Android is a mobile operating system based on a modified version of the Linux kernel and other open-source software, designed primarily for touchscreen mobile devices such as smartphones and tablets.

The core Android source code is known as the Android Open Source Project (AOSP), which is primarily licensed under the Apache License.

This has allowed variants of Android to be developed on a range of other electronics, such as game consoles, digital cameras, PCs and others, each with a specialized user interface.

For projects, it is ideal to follow the Android Gradle project structure that is defined on the Android Gradle plugin user guide.

The ribot boilerplate project is a good reference to start from.

1. File Naming

Class files

Class names are written in UpperCamelCase.
For classes that extend an Android component, the name of the class should end with the name of the component; for example SignInActivity, SignInFragment, ImageUploaderService, ChangePasswordDialog.

Resources files

Resources file names are written in lowercase_underscore.

Drawable files

Naming conventions for drawables:

Asset Type	Prefix	Example
Action bar	ab_	ab_stacked.9.png
Button	btn_	btn_send_pressed.9.png
Dialog	dialog_	dialog_top.9.png
Divider	divider_	divider_horizontal.9.png
Icon	ic_	ic_star.png
Menu	menu_	menu_submenu_bg.9.png
Notification	notification_	notification_bg.9.png
Tabs	tab_	tab_pressed.9.png

Naming conventions for icons (taken from Android iconography guidelines):

Asset Type	Prefix	Example
Icons	ic_	ic_star.png
Launcher icons	ic_launcher	ic_launcher_calendar.png
Menu icons and Action Bar icons	ic_menu	ic_menu_archive.png
Status bar icons	ic_stat_notify	ic_stat_notify_msg.png
Tab icons	ic_tab	ic_tab_recent.png
Dialog icons	ic_dialog	ic_dialog_info.png

Naming conventions for selector states:

State	Suffix	Example
Normal	_normal	btn_order_normal.9.png
Pressed	_pressed	btn_order_pressed.9.png
Focused	_focused	btn_order_focused.9.png
Disabled	_disabled	btn_order_disabled.9.png
Selected	_selected	btn_order_selected.9.png

Layout files

Layout files should match the name of the Android components that they are intended for but moving the top level component name to the beginning. For example, if we are creating a layout for the SignInActivity, the name of the layout file should be activity_sign_in.xml.

Component	Class Name	Layout Name
Activity	UserProfileActivity	module_name_user_profile_activity.xml
Fragment	SignUpFragment	sign_up_fragment.xml
Dialog	ChangePasswordDialog	change_password_dialog.xml
AdapterView item	—	person_view_holder_item.xml
Partial layout	—	partial_stats_bar.xml

A slightly different case is when we are creating a layout that is going to be inflated by an Adapter, e.g to populate a ListView. In this case, the name of the layout should start with item_.

Note that there are cases where these rules will not be possible to apply. For example, when creating layout files that are intended to be part of other layouts. In this case you should use the prefix partial_.

Menu files

Similar to layout files, menu files should match the name of the component. For example, if we are defining a menu file that is going to be used in the UserActivity, then the name of the file should be activity_user.xml

A good practice is to not include the word menu as part of the name because these files are already located in the menu directory.

Values files

Resource files in the values folder should be plural, e.g. strings.xml, styles.xml, colors.xml, dimens.xml, attrs.xml

Code guidelines

2. Kotlin

This document serves as the complete definition of Google’s Android coding standards for source code in the Kotlin Programming Language.

Naming

If a source file contains only a single top-level class, the file name should reflect the case-sensitive name plus the .kt extension.

Otherwise, if a source file contains multiple top-level declarations, choose a name that describes the contents of the file, apply PascalCase, and append the .kt extension.

