Hedera Java Style Guide

# Hedera Java Style Guide

## 1 Introduction

This document serves as the **complete** definition of Hedera's coding standards for source code in
the Java™ Programming Language. A Java source file is described as being _in Hedera Style_ if and
only if it adheres to the rules herein.

This document is a derivative of the
[Google Java Style Guide](https://google.github.io/styleguide/javaguide.html) with modifications
used when developing for Hedera. We adhere to the language, style, and substance of the Google Java
Style Guide, and owe much to the authors. By hewing closely to the Google standard, we benefit from
an industry of tools and commonality in style.

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

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", and instead use the common term "Javadoc."

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

### 1.2 Guide notes

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

### 2.1 File name

The source file name consists of the case-sensitive name of the top-level class it contains (of
which there is [exactly one](#s3.4.1-one-top-level-class)), plus the `.java` extension.

### 2.2 File encoding: UTF-8

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

### 2.3 Special characters

#### 2.3.1 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.

As an exception to this rule, Java text blocks (introduced in Java 15) may use arbitrary whitespace
characters.

#### 2.3.2 Special escape sequences

For any character that has a
[special escape sequence](http://docs.oracle.com/javase/tutorial/java/data/characters.html) (`\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

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:

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

> **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

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

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

### 3.2 Package statement

The package statement is **not line-wrapped**. The column limit (Section 4.4,
[Column limit: 120](#s4.4-column-limit)) does not apply to package statements.

### 3.3 Import statements

#### 3.3.1 No wildcard imports

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

#### 3.3.2 No line-wrapping

Import statements are **not line-wrapped**. The column limit (Section 4.4,
[Column limit: 120](#s4.4-column-limit)) does not apply to import statements.

#### 3.3.3 Ordering and spacing

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

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

### 3.4 Class declaration

#### 3.4.1 Exactly one top-level class declaration

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

#### 3.4.2 Ordering of class contents

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.

One suggested way to organize code consistent with this section would be the following:

> static fields (public, protected, package private, private) constructors public API methods
> protected API methods package API methods private API methods private inner classes

For each method and constructor, you may sort them by the least specific to most specific.

##### 3.4.2.1 Overloads: never split

Methods of a class that share the same name appear in a single contiguous group with no other
members in between. The same applies to multiple constructors (which always have the same name).
This rule applies even when modifiers such as `static` or `private` differ between the methods.

## 4 Formatting

> **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](#s4.8.3.1-array-initializers), any array
> initializer _may_ optionally be treated as if it were a block-like construct.

### 4.1 Braces

#### 4.1.1 Use of optional 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.

#### 4.1.2 Nonempty blocks: K & R style

Braces follow the Kernighan and Ritchie style
("[Egyptian brackets](http://www.codinghorror.com/blog/2012/07/new-programming-jargon.html)") 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. For example, there is _no_ line break after the brace if it is
  followed by `else` or a
- comma.

Exception: In places where these rules allow a single statement ending with a semicolon (`;`), a
block of statements can appear, and the opening brace of this block is preceded by a line break.
Blocks like these are typically introduced to limit the scope of local variables, for example inside
switch statements.

Examples:

```java
return () -> {
    while (condition()) {
        method();
    }
};
```

```java
return new MyClass() {
    @Override public void method() {
        if (condition()) {
            try {
                something();
            } catch (ProblemException e) {
                recover();
            }
        } else if (otherCondition()) {
            somethingElse();
        } else {
            lastThing();
        }
        {
            int x = foo();
            frob(x);
        }
    }
};
```

A few exceptions for enum classes are given in Section 4.8.1, [Enum classes](#s4.8.1-enum-classes).

