Android API Guidelines

This document is intended to be a general guide for developers to use to adhere to the general principles that the API Council enforces in API reviews. Besides understanding and following these guidelines when writing APIs, developers should also run the API Lint tool, which encodes many of these rules in checks that it runs against APIs and flags errors that it sees. Think of this as the guide to the rules that are obeyed by that Lint Tool, plus guidelines and general advice on rules that cannot be easily codified into that tool.

API Lint tool

The API Lint tool is part of the Android source tree here

API Lint runs as part of the platform build. You can run it manually with:

# Run lint on public API
$ make api-stubs-docs-api-lint

# Run lint on system API
$ make system-api-stubs-docs-api-lint

API rules

One of the difficulties in concrete rules is applying them to a platform that was developed without strict guidelines from the beginning, so some of the existing APIs may not adhere. In some cases, the right choice might be to go with what is consistent with APIs in the same general area of the code, rather than in the ideal rules laid out herein.

The rules are a work in progress and will be added to in the future as other patterns emerge from future API reviews.

Coding style [S] {#style .numbered}

This category pertains to the general coding style that developers should use, especially in the public API.

Follow standard Java coding conventions, except where noted {#style-conventions .numbered}

There are Android coding conventions posted for external contributors here:

https://source.android.com/source/code-style.html

Overall, we tend to follow standard Java coding conventions.

Please also review the Kotlin-Java interop guide for best practices related to writing Kotlin-friendly APIs in Java. Some of these guidelines are reflected in the recommendations on this site; however, the link may contain slightly newer information that has not yet been propagated.

Classes [CL] {#classes .numbered}

These are rules about classes, interfaces, and inheritance.

Inherit new public classes from the appropriate base class {#classes-inheritance .numbered}

Inheritance exposes API in your subclass which may not be appropriate. For example, a new public subclass of FrameLayout will look like a FrameLayout (plus the new functionality/API). If that inherited API is not appropriate for your use case, inherit from something further up the tree (for example, ViewGroup or even View, instead of FrameLayout).

Use the base collections classes {#classes-collections .numbered}

Whether taking a collection as an argument or returning it as a value, always prefer the base class over the specific implementation (e.g. return List<Foo> rather than ArrayList<Foo>).

Abstract classes versus interfaces {#classes-interfaces .numbered}

Java 8 adds support for default interface methods, which allows API designers to add methods to interfaces while maintaining binary compatibility.

For libraries targeting Java 7 or earlier, API designers should prefer abstract classes in cases where multiple inheritance is not required and flexibility to add methods in later versions is desirable, ex. callbacks. Interfaces may still be used in cases where flexibility is not required, ex. single-method listeners.

For platform code and libraries targeting Java 8 or later, API designers should prefer interfaces in cases where the default implementation is stateless -- that is, default interface methods can be implemented as calls to other interface methods. Abstract classes must be used in cases where a constructor or internal state is required by the default implementation.

In both cases, API designers may choose to leave a single method abstract to simplify usage as a lambda.

public interface AnimationEndCallback {
  // Always called, must be implemented.
  public void onFinished(Animation anim);
  // Optional callbacks.
  public default void onStopped(Animation anim) { }
  public default void onCanceled(Animation anim) { }
}

@see Use Listener for single-method interfaces only, Callback otherwise

Class names should reflect what they extend {#classes-subclass-naming .numbered}

For example, classes which extend “Service” should be named “FooService” for clarity.

public class IntentHelper extends Service {}
public class IntentService extends Service {}

Do not use CompletableFuture or Future {#classes-avoid-completablefuture .numbered}

java.util.concurrent.CompletableFuture has a large API surface that permits arbitrary mutation of the future's value and has error-prone defaults .

Conversely, java.util.concurrent.Future is missing non-blocking listening, making it hard to use with asynchronous code.