// MyClass.kt

class MyClass { }

Formatting

Braces

Braces are not required for when branches and if statement bodies which have no else if/else branches and which fit on a single line.

if (string.isEmpty()) return

when (value) {

0 -> return

// …

}

Braces are otherwise required for any if, for, when branch, do, and while statements, even when the body is empty or contains only a single statement.

if (string.isEmpty())

return // WRONG!

if (string.isEmpty()) {

return // Okay

}

Non-empty blocks

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 function, constructor, or named class. For example, there is no line break after the brace if it is followed byelse or a comma.

return Runnable {

while (condition()) {

foo()

}

return object : MyClass() {

override fun foo() {

if (condition()) {

try {

something()

} catch (e: ProblemException) {

recover()

}

} else if (otherCondition()) {

somethingElse()

} else {

lastThing()

}

A few exceptions for enum classes are given below.

Empty blocks

An empty block or block-like construct must be in K&R style.

try {

doSomething()

} catch (e: Exception) {} // WRONG!

try {

doSomething()

} catch (e: Exception) {

} // Okay

Expressions

An if/else conditional that is used as an expression may omit braces only if the entire expression fits on one line.

val value = if (string.isEmpty()) 0 else 1 // Okay

val value = if (string.isEmpty()) // WRONG!

else

val value = if (string.isEmpty()) { // Okay

} else {

}

Indentation

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.

One statement per line

Each statement is followed by a line break. Semicolons are not used.

Line wrapping

Code has a column limit of 100 characters. Except as noted below, any line that would exceed this limit must be line-wrapped, as explained below.

Exceptions:

Lines where obeying the column limit is not possible (for example, a long URL in KDoc)
package and import statements
Command lines in a comment that may be cut-and-pasted into a shell

Where to break

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

When a line is broken at a non-assignment operator the break comes before the symbol.
- This also applies to the following “operator-like” symbols:
- The dot separator (.).
- The two colons of a member reference (::).
When a line is broken at an assignment operator the break comes after the symbol.
A method or constructor name stays attached to the open parenthesis (() that follows it.
A comma (,) stays attached to the token that precedes it.
A lambda arrow (->) stays attached to the argument list that precedes it.

Nullability

As a rule of thumb, !! should never be used and ? should be used rarely.

Theses checks can often be avoided and doing so will provide for a more stable code-base. Whenever possible, objects and properties should be made to be not nullable.

3. Java language rules

Don’t ignore exceptions

You must never do the following:

void setServerPort(String value) {

try {

serverPort = Integer.parseInt(value);

} catch (NumberFormatException e) { }

}

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. – (Android code style guidelines)

Don’t catch generic exception

You should not do this:

try {

someComplicatedIOFunction(); // may throw IOException

someComplicatedParsingFunction(); // may throw ParsingException

someComplicatedSecurityFunction(); // may throw SecurityException

// phew, made it all the way

} catch (Exception e) { // I’ll just catch all exceptions

handleError(); // with one generic handler!

}

See the reason why and some alternatives here

Don’t use finalizers

There are no guarantees as to when a finalizer will be called, or even that it will be called at all. In most cases, you can do what you need from a finalizer with good exception handling. If you absolutely need it, define a close() method (or the like) and document exactly when that method needs to be called. See InputStream for an example. In this case, it is appropriate but not required to print a short log message from the finalizer, as long as it is not expected to flood the logs. – (Android code style guidelines)

Fully qualify imports

This is bad: import foo.*;

This is good: import foo.Bar;

3. Java style rules

Fields definition and naming

Fields should be defined at the top of the file and they should follow the naming rules listed below.

Private, non-static field names start with m.
Private, static field names start with s.
Other fields start with a lower case letter.
Static final fields (constants) are ALL_CAPS_WITH_UNDERSCORES.

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;

}

Treat acronyms as words

Good	Bad
XmlHttpRequest	XMLHTTPRequest
getCustomerId	getCustomerID
String url	String URL
long id	long ID

Use spaces for indentation

Use 4 space indents for blocks:

if (x == 1) {

x++;

}

Use 8 space indents for line wraps:

Instrument i =

someLongExpression(that, wouldNotFit, on, one, line);

Use standard brace style

Braces go on the same line as the code before them.

class MyClass {

int func() {

if (something) {

// …

} else if (somethingElse) {

// …

} else {

// …

}

Braces around the statements are required unless the condition and the body fit on one line.

