Conventions
We follow standard Java
coding conventions. We add a few Android specific rules.
Package and Import Statements
The
first non-comment line of most Java source files is a package statement. After
that, import statements can follow. For example:
package java.awt;
import java.awt.peer.CanvasPeer;
Order Import Statements
The ordering of import
statements is:
1.
Android imports
2.
Imports from third parties (com, junit, net, org)
3.
java and javax
To exactly match the IDE
settings, the imports should be:
·
Alphabetical within each grouping, with
capital letters before lower case letters (e.g. Z before a).
·
There
should be a blank line between each major grouping (android, com, junit, net, org, java, javax).
Fully Qualify Imports
When you want to use class
Bar from package foo, there are two possible ways to import it:
import foo.*;
import foo.Bar;
Use
the latter for importing all Android code. An explicit exception is made for
java standard libraries (java.util.*, java.io.*, etc.) and unit test
code (junit.framework.*)
Number per Line
One declaration per line is recommended since
it encourages commenting. In other words,
int level; //
indentation level int size; // size of table
Class and Interface Declarations
When coding Java classes and
interfaces, the following formatting rules should be followed:
• No
space between a method name and the parenthesis “(“ starting its parameter list
• Open brace “{”
appears at the end of the same line as the declaration statement
• Closing
brace “}” starts a line by itself indented to match its corresponding opening
statement, except when it is a null statement the “}” should appear immediately
after the “{“
class Sample extends Object { int ivar1;
int ivar2;
Sample(int i, int j) { ivar1 = i;
ivar2 = j;
}
int emptyMethod() {}
...
}
·
Methods
are separated by a blank line
Use Spaces for
Indentation
We
use 4 space indents for blocks. We never use tabs. When in doubt, be consistent
with code around you.
We
use 8 space indents for line wraps, including function calls and assignments.
For example, this is correct:
Instrument i =
someLongExpression(that,
wouldNotFit, on, one, line);
and
this is not correct:
Instrument i =
someLongExpression(that,
wouldNotFit, on, one, line);
Follow Field
Naming Conventions
·
Non-public,
non-static field names start with m.
·
Static
field names start with s.
·
Other
fields start with a lower case letter.
·
Public
static final fields (constants) are ALL_CAPS_WITH_UNDERSCORES.
For
example:
public class MyClass {
public static
final int SOME_CONSTANT = 42; public int publicField;
private
static MyClass sSingleton; int mPackagePrivate;
private int
mPrivate; protected int mProtected;
}
Naming Conventions
Naming
conventions make programs more understandable by making them easier to read.
They can also give information about the function of the identifier-for
example, whether it's a constant, package, or class-which can be helpful in
understanding the code.
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Rules for Naming
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Examples
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Type
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The prefix of a unique package name
is always written in all-
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lowercase ASCII letters and should be
one of the top-level
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domain names,
currently com, edu, gov, mil, net, org, or one
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of the English two-letter codes
identifying countries as
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specified in ISO
Standard 3166, 1981.
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com.sun.eng
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Packages
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Subsequent
components of the package name vary
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com.apple.quicktime.v2
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edu.cmu.cs.bovik.cheese
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according to an
organization's own internal naming
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conventions. Such
conventions might specify that certain
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directory name
components be division, department, project,
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machine, or login names.
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Class names should
be nouns, in mixed case with the first
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letter of each
internal word capitalized. Try to keep your class
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class Raster;
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names simple and descriptive. Use
whole words-avoid
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class ImageSprite;
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Classes
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acronyms and
abbreviations (unless the abbreviation is much
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more widely used
than the long form, such as URL or HTML).
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Interfaces
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Interface names
should be capitalized like class names and
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interface IDelegate;
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always prefix with
capital “I”.
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interface IStoring;
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Methods should be verbs, in mixed
case with the first letter
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run();
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Methods
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lowercase, with
the first letter of each internal word
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runFast();
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capitalized.
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getBackground();
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Except for variables, all instance, class, and class
constants
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are in mixed case with a lowercase first letter. Internal
words
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start with capital
letters. Variable names should not start with
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underscore _ or
dollar sign $ characters, even though both
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are allowed.