In platform code, prefer a combination of a completion callback, Executor, and if the API supports cancellation CancellationSignal.

public interface LoadFooCallback {
  void onSuccess(Foo result);
  void onFailure(Throwable t);
}

public void asyncLoadFoo(android.os.CancellationSignal cancellationSignal,
    Executor callbackExecutor, LoadFooCallback callback);

In libraries and apps, prefer Guava's ListenableFuture.

public com.google.common.util.concurrent.ListenableFuture<Foo> asyncLoadFoo();

If you are targeting Kotlin, prefer suspend functions.

suspend fun asyncLoadFoo(): Foo

Do not use Optional {#classes-avoid-optional .numbered}

While Optional can have advantages in some API surfaces it is inconsistent with the existing Android API surface area. @Nullable and @NonNull provide tooling assistance for null safety and Kotlin enforces nullability contracts at the compiler level, making Optional unnecessary API cruft.

Use private constructors for non-instantiable classes {#classes-non-instantiable .numbered}

Classes that can only be created by Builders, classes containing only constants or static methods, or otherwise non-instantiable classes should include at least one private constructor to prevent instantiation via the default no-arg constructor.

public final class Log {
  // Not instantiable.
  private Log() {
  }
}

Fields [F] {#fields .numbered}

These rules are about public fields on classes.

Do not expose raw fields {#avoid-raw-fields .numbered}

In general, non-final fields should be private and accessible only via public getters and setters.

Rare exceptions include simple data structures where there will never be a need to enhance the functionality of specifying or retrieving a field. In such cases, the fields should be named using standard variable naming conventions, ex. Point.x and Point.y.

Exposed fields should be marked final {#public-final-fields .numbered}

Raw fields are strongly discouraged (@see Do not expose raw fields). But in the rare situation where a field is exposed as a public field, mark that field final.

Use public instead of protected {#public-fields .numbered}

@see Use public instead of protected

Constants [C] {#constants .numbered}

These are rules about public constants.

Flag constants should be non-overlapping int or long values {.numbered}

“Flags” implies bits that can be combined into some union value. If this is not the case, do not call the variable/constant “flag”.

public static final int FLAG_SOMETHING = 2;
public static final int FLAG_SOMETHING = 3;
public static final int FLAG_PRIVATE = 1 << 2;
public static final int FLAG_PRESENTATION = 1 << 3;

static final constants should use all-cap, underscore-separated naming convention {#constant-naming .numbered}

All words in the constant should be capitalized and multiple words should be separated by _. For example:

public static final int fooThing = 5
public static final int FOO_THING = 5

Use standard prefixes for constants {#3-use-standard-prefixes-for-constants .numbered}

Many of the constants used in Android are for standard things, such as flags, keys, and actions. These constants should have standard prefixes to make them more identifiable as these things.

For example, intent extras should start with EXTRA_. Intent actions should start with ACTION_. Constants used with Context.bindService() should start with BIND_.

Naming and scoping of key constants {#4-naming-and-scoping-of-key-constants .numbered}

String constants values should be consistent with the constant name itself, and should generally be scoped to the package or domain. For example:

public static final String FOO_THING = foo

is neither named consistently nor appropriately scoped. Instead, consider:

public static final String FOO_THING = android.fooservice.FOO_THING

Prefixes of “android” in scoped string constants are reserved for the Android Open Source Project.

Intent actions and extras should be namespaced using the package name they are defined within.

package android.foo.bar {

public static final String ACTION_BAZ = android.foo.bar.action.BAZ

public static final String EXTRA_BAZ = android.foo.bar.extra.BAZ

}

Use public instead of protected {#public-constants .numbered}

@see Use public instead of protected

Use consistent prefixes {#use-consistent-prefixes .numbered}

Related constants should all start with the same prefix. For example, for a set of constants to use with flag values:

public static final int SOME_VALUE = 0x01;

public static final int SOME_OTHER_VALUE = 0x10;

public static final int SOME_THIRD_VALUE = 0x100;
public static final int FLAG_SOME_VALUE = 0x01;

public static final int FLAG_SOME_OTHER_VALUE = 0x10;

public static final int FLAG_SOME_THIRD_VALUE = 0x100;

@see Use standard prefixes for constants

Use consistent resource names {#resource-names .numbered}

Public identifiers, attributes, and values must be named using the camelCase naming convention, e.g. @id/accessibilityActionPageUp or @attr/textAppearance, similar to public fields in Java.