If the condition and the body fit on one line and that line is shorter than the max line length, then braces are not required, e.g.

This is good:

if (condition) body();

This is bad:

if (condition)

body(); // bad!

Annotations in Java language

Annotations practices

According to the Android code style guide, the standard practices for some of the predefined annotations in Java are:

@Override: The @Override annotation must be used whenever a method overrides the declaration or implementation from a super-class. For example, if you use the @inheritdocs Javadoc tag, and derive from a class (not an interface), you must also annotate that the method @Overrides the parent class’s method.

@SuppressWarnings: The @SuppressWarnings annotation should only be used under circumstances where it is impossible to eliminate a warning. If a warning passes this “impossible to eliminate” test, the @SuppressWarnings annotation must be used, so as to ensure that all warnings reflect actual problems in the code.

More information about annotation guidelines can be found here.

Annotations style

Classes, Methods and Constructors

When annotations are applied to a class, method, or constructor, they are listed after the documentation block and should appear as one annotation per line.

/* This is the documentation block about the class */

@AnnotationA

@AnnotationB

public class MyAnnotatedClass { }

Fields

Annotations applying to fields should be listed on the same line, unless the line reaches the maximum line length.

@Nullable @Mock DataManager mDataManager;

Limit variable scope

The scope of local variables should be kept to a minimum (Effective Java Item 29). By doing so, you increase the readability and maintainability of your code and reduce the likelihood of error. 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. – (Android code style guidelines)

Order import statements

If you are using an IDE such as Android Studio, you don’t have to worry about this because your IDE is already obeying these rules. If not, have a look below.

The ordering of import statements is:

Android imports
Imports from third parties (com, junit, net, org)
java and javax
Same project imports

To exactly match the IDE settings, the imports should be:

Alphabetically ordered 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).

More info here

Logging guidelines

Use the logging methods provided by the Log class to print out error messages or other information that may be useful for developers to identify issues:

Log.v(String tag, String msg) (verbose)
Log.d(String tag, String msg) (debug)
Log.i(String tag, String msg) (information)
Log.w(String tag, String msg) (warning)
Log.e(String tag, String msg) (error)

As a general rule, we use the class name as tag and we define it as a static final field at the top of the file. For example:

public class MyClass {

private static final String TAG = MyClass.class.getSimpleName();

public myMethod() {

Log.e(TAG, “My error message”);

}

VERBOSE and DEBUG logs must be disabled on release builds. It is also recommended to disable INFORMATION, WARNING and ERROR logs but you may want to keep them enabled if you think they may be useful to identify issues on release builds.

If you decide to leave them enabled, you have to make sure that they are not leaking private information such as email addresses, user ids, etc.

To only show logs on debug builds:

if (BuildConfig.DEBUG) Log.d(TAG, “The value of x is “ + x);

Class member ordering

There is no single correct solution for this but using a logical and consistent order will improve code learnability and readability.

It is recommendable to use the following order:

Constants
Fields
Constructors
Override methods and callbacks (public or private)
Public methods
Private methods
Inner classes or interfaces

Example:

public class MainActivity extends Activity {

private static final String TAG = MainActivity.class.getSimpleName();

private String mTitle;

private TextView mTextViewTitle;

@Override

public void onCreate() {

…

}

public void setTitle(String title) {

mTitle = title;

}

private void setUpView() {

…

}

static class AnInnerClass {

}

If your class is extending an Android component such as an Activity or a Fragment, it is a good practice to order the override methods so that they match the component’s lifecycle.