#### 4.1.3 Empty blocks: may be concise

An empty block or block-like construct may be in K & R style (as described in
[Section 4.1.2](#s4.1.2-blocks-k-r-style)). 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:

```java
// This is acceptable
void doNothing() {}

// This is equally acceptable
void doNothingElse() {
}
```

```java
// This is not acceptable: No concise empty blocks in a multi-block statement
try {
    doSomething();
} catch (Exception e) {}
```

### 4.2 Block indentation: +4 spaces

Each time a new block or block-like construct is opened, the indent increases by four 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](#s4.1.2-blocks-k-r-style).)

### 4.3 One statement per line

Each statement is followed by a line break.

### 4.4 Column limit: 120

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, as explained in Section
4.5, [Line-wrapping](#s4.5-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](https://en.wikipedia.org/wiki/Halfwidth_and_fullwidth_forms), 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](#s3.2-package-statement)
   and 3.3 [Import statements](#s3.3-import-statements)).
3. Command lines in a comment that may be copied-and-pasted into a shell.
4. Very long identifiers, on the rare occasions they are called for, are allowed to exceed the
   column limit. In that case, the valid wrapping for the surrounding code is as produced by
   [google-java-format](https://github.com/google/google-java-format).

### 4.5 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

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:

   ```java
   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 +8 spaces

When line-wrapping, each line after the first (each _continuation line_) is indented at least +9
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.

Section 4.6.3 on [Horizontal alignment](#s4.6.3-horizontal-alignment) addresses the discouraged
practice of using a variable number of spaces to align certain tokens with previous lines.

### 4.6 Whitespace

#### 4.6.1 Vertical Whitespace

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](#s4.8.1-enum-classes).
2. As required by other sections of this document (such as Section 3,
   [Source file structure](#s3-source-file-structure), and Section 3.3,
   [Import statements](#s3.3-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).

#### 4.6.2 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:
   - `@SomeAnnotation({a, b})` (no space is used)
   - `String[][] x = {{"foo"}};` (no space is required between `{{`, by item 9 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. Between any content and a double slash (`//`) which begins a comment. Multiple spaces are
   allowed.
7. Between a double slash (`//`) which begins a comment and the comment's text. Multiple spaces are
   allowed.
8. Between the type and variable of a declaration: `List<String> list`
9. _Optional_ just inside both braces of an array initializer
   - `new int[] {5, 6}` and `new int[] { 5, 6 }` are both valid
10. 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

**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

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

#### 4.8.1 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](#s4.8.3.1-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

##### 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

##### 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

**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 Presence of the `default` label

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

Do not use the annotations defined in the `javax.annotation` or `org.jetbrains.annotations` packages anywhere,
instead use only annotations from the Spotbugs package
(defined here: https://spotbugs.readthedocs.io/en/latest/annotations.html)

##### 4.8.5.1 Type-use annotations

Type-use annotations appear immediately before the annotated type. An annotation is a type-use
annotation if it is meta-annotated with `@Target(ElementType.TYPE_USE)`. Example:

final @Nullable String name;

public @Nullable Person getPersonByName(String name);

##### 4.8.5.2 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). These line breaks do
not constitute line-wrapping (Section 4.5, [Line-wrapping](#s4.5-line-wrapping)), so the indentation
level is not increased. Example:

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

##### 4.8.5.3 Method and constructor annotations

The rules for annotations on method and constructor declarations are the same as the
[previous section](#s4.8.5.2-class-annotation-style). Example:

@Deprecated @Override 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() { ... }

##### 4.8.5.4 Field annotations

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;

##### 4.8.5.5 Parameter and local variable annotations

There are no specific rules for formatting annotations on parameters or local variables (except, of
course, when the annotation is a type-use annotation).

#### 4.8.6 Comments

This section addresses _implementation comments_. Javadoc is addressed separately in Section 7,