In some cases, a public identifier or attribute may include a common prefix separated by an underscore:

  • Platform config values such as @string/config_recentsComponentName in config.xml
  • Layout-specific view attributes such as @attr/layout_marginStart in attrs.xml

Public themes and styles must follow the hierarchical PascalCase naming convention, e.g. @style/Theme.Material.Light.DarkActionBar or @style/Widget.Material.SearchView.ActionBar, similar to nested classes in Java.

Layout and drawable resources should not be exposed as public APIs. If they must be exposed, however, then public layouts and drawables must be named using the under_score naming convention, e.g. layout/simple_list_item_1.xml or drawable/title_bar_tall.xml.

When min/max constants could change, make them dynamic {#min-max-constants .numbered}

If a MIN_FOO or MAX_FOO constant could change in the future, consider making them dynamic methods instead.

CameraManager.MAX_CAMERAS
CameraManager.getMaxCameras()

Consider forward-compatibility for callbacks {#callbacks-compatibility .numbered}

Constants defined in future API versions are not known to apps that target older APIs. For this reason, constants delivered to apps should take into consideration that app’s target API version and map newer constants to a consistent value. Consider the following scenario:

Hypothetical SDK source:

// Added in API level 22
public static final int STATUS_SUCCESS = 1;
public static final int STATUS_FAILURE = 2;
// Added in API level 23
public static final int STATUS_FAILURE_RETRY = 3;
// Added in API level 26
public static final int STATUS_FAILURE_ABORT = 4;

Hypothetical app with targetSdkVersion="22":

if (result == STATUS_FAILURE) {
  // Oh no!
} else {
  // Success!
}

In this case, the app was designed within the constraints of API level 22 and made a (somewhat) reasonable assumption that there were only two possible states. If the app receives the newly-added STATUS_FAILURE_RETRY, however, it will interpret this as success.

Methods that return constants can safely handle cases like this by constraining their output to match the API level targeted by the app:

private int mapResultForTargetSdk(Context context, int result) {
  int targetSdkVersion = context.getApplicationInfo().targetSdkVersion;
  if (targetSdkVersion < 26) {
    if (result == STATUS_FAILURE_ABORT) {
      return STATUS_FAILURE;
    }
    if (targetSdkVersion < 23) {
      if (result == STATUS_FAILURE_RETRY) {
        return STATUS_FAILURE;
      }
    }
  }
  return result;
}

It’s unreasonable to expect developers and their published applications to be clairvoyant. If you define an API with an UNKNOWN or UNSPECIFIED constant that looks like a catch-all, developers will assume that the published constants when they wrote their app are exhaustive. If you’re unwilling to set this expectation, reconsider whether a catch-all constant is a good idea for your API.

Integer or String constant? {#integer-or-string-constant .numbered}

Use integer constants if the namespace for values is not extensible outside of your package. Use string constants if the namespace is shared or can be extended by code outside of your package.

Methods [M] {#methods .numbered}

These are rules about various specifics in methods, around parameters, method names, return types, and access specifiers.

Time {#time .numbered}

Methods expressing durations should be named duration, and use long type {#long-durations .numbered}

If a time value expresses the duration of time involved, name the parameter “duration”, not “time”. Durations in the platform use milliseconds as long (not int).

ValueAnimator.setTime(long);
ValueAnimator.setDuration(long);

Exceptions: “timeout” is appropriate when the duration specifically applies to a timeout value. And “time” is appropriate when it is referring to a specific point in time, not a duration.

Methods expressing units of time should prefer non-abbreviated shorthand for unit names {#unit-names .numbered}

public void setIntervalNs(long intervalNs);

public void setTimeoutUs(long timeoutUs);
public void setIntervalNanos(long intervalNanos);

public void setTimeoutMicros(long timeoutMicros);

Units of measurement {#measurement .numbered}

For all methods expressing a unit of measurement other than time, prefer CamelCased SI unit prefixes.

public  long[] getFrequenciesKhz();

public  float getStreamVolumeDb();

Put optional parameters at end in overloads {#optional-params-last .numbered}

If you have overloads of a method with optional parameters, keep those parameters at the end and keep consistent ordering with the other parameters:

public int doFoo(boolean flag);

public int doFoo(int id, boolean flag);
public int doFoo(boolean flag);

public int doFoo(boolean flag, int id);

When adding overloads for optional arguments, the behavior of the simpler methods should behave in exactly the same way as if default arguments had been provided to the more elaborate methods.