For example, if you have an Activity that implements onCreate(), onDestroy(), onPause() and onResume(), then the correct order is:

public class MainActivity extends Activity {

//Order matches Activity lifecycle

@Override

public void onCreate() {}

@Override

public void onResume() {}

@Override

public void onPause() {}

@Override

public void onDestroy() {}

}

Parameter ordering in methods

When programming for Android, it is quite common to define methods that take a Context. If you are writing a method like this, then the Context must be the first parameter.

The opposite case is callback interfaces that should always be the last parameter.

Examples:

// Context always goes first

public User loadUser(Context context, int userId);

// Callbacks always go last

public void loadUserAsync(Context context, int userId, UserCallback callback);

String constants, naming, and values

Many elements of the Android SDK such as SharedPreferences, Bundle, or Intent use a key-value pair approach so it’s very likely that even for a small app you end up having to write a lot of String constants.

When using one of these components, you must define the keys as static final fields and they should be prefixed as indicated below.

Element	Field Name Prefix
SharedPreferences	PREF_
Bundle	BUNDLE_
Fragment Arguments	ARGUMENT_
Intent Extra	EXTRA_
Intent Action	ACTION_

Note that the arguments of a Fragment – Fragment.getArguments() – are also a Bundle. However, because this is a quite common use of Bundles, we define a different prefix for them.

Example:

// Note the value of the field is the same as the name to avoid duplication issues

static final String PREF_EMAIL = “PREF_EMAIL”;

static final String BUNDLE_AGE = “BUNDLE_AGE”;

static final String ARGUMENT_USER_ID = “ARGUMENT_USER_ID”;

// Intent-related items use full package name as value

static final String EXTRA_SURNAME = “com.myapp.extras.EXTRA_SURNAME”;

static final String ACTION_OPEN_USER = “com.myapp.action.ACTION_OPEN_USER”;

Arguments in Fragments and Activities

When data is passed into an Activity or Fragment via an Intent or a Bundle, the keys for the different values must follow the rules described in the section above.

When an Activity or Fragment expects arguments, it should provide a public static method that facilitates the creation of the relevant Intent or Fragment.

In the case of Activities the method is usually called getStartIntent():

public static Intent getStartIntent(Context context, User user) {

Intent intent = new Intent(context, ThisActivity.class);

intent.putParcelableExtra(EXTRA_USER, user);

return intent;

}

For Fragments it is named newInstance() and handles the creation of the Fragment with the right arguments:

public static UserFragment newInstance(User user) {

UserFragment fragment = new UserFragment();

Bundle args = new Bundle();

args.putParcelable(ARGUMENT_USER, user);

fragment.setArguments(args)

return fragment;

}

Note 1: These methods should go at the top of the class before onCreate().

Note 2: If we provide the methods described above, the keys for extras and arguments should be private because there is no need for them to be exposed outside the class.

Line length limit

Code lines should not exceed 100 characters. If the line is longer than this limit there are usually two options to reduce its length:

Extract a local variable or method (preferable).
Apply line-wrapping to divide a single line into multiple ones.

There are two exceptions where it is possible to have lines longer than 100:

Lines that are not possible to split, e.g. long URLs in comments.
package and import statements.

Line-wrapping strategies

There isn’t an exact formula that explains how to line-wrap and quite often different solutions are valid. However, there are a few rules that can be applied to common cases.

-Break at operators

When the line is broken at an operator, the break comes before the operator. For example:

int longName = anotherVeryLongVariable + anEvenLongerOne – thisRidiculousLongOne

+ theFinalOne;

Assignment Operator Exception

An exception to the break at operators rule is the assignment operator =, where the line break should happen after the operator.

int longName =

anotherVeryLongVariable + anEvenLongerOne – thisRidiculousLongOne + theFinalOne;

Method chain case

When multiple methods are chained in the same line – for example when using Builders – every call to a method should go in its own line, breaking the line before the .

