Google Java Style Guide

ゞ浴缸里的玫瑰 2023-06-05 05:28 143阅读 0赞

Google诸类变成语言代码风格指南：[http://google.github.io/styleguide/][http_google.github.io_styleguide]

Google Java代码风格指南：[https://google.github.io/styleguide/javaguide.html][https_google.github.io_styleguide_javaguide.html]

### Table of Contents ###

<table> 
 <tbody> 
 <tr> 
 <td> <a href="https://google.github.io/styleguide/javaguide.html#s1-introduction" rel="nofollow">1 Introduction</a> <a href="https://google.github.io/styleguide/javaguide.html#s1.1-terminology" rel="nofollow">1.1 Terminology notes</a> <a href="https://google.github.io/styleguide/javaguide.html#s1.2-guide-notes" rel="nofollow">1.2 Guide notes</a> <a href="https://google.github.io/styleguide/javaguide.html#s2-source-file-basics" rel="nofollow">2 Source file basics</a> <a href="https://google.github.io/styleguide/javaguide.html#s2.1-file-name" rel="nofollow">2.1 File name</a> <a href="https://google.github.io/styleguide/javaguide.html#s2.2-file-encoding" rel="nofollow">2.2 File encoding: UTF-8</a> <a href="https://google.github.io/styleguide/javaguide.html#s2.3-special-characters" rel="nofollow">2.3 Special characters</a> <a href="https://google.github.io/styleguide/javaguide.html#s3-source-file-structure" rel="nofollow">3 Source file structure</a> <a href="https://google.github.io/styleguide/javaguide.html#s3.1-copyright-statement" rel="nofollow">3.1 License or copyright information, if present</a> <a href="https://google.github.io/styleguide/javaguide.html#s3.2-package-statement" rel="nofollow">3.2 Package statement</a> <a href="https://google.github.io/styleguide/javaguide.html#s3.3-import-statements" rel="nofollow">3.3 Import statements</a> <a href="https://google.github.io/styleguide/javaguide.html#s3.4-class-declaration" rel="nofollow">3.4 Class declaration</a> <a href="https://google.github.io/styleguide/javaguide.html#s4-formatting" rel="nofollow">4 Formatting</a> <a href="https://google.github.io/styleguide/javaguide.html#s4.1-braces" rel="nofollow">4.1 Braces</a> <a href="https://google.github.io/styleguide/javaguide.html#s4.2-block-indentation" rel="nofollow">4.2 Block indentation: +2 spaces</a> <a href="https://google.github.io/styleguide/javaguide.html#s4.3-one-statement-per-line" rel="nofollow">4.3 One statement per line</a> <a href="https://google.github.io/styleguide/javaguide.html#s4.4-column-limit" rel="nofollow">4.4 Column limit: 100</a> <a href="https://google.github.io/styleguide/javaguide.html#s4.5-line-wrapping" rel="nofollow">4.5 Line-wrapping</a> <a href="https://google.github.io/styleguide/javaguide.html#s4.6-whitespace" rel="nofollow">4.6 Whitespace</a> <a href="https://google.github.io/styleguide/javaguide.html#s4.7-grouping-parentheses" rel="nofollow">4.7 Grouping parentheses: recommended</a> <a href="https://google.github.io/styleguide/javaguide.html#s4.8-specific-constructs" rel="nofollow">4.8 Specific constructs</a> <a href="https://google.github.io/styleguide/javaguide.html#s5-naming" rel="nofollow">5 Naming</a> <a href="https://google.github.io/styleguide/javaguide.html#s5.1-identifier-names" rel="nofollow">5.1 Rules common to all identifiers</a> <a href="https://google.github.io/styleguide/javaguide.html#s5.2-specific-identifier-names" rel="nofollow">5.2 Rules by identifier type</a> <a href="https://google.github.io/styleguide/javaguide.html#s5.3-camel-case" rel="nofollow">5.3 Camel case: defined</a> <a href="https://google.github.io/styleguide/javaguide.html#s6-programming-practices" rel="nofollow">6 Programming Practices</a> <a href="https://google.github.io/styleguide/javaguide.html#s6.1-override-annotation" rel="nofollow">6.1 @Override: always used</a> <a href="https://google.github.io/styleguide/javaguide.html#s6.2-caught-exceptions" rel="nofollow">6.2 Caught exceptions: not ignored</a> <a href="https://google.github.io/styleguide/javaguide.html#s6.3-static-members" rel="nofollow">6.3 Static members: qualified using class</a> <a href="https://google.github.io/styleguide/javaguide.html#s6.4-finalizers" rel="nofollow">6.4 Finalizers: not used</a> <a href="https://google.github.io/styleguide/javaguide.html#s7-javadoc" rel="nofollow">7 Javadoc</a> <a href="https://google.github.io/styleguide/javaguide.html#s7.1-javadoc-formatting" rel="nofollow">7.1 Formatting</a> <a href="https://google.github.io/styleguide/javaguide.html#s7.2-summary-fragment" rel="nofollow">7.2 The summary fragment</a> <a href="https://google.github.io/styleguide/javaguide.html#s7.3-javadoc-where-required" rel="nofollow">7.3 Where Javadoc is used</a> </td> 
 </tr> 
 </tbody> 
</table>