Corollary: Don’t overload methods other than to add optional arguments or to accept different types of arguments if the method is polymorphic. If the overloaded method does something fundamentally different, then give it a new name.

Note: The guideline on placement of single abstract method parameters (ex. Runnable, listeners) overrides this guideline. In cases where a developer could reasonably expected to write the body of a SAM class as a lambda, the SAM class parameter should be placed last.

The most distinctive and identifying method parameters should be first {#distinctive-params-first .numbered}

If you have a method with multiple parameters, put the most relevant ones first. Parameters that specify flags and other options are less important than those that describe the object that is being acted upon. If there is a completion callback, put it last.

public void openFile(int flags, String name);

public void openFileAsync(OnFileOpenedListener listener, String name, int flags);

public void setFlags(int mask, int flags);
public void openFile(String name, int flags);

public void openFileAsync(String name, int flags, OnFileOpenedListener listener);

public void setFlags(int flags, int mask);

@see Put optional parameters at end in overloads

Builders {#builders .numbered}

Builder classes must return the builder {#builders-return-builder .numbered}

Builder classes must enable method chaining by returning the Builder object (e.g. this) from every method. Additional built objects should be passed as arguments -- do not return a different object’s builder. For example:

public static class Builder {
  public void setDuration(long);
  public void setFrequency(int);
  public DtmfConfigBuilder addDtmfConfig();
  public Tone build();
}
public class Tone {
  public static class Builder {
    public Builder setDuration(long);
    public Builder setFrequency(int);
    public Builder addDtmfConfig(DtmfConfig);
    public Tone build();
  }
}

All arguments to builder constructors must be required (e.g. @NonNull) {#builders-nonnull-constructors .numbered}

Optional, e.g. @Nullable, arguments should be moved to setter methods. The Builder constructor should throw an NullPointerException (consider using Preconditions.checkNotNull) if any required arguments are not specified.

Builder classes should be a static inner class of their built type {#builders-static-inner .numbered}

For the sake of logical organization within a package, builder classes should typically be exposed as inner classes of their built types, ex. Tone.Builder rather than ToneBuilder.

Builder setters should take @Nullable arguments if the builder has newBuilder constructor {#builders-nullable-setters .numbered}

Resetting is essential if there is newBuilder functionality where you can only re-build an instance based on an existing one. If newBuilder does not exist then you can have either @Nullable or @NonNullable arguments.

Builder setters may be provided for mutable properties where setters are available on the build class {#builders-mutable-setters .numbered}

If your class has mutable properties and needs a Builder class, first ask yourself whether your class should actually have mutable properties.

Next, if you're certain that you need mutable properties, decide which of the following scenarios works better for your expected use case:

  1. The built object should be immediately usable, thus setters should be provided for all relevant properties whether mutable or immutable.

    map.put(key, new Value.Builder(requiredValue)
        .setImmutableProperty(immutableValue)
        .setUsefulMutableProperty(usefulValue)
        .build());
    
  2. Some additional calls may need to be made before the built object can be useful, thus setters should not be provided for mutable properties.

    Value v = new Value.Builder(requiredValue)
        .setImmutableProperty(immutableValue)
        .build();
    v.setUsefulMutableProperty(usefulValue)
    Result r = v.performSomeAction();
    Key k = callSomeMethod(r);
    map.put(k, v);
    

Don't mix the two scenarios.