Picasso.with(context).load(“http://ribot.co.uk/images/sexyjoe.jpg”).into(imageView);

Picasso.with(context)

.load(“http://ribot.co.uk/images/sexyjoe.jpg”)

.into(imageView);

Long parameters case

When a method has many parameters or its parameters are very long, we should break the line after every comma ,

loadPicture(context, “http://ribot.co.uk/images/sexyjoe.jpg”, mImageViewProfilePicture, clickListener, “Title of the picture”);

loadPicture(context,

“http://ribot.co.uk/images/sexyjoe.jpg”,

mImageViewProfilePicture,

clickListener,

“Title of the picture”);

RxJava chains styling

Rx chains of operators require line-wrapping. Every operator must go in a new line and the line should be broken before the .

public Observable<Location> syncLocations() {

return mDatabaseHelper.getAllLocations()

.concatMap(new Func1<Location, Observable<? extends Location>>() {

4. XML style rules

Use self closing tags

When an XML element doesn’t have any contents, you must use self closing tags.

This is good:

<TextView

android:id=“@+id/text_view_profile”

android:layout_width=“wrap_content”

android:layout_height=“wrap_content” />

This is bad :

<!– Don\’t do this! –>

<TextView

android:id=“@+id/text_view_profile”

android:layout_width=“wrap_content”

android:layout_height=“wrap_content” >

</TextView>

Resources naming

Resource IDs and names are written in lowercase_underscore.

ID naming

IDs should be prefixed with the name of the element in lowercase underscore. For example:

Element	Prefix
TextView	text_
ImageView	image_
Button	button_
Menu	menu_

Image view example:

<ImageView

android:id=“@+id/image_profile”

android:layout_width=“wrap_content”

android:layout_height=“wrap_content” />

Menu example:

<menu>

<item

android:id=“@+id/menu_done”

android:title=“Done” />

</menu>

Strings

String names start with a prefix that identifies the section they belong to. For example registration_email_hint or registration_name_hint. If a string doesn’t belong to any section, then you should follow the rules below:

Prefix	Description
error_	An error message
msg_	A regular information message
title_	A title, i.e. a dialog title
action_	An action such as “Save” or “Create”

Styles and Themes

Unlike the rest of the resources, style names are written in UpperCamelCase.

Attributes ordering in XML

As a general rule, you should try to group similar attributes together. A good way of ordering the most common attributes is:

View Id
Style
Layout width and layout height
Other layout attributes, sorted alphabetically
Remaining attributes sorted alphabetically

5. Tests style rules

Unit tests

Test classes should match the name of the class the tests are targeting, followed by Test. For example, if we create a test class that contains tests for the DatabaseHelper, we should name it DatabaseHelperTest.

Test methods are annotated with @Test and should generally start with the name of the method that is being tested, followed by a precondition and/or expected behaviour.

Template: @Test void methodNamePreconditionExpectedBehaviour()
Example: @Test void signInWithEmptyEmailFails()

Precondition and/or expected behaviour may not always be required if the test is clear enough without them.

Sometimes a class may contain a large number of methods, that at the same time require several tests for each method.

In this case, it’s recommendable to split up the test class into multiple ones.

For example, if the DataManager contains a lot of methods we may want to divide it into DataManagerSignInTest, DataManagerLoadUsersTest, etc. Generally, you will be able to see what tests belong together because they have common test fixtures.

Espresso tests

Every Espresso test class usually targets an Activity, therefore the name should match the name of the targeted Activity followed by Test, e.g. SignInActivityTest

When using the Espresso API it is a common practice to place chained methods in new lines.

onView(withId(R.id.view))

.perform(scrollTo())

.check(matches(isDisplayed()))

@Override

public Observable<? extends Location> call(Location location) {

return mRetrofitService.getLocation(location.id);

}

})

.retry(new Func2<Integer, Throwable, Boolean>() {

@Override

public Boolean call(Integer numRetries, Throwable throwable) {

return throwable instanceof RetrofitError;

}

});

}

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Best Practices in Android Coding: A Developer's Guide