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int i;
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Variable
names should be short yet meaningful. The choice
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char c;
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Variables
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of
a variable name should be mnemonic- that is, designed to
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float myWidth;
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indicate to the casual observer the intent of its use.
One-
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character variable names should be avoided except for
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temporary
"throwaway" variables. Common names for
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temporary variables are i, j, k, m, and n for integers; c, d,
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and e for characters.
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The names of variables declared class constants and of
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static final int
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Constants
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ANSI constants should be all uppercase with words
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MIN_WIDTH = 4;
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separated by
underscores ("_"). (ANSI constants should be
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static final int
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avoided, for ease
of debugging.)
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MAX_WIDTH = 999;
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Use Standard Brace Style
Braces do not go on their own
line; they go on the same line as the code before them. So:
class MyClass { int func() {
if (something) {
// ...
} else if (somethingElse) {
// ...
} else {
// ...
}
}
}
Comment Formats
·
Block comments are used to provide
descriptions of files, methods, data structures and algorithms.
·
Short
comments can appear on a single line indented to the level of the code that
follows.
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Very short comments can appear on the same
line as the code they describe, but should be shifted far enough to separate
them from the statements.
·
The //
comment delimiter can comment out a complete line or only a partial line. It
shouldn't be used on consecutive multiple lines for text comments; however, it
can be used in consecutive multiple lines for commenting out sections of code.
Sometimes
it is tempting to write code that completely ignores an exception like this:
void setServerPort(String value) { try {
serverPort = Integer.parseInt(value); } catch
(NumberFormatException e) { }
}
You must never do this. While you may think
that your code will never encounter this error condition or that it is not
important to handle it, ignoring exceptions like above creates mines in your
code for someone else to trip over some day. You must handle every Exception in
your code in some principled way. The specific handling varies depending on the
case.
Acceptable
alternatives (in order of preference) are:
·
Throw
the exception up to the caller of your method.
void
setServerPort(String value) throws NumberFormatException { serverPort =
Integer.parseInt(value);
}
·
Throw
a new exception that's appropriate to your level of abstraction.
void setServerPort(String value) throws
ConfigurationException {
try {
serverPort = Integer.parseInt(value); } catch
(NumberFormatException e) {
throw new
ConfigurationException("Port " + value + " is not valid.");
}
}
·
Handle
the error gracefully and substitute an appropriate value in the catch {} block.
/** Set port. If value is not a valid number, 80 is
substituted. */
void
setServerPort(String value) { try {
serverPort =
Integer.parseInt(value); } catch (NumberFormatException e) {
serverPort = 80; // default port for server
}
}
·
Last resort: if you are confident that
actually ignoring the exception is appropriate then you may ignore it, but you
must also comment why with a good reason:
/** If value is not a valid number,
original port number is used. */ void setServerPort(String value) {
try {
serverPort =
Integer.parseInt(value); } catch (NumberFormatException e) {
// Method is
documented to just ignore invalid user input.
// serverPort
will just be unchanged.
}
}
Don't
Catch Generic Exception
Sometimes
it is tempting to be lazy when catching exceptions.
Alternatives
to catching generic Exception:
·
Catch each exception separately as separate
catch blocks after a single try. This can be awkward but is still preferable to
catching all Exceptions. Beware repeating too much code in the catch blocks.
·
Refactor your code to have more fine-grained
error handling, with multiple try blocks. Split up the IO from the parsing,
handle errors separately in each case.
·
Rethrow the exception. Many times you don't
need to catch the exception at this level anyway, just let the method throw it.
Use Javadoc Standard Comments
Every
file should have a copyright statement at the top. Then a package statement and
import statements should follow, each block separated by a blank line. And then
there is the class or interface declaration. In the Javadoc comments, describe
what the class or interface does.
/*
* Copyright (C)
2010 The Android Open Source Project
*
* Licensed
under the Apache License, Version 2.0 (the "License");
* you may not
use this file except in compliance with the License.