Value v = new Value.Builder(requiredValue)
    .setImmutableProperty(immutableValue)
    .setUsefulMutableProperty(usefulValue)
    .build();
Result r = v.performSomeAction();
Key k = callSomeMethod(r);
map.put(k, v);

Builder setters must have corresponding getters on the built class {#builders-symmetric-setters .numbered}

public class Tone {
  public static class Builder {
    public Builder setDuration(long);
    public Builder setFrequency(int);
    public Builder addDtmfConfig(DtmfConfig);
    public Tone build();
  }
}
public class Tone {
  public static class Builder {
    public Builder setDuration(long);
    public Builder setFrequency(int);
    public Builder addDtmfConfig(DtmfConfig);
    public Tone build();
  }

  public long getDuration();
  public int getFrequency();
  public @NonNull List<DtmfConfig> getDtmfConfigs();
}

Do not expose internal locks {#avoid-synchronized .numbered}

Methods in the public API should not use the synchronized keyword. This keyword causes your object/class to be used as the lock, and since it’s exposed to others, you may encounter unexpected side effects if other code outside your class starts using it for locking purposes.

Instead, perform any required locking against an internal, private object.

Use is prefix for boolean accessor methods {#boolean-methods .numbered}

This is the standard naming convention for boolean methods and variables in Java.

Setter methods must have the set prefix, as in:

// Visibility is a direct property. The object "is" visible:
void setVisible(boolean visible);
boolean isVisible();

In cases where the boolean describes inclusion or ownership of a property, use has rather than is:

// Transient state is an indirect property used to track state
// related to the object. The object is not transient; rather,
// the object "has" transient state associated with it:
void setHasTransientState(boolean hasTransientState);
boolean hasTransientState();

Some alternative prefixes that may be more suitable include can and should:

// "Can" describes a behavior that the object may provide,
// and here is more concise than setRecordingEnabled or
// setRecordingAllowed. The object "can" record:
void setCanRecord(boolean canRecord);
boolean canRecord();

// "Should" describes a hint or property that is not strictly
// enforced, and here is more explicit than setFitWidthEnabled.
// The object "should" fit width:
void setShouldFitWidth(boolean shouldFitWidth);
boolean shouldFitWidth();

As a last resort, methods that toggle behaviors or features may use the is prefix and Enabled suffix:

// "Enabled" describes the availability of a property, and is
// more appropriate here than "can use" or "should use" the
// property:
void setWiFiRoamingSettingEnabled(boolean enabled)
boolean isWiFiRoamingSettingEnabled()

Generally, method names should be written as questions that are answered by the return value.

Bitmask accessors {#bitmask-accessors .numbered}

See Use @IntDef for bitmask flags for API guidelines regarding defining bitmask flags.

Setters

Two setter methods should be provided: one that takes a full bitstring and overwrites all existing flags and another that takes a custom bitmask to allow more flexibility.

/**
 * Sets the state of all scroll indicators.
 * <p>
 * See {@link #setScrollIndicators(int, int)} for usage information.
 *
 * @param indicators a bitmask of indicators that should be enabled, or
 *                   {@code 0} to disable all indicators
 * @see #setScrollIndicators(int, int)
 * @see #getScrollIndicators()
 */
public void setScrollIndicators(@ScrollIndicators int indicators);

/**
 * Sets the state of the scroll indicators specified by the mask. To change
 * all scroll indicators at once, see {@link #setScrollIndicators(int)}.
 * <p>
 * When a scroll indicator is enabled, it will be displayed if the view
 * can scroll in the direction of the indicator.
 * <p>
 * Multiple indicator types may be enabled or disabled by passing the
 * logical OR of the desired types. If multiple types are specified, they
 * will all be set to the same enabled state.
 * <p>
 * For example, to enable the top scroll indicator:
 * {@code setScrollIndicators(SCROLL_INDICATOR_TOP, SCROLL_INDICATOR_TOP)}
 * <p>
 * To disable the top scroll indicator:
 * {@code setScrollIndicators(0, SCROLL_INDICATOR_TOP)}
 *
 * @param indicators a bitmask of values to set; may be a single flag,
 *                   the logical OR of multiple flags, or 0 to clear
 * @param mask a bitmask indicating which indicator flags to modify
 * @see #setScrollIndicators(int)
 * @see #getScrollIndicators()
 */
public void setScrollIndicators(@ScrollIndicators int indicators, @ScrollIndicators int mask);

Getters

One getter should be provided to obtain the full bitmask.

