New projects should follow the Android Gradle project structure that is defined on the Android Gradle plugin user guide.
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 file names are written in lowercase_underscore.
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 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 |
activity_user_profile.xml |
| Fragment | SignUpFragment |
fragment_sign_up.xml |
| Dialog | ChangePasswordDialog |
dialog_change_password.xml |
| AdapterView item | --- | item_person.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_.
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.
Resource files in the values folder should be plural, e.g. strings.xml, styles.xml, colors.xml, dimens.xml, attrs.xml
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)
See alternatives here.
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
We 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)
This is bad: import foo.*;
This is good: import foo.Bar;
See more info here
Fields should be defined at the top of the file and they should follow the naming rules listed below.
- 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 singleton;
int packagePrivate;
private int private;
protected int protected;
}| Good | Bad |
|---|---|
XmlHttpRequest |
XMLHTTPRequest |
getCustomerId |
getCustomerID |
String url |
String URL |
long id |
long ID |
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);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.
if (condition) body();This is bad:
if (condition)
body(); // bad!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.
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;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)
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
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);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 String mTitle;
private TextView mTextViewTitle;
public void setTitle(String title) {
mTitle = title;
}
@Override
public void onCreate() {
...
}
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() {}
}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 are 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);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 a 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";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 not need for them to be exposed outside the class.
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.
packageandimportstatements.
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("https://tango.agency/images/hello.jpg").into(imageView);Picasso.with(context)
.load("https://tango.agency/images/hello.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, "https://tango.agency/images/hello.jpg", mImageViewProfilePicture, clickListener, "Title of the picture");loadPicture(context,
"https://tango.agency/images/hello.jpg",
mImageViewProfilePicture,
clickListener,
"Title of the picture");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>>() {
@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;
}
});
}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>Resource IDs and names are written in lowercase_underscore.
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>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" |
Unless the rest of resources, style names are written in UpperCamelCase.
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
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 amount 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.
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()))Copyright 2015 Ribot Ltd.
Modifications copyright 2017 Tango Agency
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.