## 1 Introduction[![format_png][]][format_png 1] ##

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).

### 1.1 Terminology notes[![format_png][]][format_png 2] ###

In this document, unless otherwise clarified:

1.  The term *class* is used inclusively to mean an "ordinary" class, enum class, interface or annotation type (`@interface`).
2.  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.
3.  The term *comment* always refers to *implementation* comments. We do not use the phrase "documentation comments", instead using the common term "Javadoc."

Other "terminology notes" will appear occasionally throughout the document.

### 1.2 Guide notes[![format_png][]][format_png 3] ###

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.

## 2 Source file basics[![format_png][]][format_png 4] ##

### 2.1 File name[![format_png][]][format_png 5] ###

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.

### 2.2 File encoding: UTF-8[![format_png][]][format_png 6] ###

Source files are encoded in **UTF-8**.

### 2.3 Special characters[![format_png][]][format_png 7] ###

2.3.1 Whitespace characters[![format_png][]][format_png 8]

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.

2.3.2 Special escape sequences[![format_png][]][format_png 9]

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.

2.3.3 Non-ASCII characters[![format_png][]][format_png 10]

For the remaining non-ASCII characters, either the actual Unicode character (e.g. `∞`) or the equivalent Unicode escape (e.g. `\u221e`) is used. The choice depends only on which makes the code **easier to read and understand**, although Unicode escapes outside string literals and comments are strongly discouraged.

**Tip:** In the Unicode escape case, and occasionally even when actual Unicode characters are used, an explanatory comment can be very helpful.

Examples:

<table> 
 <tbody> 
 <tr> 
 <th>Example</th> 
 <th>Discussion</th> 
 </tr> 
 <tr> 
 <td><code>String unitAbbrev = "μs";</code></td> 
 <td>Best: perfectly clear even without a comment.</td> 
 </tr> 
 <tr> 
 <td><code>String unitAbbrev = "\u03bcs"; // "μs"</code></td> 
 <td>Allowed, but there's no reason to do this.</td> 
 </tr> 
 <tr> 
 <td><code>String unitAbbrev = "\u03bcs"; // Greek letter mu, "s"</code></td> 
 <td>Allowed, but awkward and prone to mistakes.</td> 
 </tr> 
 <tr> 
 <td><code>String unitAbbrev = "\u03bcs";</code></td> 
 <td>Poor: the reader has no idea what this is.</td> 
 </tr> 
 <tr> 
 <td><code>return '\ufeff' + content; // byte order mark</code></td> 
 <td>Good: use escapes for non-printable characters, and comment if necessary.</td> 
 </tr> 
 </tbody> 
</table>

**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 structure[![format_png][]][format_png 11] ##

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.

### 3.1 License or copyright information, if present[![format_png][]][format_png 12] ###

If license or copyright information belongs in a file, it belongs here.

