It can be tempting to write code that completely ignores an exception, such as:
void setServerPort(String value) {
try {
serverPort = Integer.parseInt(value);
} catch (NumberFormatException e) { }
}Do not do this. While you may think your code will never encounter this error condition or that it is not important to handle it, ignoring exceptions as above creates mines in your code for someone else to trigger some day. You must handle every Exception in your code in a principled way; the specific handling varies depending on the case.
Anytime somebody has an empty catch clause they should have a creepy feeling. There are definitely times when it is actually the correct thing to do, but at least you have to think about it. In Java you can't escape the creepy feeling. -James Gosling
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
}
}- Catch the Exception and throw a new RuntimeException. This is dangerous, so do it only if you are positive that if this error occurs the appropriate thing to do is crash.
/** Set port. If value is not a valid number, die. */
void setServerPort(String value) {
try {
serverPort = Integer.parseInt(value);
} catch (NumberFormatException e) {
throw new RuntimeException("port " + value " is invalid, ", e);
}
}- As a last resort, if you are confident that 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.
}
}Try to avoid doing 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!
}Multiple Exceptions can mean multiple errors, it can be a network error, a parsing, etc, so ideally we should act accordingly.
See more reasons why and some alternatives here
This is bad: import foo.*;
This is good: import foo.Bar;
See more info here
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+ .
Special prefixes or suffixes, like those seen in the examples name_, mName, s_name and kName, are not used.
Package names are all lowercase, with consecutive words simply concatenated together (no underscores). For example, com.example.deepspace, not com.example.deepSpace or com.example.deep_space.
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).
There are no specific rules or even well-established conventions for naming annotation types.
Test classes are named starting with the name of the class they are testing, and ending with Test. For example, HashTest or HashIntegrationTest.
Method names are written in lowerCamelCase.
Method names are typically verbs or verb phrases. For example, sendMessage or stop.
Underscores may appear in JUnit test method names to separate logical components of the name. One typical pattern is test<MethodUnderTest>_<state>, for example testPop_emptyStack. There is no One Correct Way to name test methods.
Constant names use CONSTANT_CASE: all uppercase letters, with words separated by underscores. 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.
Non-constant field names (static or otherwise) are written in lowerCamelCase.
These names are typically nouns or noun phrases. For example, computedValues or index.
The greater the scope, more indicative must be the variable name. For example a variable with a loop scope could be called just i, while on function scope it could be index, and a class member variable would be currentIndex.
Parameter names are written in lowerCamelCase.
One-character parameter names in public methods should be avoided.
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.
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).
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 {
private String title;
private TextView textViewTitle;
public void setTitle(String title) {
this.title = 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";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, Conding Style specifies the following (nearly) deterministic scheme.
Beginning with the prose form of the name:
-
Convert the phrase to plain ASCII and remove any apostrophes. For example, "Müller's algorithm" might become "Muellers algorithm".
-
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. -
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 -
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.
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!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("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");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 databaseHelper.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;
}
});
}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 one line, unless the line reaches the maximum line length, and the field declaration on the line bellow.
@Nullable @Mock
DataManager dataManager;#XML style rules
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
If in doubt, default to the Java Coding Conventions such as:
- use of camelCase for names (and avoid underscore in names)
- types start with upper case
- methods and properties start with lower case
- use 4 space indentation
- public functions should have documentation such that it appears in Kotlin Doc
There is a space before colon where colon separates type and supertype and there's no space where colon separates instance and type:
interface Foo<out T : Any> : Bar {
fun foo(a: Int): T
}In lambda expressions, spaces should be used around the curly braces, as well as around the arrow which separates the parameters from the body. Whenever possible, a lambda should be passed outside of parentheses.
list.filter { it > 10 }.map { element -> element * 2 }In lambdas which are short and not nested, it's recommended to use the it convention instead of declaring the parameter explicitly. In nested lambdas with parameters, parameters should be always declared explicitly.
Classes with a few arguments can be written in a single line:
class Person(id: Int, name: String)Classes with longer headers should be formatted so that each primary constructor argument is in a separate line with indentation. Also, the closing parenthesis should be on a new line. If we use inheritance, then the superclass constructor call or list of implemented interfaces should be located on the same line as the parenthesis:
class Person(
id: Int,
name: String,
surname: String
) : Human(id, name) {
// ...
}For multiple interfaces, the superclass constructor call should be located first and then each interface should be located in a different line:
class Person(
id: Int,
name: String,
surname: String
) : Human(id, name),
KotlinMaker {
// ...
}Constructor parameters can use either the regular indent or the continuation indent (double the regular indent).
If a function returns Unit, the return type should be omitted:
fun foo() { // ": Unit" is omitted here
}In some cases functions with no arguments might be interchangeable with read-only properties. Although the semantics are similar, there are some stylistic conventions on when to prefer one to another.
Prefer a property over a function when the underlying algorithm:
- does not throw
- has a O(1) complexity
- is cheap to calculate (or caсhed on the first run)
- returns the same result over invocations
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 2017 Rocket Chat
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.