/**
 * Returns a bitmask representing the enabled scroll indicators.
 * <p>
 * For example, if the top and left scroll indicators are enabled and all
 * other indicators are disabled, the return value will be
 * {@code View.SCROLL_INDICATOR_TOP | View.SCROLL_INDICATOR_LEFT}.
 * <p>
 * To check whether the bottom scroll indicator is enabled, use the value
 * of {@code (getScrollIndicators() & View.SCROLL_INDICATOR_BOTTOM) != 0}.
 *
 * @return a bitmask representing the enabled scroll indicators
 */
@ScrollIndicators
public int getScrollIndicators();

Use public instead of protected {#avoid-protected .numbered}

Always prefer public to protected in public API. Protected access ends up being painful in the long run, because implementers have to override to implement the functionality in cases where external access would have been just as good.

Implement neither or both of equals() and hashCode() {#equals-and-hashcode .numbered}

If you override one, you must override the other.

Use createFoo when returning newly created objects {#create-methods .numbered}

Use create, not get, for methods that will create return values.

When the method will create an object to return, make that clear in the method name.

public FooThing getFooThing() {
  return new FooThing();
}
public FooThing createFooThing() {
  return new FooThing();
}

Methods accepting File objects should also accept streams {#files-and-streams .numbered}

Data storage locations on Android are not always files on disk. For example, content passed across user boundaries is represented as content:// Uris. To enable processing of various data sources, APIs which accept File objects should also accept InputStream and/or OutputStream.

public void setDataSource(File file)
public void setDataSource(InputStream stream)

Take and return raw primitives instead of boxed versions {#raw-primitives .numbered}

If you need to communicate missing or null values, consider using -1, Integer.MAX_VALUE, or Integer.MIN_VALUE.

public java.lang.Integer getLength()
public void setLength(java.lang.Integer)
public int getLength()
public void setLength(int value)

Avoiding class equivalents of primitive types avoids the memory overhead of these classes, method access to values, and, more importantly, autoboxing that comes from casting between primitive and object types. Avoiding these behaviors saves on memory and on temporary allocations that can lead to expensive and more frequent garbage collections.

Use annotations to clarify valid parameter and return values {#annotations .numbered}

Developer annotations were added to help clarify allowable values in various situations. This makes it easier for tools to help developers when they supply incorrect values (for example, passing an arbitrary int when the framework requires one of a specific set of constant values). Use any and all of the following annotations when appropriate:

Nullability

@Nullable: Indicates that a given return value, parameter, or field can be null:

@Nullable
public String getName()

public void setName(@Nullable String name)

@NonNull: Indicates that a given return value, parameter, or field cannot be null. Marking things as @Nullable is relatively new to Android, so most of Android's API methods are not consistently documented. Therefore we have a tri-state of “unknown, @Nullable, @NonNull” which is why @NonNull is part of the API guidelines.:

@NonNull
public String getName()

public void setName(@NonNull String name)

Users of IntelliJ will want to set up their IDE to understand these annotations (for code-completion and error checking purposes). To do this:

  • Open Preferences...
  • Search for “Nullable problems”
  • Select the highlighted item (under “Probable bugs”)
  • Click the “Configure annotations...” button
  • Add the item “android.annotation.Nullable” to the top (Nullable) section
  • Add the item “android.annotation.NonNull” to the bottom (NotNull) section

Existing “not really nullable” methods: Existing methods in the API without a declared @Nullable annotation if the method can return null under specific, obvious circumstances to the developer (e.g. findViewById()) should be annotated @Nullable once we have migration tools in place, and companion @NotNull requireFoo() methods that throw IllegalStateException should be added for developers who do not want to null check.