### 3.2 Package statement[![format_png][]][format_png 13] ###

The package statement is **not line-wrapped**. The column limit (Section 4.4, [Column limit: 100][Column limit_ 100]) does not apply to package statements.

### 3.3 Import statements[![format_png][]][format_png 14] ###

3.3.1 No wildcard imports[![format_png][]][format_png 15]

**Wildcard imports**, static or otherwise, **are not used**.

3.3.2 No line-wrapping[![format_png][]][format_png 16]

Import statements are **not line-wrapped**. The column limit (Section 4.4, [Column limit: 100][Column limit_ 100]) does not apply to import statements.

3.3.3 Ordering and spacing[![format_png][]][format_png 17]

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.

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 classes[![format_png][]][format_png 18]

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

### 3.4 Class declaration[![format_png][]][format_png 19] ###

3.4.1 Exactly one top-level class declaration[![format_png][]][exactly one]

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

3.4.2 Ordering of class contents[![format_png][]][format_png 20]

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.

3.4.2.1 Overloads: never split

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).

## 4 Formatting[![format_png][]][format_png 21] ##

**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.

### 4.1 Braces[![format_png][]][format_png 22] ###

4.1.1 Braces are used where optional[![format_png][]][format_png 23]

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

4.1.2 Nonempty blocks: K & R style[![format_png][]][format_png 24]

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, *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.

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][].

4.1.3 Empty blocks: may be concise[![format_png][]][format_png 25]

An empty block or block-like construct may be in K & R style (as described in [Section 4.1.2][format_png 24]). 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) {}

### 4.2 Block indentation: +2 spaces[![format_png][]][format_png 26] ###

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][format_png 24].)

### 4.3 One statement per line[![format_png][]][format_png 27] ###

Each statement is followed by a line break.

### 4.4 Column limit: 100[![format_png][]][Column limit_ 100] ###

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 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:**

1.  Lines where obeying the column limit is not possible (for example, a long URL in Javadoc, or a long JSNI method reference).
2.  `package` and `import` statements (see Sections 3.2 [Package statement][format_png 13] and 3.3 [Import statements][format_png 14]).
3.  Command lines in a comment that may be cut-and-pasted into a shell.

### 4.5 Line-wrapping[![format_png][]][Line-wrapping] ###

**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.

4.5.1 Where to break[![format_png][]][format_png 28]

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 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)`).
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.
5. 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.

4.5.2 Indent continuation lines at least +4 spaces[![format_png][]][format_png 29]

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.

### 4.6 Whitespace[![format_png][]][format_png 30] ###

4.6.1 Vertical Whitespace[![format_png][]][format_png 31]

A single blank line always appears:

1.  *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 are covered in [Section 4.8.1][Enum classes].
2.  As required by other sections of this document (such as Section 3, [Source file structure][format_png 11], and Section 3.3, [Import statements][format_png 14]).

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 whitespace[![format_png][]][format_png 32]

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:
 
 * `@SomeAnnotation({a, b})` (no space is used)
 * `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
 * 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()`
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
9. 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.

4.6.3 Horizontal alignment: never required[![format_png][]][Horizontal alignment]

**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.

### 4.7 Grouping parentheses: recommended[![format_png][]][format_png 33] ###

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.

### 4.8 Specific constructs[![format_png][]][format_png 34] ###

4.8.1 Enum classes[![format_png][]][Enum classes]

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.

4.8.2 Variable declarations[![format_png][]][format_png 35]

4.8.2.1 One variable per declaration

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.

4.8.2.2 Declared when needed

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.

4.8.3 Arrays[![format_png][]][format_png 36]

4.8.3.1 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 declarations

The square brackets form a part of the *type*, not the variable: `String[] args`, not `String args[]`.

4.8.4 Switch statements[![format_png][]][format_png 37]

**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).

4.8.4.1 Indentation

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.

4.8.4.2 Fall-through: commented

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.

4.8.4.3 The `default` case is present

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.

4.8.5 Annotations[![format_png][]][format_png 38]

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.

4.8.6 Comments[![format_png][]][format_png 39]

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.