[Javadoc](#s7-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

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

`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

### 5.1 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+` .

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

#### 5.2.1 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`.

#### 5.2.2 Class names

Class names are written in [UpperCamelCase](#s5.3-camel-case).

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`.

#### 5.2.3 Method names

Method names are written in [lowerCamelCase](#s5.3-camel-case).

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](#s5.3-camel-case), for example
`transferMoney_deductsFromSource`. There is no One Correct Way to name test methods.

#### 5.2.4 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. Examples:

// Constants static final int NUMBER = 5; static final ImmutableList<String> NAMES =
ImmutableList.of("Ed", "Ann"); static final Map<String, Integer> AGES = ImmutableMap.of("Ed", 35,
"Ann", 32); static final Joiner COMMA_JOINER = Joiner.on(','); // because Joiner is immutable static
final SomeMutableType\[\] EMPTY_ARRAY = {};

// Not constants static String nonFinal = "non-final"; final String nonStatic = "non-static"; static
final Set<String> mutableCollection = new HashSet<String>(); static final
ImmutableSet<SomeMutableType> mutableElements = ImmutableSet.of(mutable); static final
ImmutableMap<String, SomeMutableType> mutableValues = ImmutableMap.of("Ed", mutableInstance, "Ann",
mutableInstance2); static final Logger logger = Logger.getLogger(MyClass.getName()); static final
String\[\] nonEmptyArray = {"these", "can", "change"};

These names are typically nouns or noun phrases.

#### 5.2.5 Non-constant field names

Non-constant field names (static or otherwise) are written in [lowerCamelCase](#s5.3-camel-case).

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

#### 5.2.6 Parameter names

Parameter names are written in [lowerCamelCase](#s5.3-camel-case).

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

#### 5.2.7 Local variable names

Local variable names are written in [lowerCamelCase](#s5.3-camel-case).

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

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](#s5.2.2-class-names)),
  followed by the capital letter `T` (examples: `RequestT`, `FooBarT`).

### 5.3 Camel case: defined

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:

Prose form

Correct

Incorrect

"XML HTTP request"

`XmlHttpRequest`

`XMLHTTPRequest`

"new customer ID"

`newCustomerId`

`newCustomerID`

"inner stopwatch"

`innerStopwatch`

`innerStopWatch`

"supports IPv6 on iOS?"

`supportsIpv6OnIos`

`supportsIPv6OnIOS`

"YouTube importer"

`YouTubeImporter` `YoutubeImporter`\*

\*Acceptable, but not recommended.

**Note:** Some words are ambiguously hyphenated in the English language: for example "nonempty" and
"non-empty" are both correct, so the method names `checkNonempty` and `checkNonEmpty` are likewise
both correct.

## 6 Programming Practices

### 6.1 `@Override`: always used

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

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

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

It is **extremely rare** to override `Object.finalize`.

**Tip:** Don't do it. If you absolutely must, first read and understand
[_Effective Java_ Item 8](http://books.google.com/books?isbn=0134686047), "Avoid finalizers and
cleaners" very carefully, and _then_ don't do it.

## 7 Javadoc

### 7.1 Formatting

#### 7.1.1 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 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

One blank line—that is, a line containing only the aligned leading asterisk (`*`)—appears between
paragraphs, and before the group of block tags if present. Each paragraph except the first has `<p>`
immediately before the first word, with no space after it. HTML tags for other block-level elements,
such as `<ul>` or `<table>`, are _not_ preceded with `<p>`.

#### 7.1.3 Block tags

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

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

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](#s7.3.4-javadoc-non-required).

#### 7.3.1 Exception: self-explanatory members

Javadoc is optional for "simple, obvious" members like `getFoo()`, in cases where there _really and
truly_ is nothing else worthwhile to say but "Returns the foo".

**Important:** it is not appropriate to cite this exception to justify omitting relevant information
that a typical reader might need to know. For example, for a method named `getCanonicalName`, don't
omit its documentation (with the rationale that it would say only
`/** Returns the canonical name. */`) if a typical reader may have no idea what the term "canonical
name" means!

#### 7.3.2 Exception: overrides

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

#### 7.3.4 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 7.1.1,
7.1.2, 7.1.3, and 7.2, though it is of course recommended.
