Google Java Style Guide, Wellesley CS 230 Edition

1 Introduction

Tip: The below document is our slightly modified version of Google's Java Style Guide. Why did we choose to use a (slightly simplified version) of a real company's guide? Well, we had to choose something, since consistent style is an important part of working in teams and being able to read other people's code. We felt that it was worth using a real style guide used by professional software developers since it illustrates what one might look like. Of course, the choices that Google has made are not inherently more "correct" than other style rules, but you can learn a lot about code readability through their choices and reasoning. You do not need to memorize this entire document, or even read it cover-to-cover. It is a reference document, like a dictionary, to which you should refer when making stylistic choices.

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 Guide Notes

1.1 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), plus the .java extension.

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.

3 Source file structure

Exactly one blank line separates each section that is present.

3.1 Metadata: Author, assignment, date, partner, others you consulted

Each file should begin with a commented block like this:

// Author: Ada Lerner
// Assignment: Assignment 3
// Date: 2018/02/15
// Partner: Orit Shaer
// Who I consulted: Jean Herbst, Stella Kakavouli

3.3 Import statements

3.3.1 No wildcard imports

Wildcard imports, static or otherwise, are not used.

For example, avoid imports such as:

import java.awt.*;   // Nooooo!

3.3.2 No line-wrapping

Import statements are not line-wrapped. The column limit (Section 4.4, Column limit: 100) 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 ';'.)

3.4 Class declaration

3.4.1 Exactly one top-level class declaration

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

A "top-level" class is just an ordinary class (not an inner class, which you will learn about later in the semester.

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.

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

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

4.1.1 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.

if (x > 5)                  // No! Do not omit these curly braces!
  doTheBigThing()
else 
  doTheSmallThing()

for (int i=0; i<10; i++)    // No! Do not omit these either!
  System.out.println(x); 

if (x > 5) {                // Yes.
  doTheBigThing()
} else {
  doTheSmallThing()
}

for (int i=0; i<10; i++) {  // Yes. Curly braces stop bugs.
  System.out.println(x); 
}

4.1.2 Nonempty blocks: K & R style

Braces follow the Kernighan and Ritchie style 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:

class MyClass() {
  public void method() {
    if (condition()) {
      try {
        something();
      } catch (ProblemException e) {
        recover();
      }
    } else if (otherCondition()) {
      somethingElse();
    } else {
      lastThing();
    }
  }
};

Note that BlueJ's templates when you create a new class use a different curly brace style, where open-curlys are on a new line. You can just make a couple edits when you create a new file to fix this.

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). 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: +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.)

Ada's note: 4 spaces is is the default indentation in BlueJ.

4.3 One statement per line

Each statement is followed by a line break.

4.4 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.

It is always permissible to line-wrap early in order to make code more readable.

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 and 3.3 Import statements).
3. Command lines in a comment that may be cut-and-pasted into a shell.

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.

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

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

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.
2. As required by other sections of this document (such as Section 3, Source file structure, and Section 3.3, 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
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.

// Examples
if(x>5){          // Very constricted and hard to read.
  foo();          // Put spaces between your keywords, parens, braces, operators.
}else{
  bar();
}

if (x > 5) {      // Now I can breathe!
  foo();
} else {
  bar();
}

5>6?x:y           // This ternary operator is quite hard to read.
5>6 ? x : y       // This one is easier to read.

name . charAt(0)  // But don't go overboard...
name.charAt(0)    // ...this is correct :)

foo();//by the way, please do not write comments like this
foo();            // This is much nicer for the reader (remember: you are often the reader! Be kind to yourself :)

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.

    // Example:
    if (a && b || c && d) {
        // The above expression is confusing, since I have to remember operator precedence.
        // Use parentheses to make it clear what you mean.
    }

4.8 Specific constructs

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}

4.8.4 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 +4.

After a switch label, there is a line break, and the indentation level is increased +4, 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.

4.8.6 Comments

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

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

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

5.2.2 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).

5.2.3 Method names

Method names are written in lowerCamelCase.

Method names are typically verbs or verb phrases. For example, sendMessage or stop.

5.2.4 Constant names

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

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

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

5.2.6 Parameter names

Parameter names are written in lowerCamelCase.

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

5.2.7 Local variable names

Local variable names are written in lowerCamelCase.

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

5.2.8 Type variable names

Tip:The discussion below is about acceptable names for generic types when writing your own generic collections and classes.

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), 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

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 but the first has <p> immediately before the first word, with no space after.

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.

7.3.1 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!

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