4.8.6.1 Block comment style

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.

4.8.7 Modifiers[![format_png][]][format_png 40]

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

4.8.8 Numeric Literals[![format_png][]][format_png 41]

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

## 5 Naming[![format_png][]][format_png 42] ##

### 5.1 Rules common to all identifiers[![format_png][]][format_png 43] ###

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`.

### 5.2 Rules by identifier type[![format_png][]][format_png 44] ###

5.2.1 Package names[![format_png][]][format_png 45]

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`.

5.2.2 Class names[![format_png][]][format_png 46]

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`.

5.2.3 Method names[![format_png][]][format_png 47]

Method names are written in [lowerCamelCase][UpperCamelCase].

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][UpperCamelCase]. One typical pattern is `<methodUnderTest>_<state>`, for example `pop_emptyStack`. There is no One Correct Way to name test methods.

5.2.4 Constant names[![format_png][]][format_png 48]

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.

5.2.5 Non-constant field names[![format_png][]][format_png 49]

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

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

5.2.6 Parameter names[![format_png][]][format_png 50]

Parameter names are written in [lowerCamelCase][UpperCamelCase].

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

5.2.7 Local variable names[![format_png][]][format_png 51]

Local variable names are written in [lowerCamelCase][UpperCamelCase].

Even when final and immutable, local variables are not considered to be constants, and should not be styled as constants.

5.2.8 Type variable names[![format_png][]][format_png 52]

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 Section 5.2.2, [Class names][format_png 46]), followed by the capital letter `T` (examples: `RequestT`, `FooBarT`).

### 5.3 Camel case: defined[![format_png][]][UpperCamelCase] ###

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:

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:

<table> 
 <tbody> 
 <tr> 
 <th>Prose form</th> 
 <th>Correct</th> 
 <th>Incorrect</th> 
 </tr> 
 <tr> 
 <td>"XML HTTP request"</td> 
 <td><code>XmlHttpRequest</code></td> 
 <td><code>XMLHTTPRequest</code></td> 
 </tr> 
 <tr> 
 <td>"new customer ID"</td> 
 <td><code>newCustomerId</code></td> 
 <td><code>newCustomerID</code></td> 
 </tr> 
 <tr> 
 <td>"inner stopwatch"</td> 
 <td><code>innerStopwatch</code></td> 
 <td><code>innerStopWatch</code></td> 
 </tr> 
 <tr> 
 <td>"supports IPv6 on iOS?"</td> 
 <td><code>supportsIpv6OnIos</code></td> 
 <td><code>supportsIPv6OnIOS</code></td> 
 </tr> 
 <tr> 
 <td>"YouTube importer"</td> 
 <td><code>YouTubeImporter</code> <code>YoutubeImporter</code>*</td> 
 <td>&nbsp;</td> 
 </tr> 
 </tbody> 
</table>

\*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[![format_png][]][format_png 53] ##

### 6.1 `@Override`: always used[![format_png][]][format_png 54] ###

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[![format_png][]][format_png 55] ###

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[![format_png][]][format_png 56] ###

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[![format_png][]][format_png 57] ###

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,][Effective Java_Item 7] "Avoid Finalizers," very carefully, and *then* don't do it.

## 7 Javadoc[![format_png][]][Javadoc] ##

### 7.1 Formatting[![format_png][]][format_png 58] ###

7.1.1 General form[![format_png][]][format_png 59]

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[![format_png][]][format_png 60]

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 `` immediately before the first word, with no space after.

7.1.3 Block tags[![format_png][]][format_png 61]

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[![format_png][]][format_png 62] ###

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[![format_png][]][format_png 63] ###

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 methods[![format_png][]][format_png 64]

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!

7.3.2 Exception: overrides[![format_png][]][format_png 65]

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

7.3.4 Non-required Javadoc[![format_png][]][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 7.1.2, 7.1.3, and 7.2, though it is of course recommended.

