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Java style rules
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;
}| 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
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
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 {
// Constants
public static final String TAG = "MainActivity";
// Fields
private String mTitle;
private TextView mTextViewTitle;
// Constructor
// override methodds
@Override
public void onCreate() {
...
}
// public methods
public void setTitle(String title){
}
// private methods
private void setUpView() {
...
}
// inner classes
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;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://developer.android.com/_static/images/android/touchicon-180.png", mImageViewProfilePicture, clickListener, "Title of the picture");loadPicture (
context,
"https://developer.android.com/_static/images/android/touchicon-180.png",
mImageViewProfilePicture,
clickListener,
"Title of the picture"
);Coding Guidelines
Android IMP