Resources

Resource identifiers: Integer parameters that denote ids for specific resources should be annotated with the appropriate resource-type definition. There is an annotation for every type of resource, such as @StringRes, @ColorRes, and @AnimRes, in addition to the catch-all @AnyRes. For example:

public void setTitle(@StringRes int resId)

@IntDef for constant sets

Magic constants: String and int parameters that are meant to receive one of a finite set of possible values denoted by public constants should be annotated appropriately with @StringDef or @IntDef. These annotations allow you to create a new annotation that you can use that works like a typedef for allowable parameters. For example:

/** @hide */
@IntDef(prefix = {“NAVIGATION_MODE_”}, value = {
  NAVIGATION_MODE_STANDARD,
  NAVIGATION_MODE_LIST,
  NAVIGATION_MODE_TABS
})
@Retention(RetentionPolicy.SOURCE)
public @interface NavigationMode {}

public static final int NAVIGATION_MODE_STANDARD = 0;
public static final int NAVIGATION_MODE_LIST = 1;
public static final int NAVIGATION_MODE_TABS = 2;

@NavigationMode
public int getNavigationMode();
public void setNavigationMode(@NavigationMode int mode);

Notice that the constants must be defined in the class that will use them, not in a subclass or interface.

@IntDef for bitmask flags

The annotation can also specify that the constants are flags, and can be combined with & and I:

/** @hide */
@IntDef(flag = true, prefix = { FLAG_ }, value = {
  FLAG_USE_LOGO,
  FLAG_SHOW_HOME,
  FLAG_HOME_AS_UP,
});
@Retention(RetentionPolicy.SOURCE)
public @interface DisplayOptions {}

@StringDef for string constant sets

There is also the @StringDef annotation, which is exactly like @IntDef above, but for String constants. You can include multiple “prefix” values which are used to automatically emit documentation for all values.

@see Support Annotations (Ignore the “support” part of this document, and don’t use it in the includes; the framework has internal annotations that you should use directly. The article is intended for external developers that would be using the support library version of annotations instead).

@SdkConstant for SDK constants

@SdkConstant Annotate public fields when they are one of these SdkConstant values: ACTIVITY_INTENT_ACTION, BROADCAST_INTENT_ACTION, SERVICE_ACTION, INTENT_CATEGORY, FEATURE.

@SdkConstant(SdkConstantType.ACTIVITY_INTENT_ACTION)
public static final String ACTION_CALL = "android.intent.action.CALL";

Annotate all non-primitive arguments and return types with nullability {#nullability .numbered}

Android APIs that do not have default nullability states interact poorly with Kotlin and static analysis tooling, unless explicitly annotated with @NonNull or @Nullable.

Additionally, annotating your method parameters will automatically generate documentation in the form “This value may be null.” unless “null” is explicitly used elsewhere in the parameter doc.

Methods are recommended to perform input validation for @NonNull parameters via Objects.requireNonNull() and throw a NullPointerException when the parameters are null.

Prefer non-Nullable (e.g. @NonNull) arguments where possible {#prefer-nonnull-arguments .numbered}

When methods are overloaded, prefer that all arguments are non-null.

public void startActivity(@NonNull Component component) { ... }
public void startActivity(@NonNull Component component, @NonNull Bundle options) { ... }

This rule applies to overloaded property setters as well. The primary argument should be non-null and clearing the property should be implemented as a separate method. This prevents “nonsense” calls where the developer must set trailing parameters even though they are not required.

public void setTitleItem(@Nullable IconCompat icon, @ImageMode mode)
public void setTitleItem(@Nullable IconCompat icon, @ImageMode mode, boolean isLoading)

// Nonsense call to clear property
setTitleItem(null, MODE_RAW, false);
public void setTitleItem(@NonNull IconCompat icon, @ImageMode mode)
public void setTitleItem(@NonNull IconCompat icon, @ImageMode mode, boolean isLoading)
public void clearTitleItem()

Prefer non-Nullable (e.g. @NonNull) return types where possible {#prefer-nonnull-return .numbered}

For container types -- Bundles, Collections, etc. -- return an empty (and immutable, where applicable) container. For objects,

@NonNull
public Bundle getExtras() { ... }

NOTE that Intent.getExtras() returns a @Nullable Bundle and specifies a case where it returns null, but this was a mistake that should be avoided in future APIs.

Nullability annotations for get/set pairs must agree {#symmetric-nullability .numbered}

Get/set method pairs for a single logical property should always agree in their nullability annotations. Failing to follow this guideline will defeat Kotlin's property syntax, and adding disagreeing nullability annotations to existing property methods is therefore a source-breaking change for Kotlin users.