[http_google.github.io_styleguide]: http://google.github.io/styleguide/
[https_google.github.io_styleguide_javaguide.html]: https://google.github.io/styleguide/javaguide.html
[format_png]: /images/20230601/036f73a7bb4640269efe536b9ab40d08.png
[format_png 1]: https://google.github.io/styleguide/javaguide.html#s1-introduction
[format_png 2]: https://google.github.io/styleguide/javaguide.html#s1.1-terminology
[format_png 3]: https://google.github.io/styleguide/javaguide.html#s1.2-guide-notes
[format_png 4]: https://google.github.io/styleguide/javaguide.html#s2-source-file-basics
[format_png 5]: https://google.github.io/styleguide/javaguide.html#s2.1-file-name
[exactly one]: https://google.github.io/styleguide/javaguide.html#s3.4.1-one-top-level-class
[format_png 6]: https://google.github.io/styleguide/javaguide.html#s2.2-file-encoding
[format_png 7]: https://google.github.io/styleguide/javaguide.html#s2.3-special-characters
[format_png 8]: https://google.github.io/styleguide/javaguide.html#s2.3.1-whitespace-characters
[format_png 9]: https://google.github.io/styleguide/javaguide.html#s2.3.2-special-escape-sequences
[special escape sequence]: http://docs.oracle.com/javase/tutorial/java/data/characters.html
[format_png 10]: https://google.github.io/styleguide/javaguide.html#s2.3.3-non-ascii-characters
[format_png 11]: https://google.github.io/styleguide/javaguide.html#s3-source-file-structure
[format_png 12]: https://google.github.io/styleguide/javaguide.html#s3.1-copyright-statement
[format_png 13]: https://google.github.io/styleguide/javaguide.html#s3.2-package-statement
[Column limit_ 100]: https://google.github.io/styleguide/javaguide.html#s4.4-column-limit
[format_png 14]: https://google.github.io/styleguide/javaguide.html#s3.3-import-statements
[format_png 15]: https://google.github.io/styleguide/javaguide.html#s3.3.1-wildcard-imports
[format_png 16]: https://google.github.io/styleguide/javaguide.html#s3.3.2-import-line-wrapping
[format_png 17]: https://google.github.io/styleguide/javaguide.html#s3.3.3-import-ordering-and-spacing
[format_png 18]: https://google.github.io/styleguide/javaguide.html#s3.3.4-import-class-not-static
[format_png 19]: https://google.github.io/styleguide/javaguide.html#s3.4-class-declaration
[format_png 20]: https://google.github.io/styleguide/javaguide.html#s3.4.2-ordering-class-contents
[format_png 21]: https://google.github.io/styleguide/javaguide.html#s4-formatting
[array initializers]: https://google.github.io/styleguide/javaguide.html#s4.8.3.1-array-initializers
[format_png 22]: https://google.github.io/styleguide/javaguide.html#s4.1-braces
[format_png 23]: https://google.github.io/styleguide/javaguide.html#s4.1.1-braces-always-used
[format_png 24]: https://google.github.io/styleguide/javaguide.html#s4.1.2-blocks-k-r-style
[Egyptian brackets]: http://www.codinghorror.com/blog/2012/07/new-programming-jargon.html
[Enum classes]: https://google.github.io/styleguide/javaguide.html#s4.8.1-enum-classes
[format_png 25]: https://google.github.io/styleguide/javaguide.html#s4.1.3-braces-empty-blocks
[format_png 26]: https://google.github.io/styleguide/javaguide.html#s4.2-block-indentation
[format_png 27]: https://google.github.io/styleguide/javaguide.html#s4.3-one-statement-per-line
[Line-wrapping]: https://google.github.io/styleguide/javaguide.html#s4.5-line-wrapping
[fullwidth characters]: https://en.wikipedia.org/wiki/Halfwidth_and_fullwidth_forms
[format_png 28]: https://google.github.io/styleguide/javaguide.html#s4.5.1-line-wrapping-where-to-break
[format_png 29]: https://google.github.io/styleguide/javaguide.html#s4.5.2-line-wrapping-indent
