 2, 3            0,
      }                       1,
                              2,
      new int[] {             3,
        0, 1,               }
        2, 3
      }                     new int[]
                                {0, 1, 2, 3}
    

Switch statements

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.

      switch (input) {
        case 1:
        case 2:
          prepareOneOrTwo();
          // fall through
        case 3:
          handleOneTwoOrThree();
          break;
      default:
          handleLargeNumber(input);
      }
    

Annotations

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 parameter and local variable annotations.

Comments

      /*
       * This is          // And so           /* Or you can
       * okay.            // is this.          * even do this. */
       */
    

Modifiers

Class and member modifiers, when present, appear in the order recommended by the Java Language Specification:

      public protected private abstract 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.

Package names

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

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

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. One typical pattern is test<MethodUnderTest>_<state>, for example testPop_emptyStack. There is no One Correct Way to name test methods.

Constant names

Constant names use CONSTANT_CASE: all uppercase letters, 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. Before choosing constant case, consider whether the field really feels like a constant. For example, if any of that instance's observable state can change, it is almost certainly not a constant. Merely intending to never mutate the object is generally not enough. 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.

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 should be avoided.

Local variable names

Local variable names are written in lowerCamelCase, and can be abbreviated 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 not be styled as constants.

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 Class names), followed by the capital letter T (examples: RequestT, FooBarT).

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 no at-clauses present, and the entirety of the Javadoc block (including comment markers) can fit on a single line.

Paragraphs

One blank line—that is, a line containing only the aligned leading asterisk (*)—appears between paragraphs, and before the group of "at-clauses" if present. Each paragraph but the first has <p> immediately before the first word, with no space after.

At-clauses

Any of the standard "at-clauses" that are used appear in the order @param, @return, @throws, @deprecated, and these four types never appear with an empty description. When an at-clause doesn't fit on a single line, continuation lines are indented four (or more) spaces from the position of the @.

Exception: self-explanatory methods

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!

Exception: overrides

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