* You may
obtain a copy of the License at
*
*
http://www.apache.org/licenses/LICENSE-2.0
*
* Unless
required by applicable law or agreed to in writing, software
* distributed
under the License is distributed on an "AS IS" BASIS,
* WITHOUT
WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the
License for the specific language governing permissions and
* limitations
under the License.
*/
package com.android.internal.foo;
import
android.os.Blah; import android.view.Yada;
import
java.sql.ResultSet; import java.sql.SQLException;
/**
* Does X and
Y and provides an abstraction for Z. */
public class Foo {
...
Every
class and nontrivial public method you write must contain a Javadoc
comment with at least one sentence describing what the class or method does.
This sentence should start with a 3rd person descriptive verb.
Examples:
/** Returns the correctly rounded
positive square root of a double value. */ static double sqrt(double a) {
...
}
or
/**
* Constructs a
new String by converting the specified array of
* bytes using
the platform's default character encoding.
*/
public String(byte[] bytes) {
...
}
You
do not need to write Javadoc for trivial get and set methods such as setFoo() if all your Javadoc
would say is "sets Foo". If the method does something more complex
(such as enforcing a constraint or having an important side effect), then you
must document it. And if it's not obvious what the property "Foo"
means, you should document it.
Every
method you write, whether public or otherwise, would benefit from Javadoc.
Public methods are part of an API and therefore require Javadoc.
Limit Variable Scope
The
scope of local variables should be kept to a minimum. Each variable should be
declared in the innermost block that encloses all uses of the variable.
Local
variables should be declared at the point they are first used. Nearly every
local variable declaration should contain an initializer. If you don't yet have
enough information to initialize a variable sensibly, you should postpone the
declaration until you do.
One
exception to this rule concerns try-catch statements. If a variable is
initialized with the return value of a method that throws a checked exception,
it must be initialized inside a try block. If the value must be used outside of
the try block, then it must be declared before the try block, where it cannot
yet be sensibly initialized:
//
Instantiate class cl, which represents some sort of Set Set s = null;
try {
s = (Set) cl.newInstance();
} catch(IllegalAccessException e) {
throw new IllegalArgumentException(cl + " not
accessible"); } catch(InstantiationException e) {
throw new
IllegalArgumentException(cl + " not instantiable");
// Exercise
the set s.addAll(Arrays.asList(args));
But
even this case can be avoided by encapsulating the try-catch block in a method:
Set createSet(Class cl) {
//
Instantiate class cl, which represents some sort of Set try {
return (Set) cl.newInstance(); } catch(IllegalAccessException e)
{
throw new IllegalArgumentException(cl + " not
accessible"); } catch(InstantiationException e) {
throw new
IllegalArgumentException(cl + " not instantiable");
}
}
...
// Exercise
the set Set s = createSet(cl);
s.addAll(Arrays.asList(args));
Loop
variables should be declared in the for statement itself unless there is a
compelling reason to do otherwise:
for (int i = 0; i
n; i++) { doSomething(i);
}
and
for (Iterator i =
c.iterator(); i.hasNext(); ) { doSomethingElse(i.next());
}
Treat Acronyms
as Words
Treat
acronyms and abbreviations as words in naming variables, methods, and classes.
The names are much more readable:
Good
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Bad
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XmlHttpRequest
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XMLHTTPRequest
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getCustomerId
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getCustomerID
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class Html
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class HTML
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String url
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String URL
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long id
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long ID
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Both
the JDK and the Android code bases are very inconsistent with regards to
acronyms, therefore, it is virtually impossible to be consistent with the code
around you.
Use TODO
Comments
Use
TODO comments for code that is temporary, a short-term solution, or good-enough
but not perfect.
TODOs
should include the string TODO in all caps, followed by a colon:
// TODO: Remove this code after the
UrlTable2 has been checked in.
and
// TODO: Change this to use a flag
instead of a constant.
If
your TODO is of the form "At a future date do something" make sure
that you either include a very specific date ("Fix by November 2005")
or a very specific event ("Remove this code after all production mixers
understand protocol V7.").