[Horizontal alignment]: https://google.github.io/styleguide/javaguide.html#s4.6.3-horizontal-alignment
[format_png 30]: https://google.github.io/styleguide/javaguide.html#s4.6-whitespace
[format_png 31]: https://google.github.io/styleguide/javaguide.html#s4.6.1-vertical-whitespace
[format_png 32]: https://google.github.io/styleguide/javaguide.html#s4.6.2-horizontal-whitespace
[format_png 33]: https://google.github.io/styleguide/javaguide.html#s4.7-grouping-parentheses
[format_png 34]: https://google.github.io/styleguide/javaguide.html#s4.8-specific-constructs
[format_png 35]: https://google.github.io/styleguide/javaguide.html#s4.8.2-variable-declarations
[format_png 36]: https://google.github.io/styleguide/javaguide.html#s4.8.3-arrays
[format_png 37]: https://google.github.io/styleguide/javaguide.html#s4.8.4-switch
[format_png 38]: https://google.github.io/styleguide/javaguide.html#s4.8.5-annotations
[format_png 39]: https://google.github.io/styleguide/javaguide.html#s4.8.6-comments
[Javadoc]: https://google.github.io/styleguide/javaguide.html#s7-javadoc
[format_png 40]: https://google.github.io/styleguide/javaguide.html#s4.8.7-modifiers
[format_png 41]: https://google.github.io/styleguide/javaguide.html#s4.8.8-numeric-literals
[format_png 42]: https://google.github.io/styleguide/javaguide.html#s5-naming
[format_png 43]: https://google.github.io/styleguide/javaguide.html#s5.1-identifier-names
[format_png 44]: https://google.github.io/styleguide/javaguide.html#s5.2-specific-identifier-names
[format_png 45]: https://google.github.io/styleguide/javaguide.html#s5.2.1-package-names
[format_png 46]: https://google.github.io/styleguide/javaguide.html#s5.2.2-class-names
[UpperCamelCase]: https://google.github.io/styleguide/javaguide.html#s5.3-camel-case
[format_png 47]: https://google.github.io/styleguide/javaguide.html#s5.2.3-method-names
[format_png 48]: https://google.github.io/styleguide/javaguide.html#s5.2.4-constant-names
[format_png 49]: https://google.github.io/styleguide/javaguide.html#s5.2.5-non-constant-field-names
[format_png 50]: https://google.github.io/styleguide/javaguide.html#s5.2.6-parameter-names
[format_png 51]: https://google.github.io/styleguide/javaguide.html#s5.2.7-local-variable-names
[format_png 52]: https://google.github.io/styleguide/javaguide.html#s5.2.8-type-variable-names
[format_png 53]: https://google.github.io/styleguide/javaguide.html#s6-programming-practices
[format_png 54]: https://google.github.io/styleguide/javaguide.html#s6.1-override-annotation
[format_png 55]: https://google.github.io/styleguide/javaguide.html#s6.2-caught-exceptions
[format_png 56]: https://google.github.io/styleguide/javaguide.html#s6.3-static-members
[format_png 57]: https://google.github.io/styleguide/javaguide.html#s6.4-finalizers
[Effective Java_Item 7]: http://books.google.com/books?isbn=8131726592
[format_png 58]: https://google.github.io/styleguide/javaguide.html#s7.1-javadoc-formatting
[format_png 59]: https://google.github.io/styleguide/javaguide.html#s7.1.1-javadoc-multi-line
[format_png 60]: https://google.github.io/styleguide/javaguide.html#s7.1.2-javadoc-paragraphs
[format_png 61]: https://google.github.io/styleguide/javaguide.html#s7.1.3-javadoc-block-tags
[format_png 62]: https://google.github.io/styleguide/javaguide.html#s7.2-summary-fragment
[format_png 63]: https://google.github.io/styleguide/javaguide.html#s7.3-javadoc-where-required
[Non-required Javadoc]: https://google.github.io/styleguide/javaguide.html#s7.3.4-javadoc-non-required
[format_png 64]: https://google.github.io/styleguide/javaguide.html#s7.3.1-javadoc-exception-self-explanatory
[format_png 65]: https://google.github.io/styleguide/javaguide.html#s7.3.2-javadoc-exception-overrides