@NonNull
public Bundle getExtras() { ... }
public void setExtras(@NonNull Bundle bundle) { ... }

Return value in failure / error conditions {#return-error .numbered}

All APIs should permit applications to react to errors. Returning false, -1, null, or other catch-all values of “something went wrong” do not tell a developer enough about the failure to set user expectations or accurately track reliability of their app in the field. When designing an API, imagine that you are building an application. If you encounter an error, does the API give you enough information to surface it to the user or react appropriately?

  1. It‘s fine (and encouraged) to include detailed information in an exception message, but developers shouldn’t have to parse it to handle the error appropriately. Verbose error codes or other information should be exposed as methods.
  2. Make sure your chosen error handling option gives you the flexibility to introduce new error types in the future. For @IntDef, that means including an OTHER or UNKNOWN value - when returning a new code, you can check the caller‘s targetSdkVersion to avoid returning an error code the application doesn’t know about. For exceptions, have a common superclass that your exceptions implement, so that any code that handles that type will also catch and handle subtypes.
  3. It should be difficult or impossible for a developer to accidentally ignore an error -- if your error is communicated by returning a value, annotate your method with @CheckResult.

Prefer throwing a ? extends RuntimeException when a failure or error condition is reached due to something that the developer did wrong, for example ignoring constraints on input parameters or failing to check observable state.

Setter or action (ex. perform) methods may return an integer status code if the action may fail as a result of asynchronously-updated state or conditions outside the developer’s control.

Status codes should be defined on the containing class as public static final fields, prefixed with ERROR_, and enumerated in an @hide @IntDef annotation.

Method names should always begin with the verb, not the subject {#method-name-verb .numbered}

The name of the method should always begin with the verb (e.g. get, create, reload, etc.), not the object you’re acting on.

public void tableReload() {
  mTable.reload();
}
public void reloadTable() {
  mTable.reload();
}

Prefer Collection<T> types over arrays as return or parameter type {#methods-prefer-collection-over-array .numbered}

Generically-typed collection interfaces provide several advantages over arrays, including stronger API guarantees around uniqueness and ordering, support for generics, and a number of developer-friendly convenience methods.

Exception for primitives

If the elements are primitives, do prefer arrays instead, in order to avoid the cost of auto-boxing. See Take and return raw primitives instead of boxed versions

Exception for Kotlin

Kotlin arrays are invariant and the Kotlin language provides ample utility APIs around arrays, so arrays are on-par with List and Collection for Kotlin APIs intended to be accessed from Kotlin.

Prefer @NonNull collections {#methods-prefer-non-null-collections .numbered}

Always prefer @NonNull for collection objects. When returning an empty collection, use the appropriate Collections.empty method to return a low-cost, correctly-typed, and immutable collection object.

Where type annotations are supported, always prefer @NonNull for collection elements.

Collection mutability {#methods-collections-mutability .numbered}

Kotlin APIs should prefer immutable (e.g. not Mutable) return types for collections by default unless the API contract specifically requires a mutable return type.

Java APIs, however, should prefer mutable return types by default since the Android platform‘s implementation of Java APIs does not yet provide a convenient implementation of immutable collections. The exception to this rule is Collections.empty return types, which are immutable. In cases where mutability could be exploited by clients -- on purpose or by mistake -- to break the API’s intended usage pattern, Java APIs should return a shallow copy of the collection.

@Nullable
public PermissionInfo[] getGrantedPermissions() {
  return mPermissions;
}
@NonNull
public Set<PermissionInfo> getGrantedPermissions() {
  if (mPermissions == null) {
    return Collections.emptySet();
  }
  return new ArraySet<>(mPermissions);
}

Use of vararg parameter type {#methods-vararg .numbered}

Both Kotlin and Java APIs are encouraged to use vararg in cases where the developer would be likely to create an array at the call site for the sole purpose of passing multiple, related parameters of the same type.

public void setFeatures(Feature[] features) { ... }

// Developer code
setFeatures(new Feature[]{Features.A, Features.B, Features.C});
public void setFeatures(Feature... features) { ... }

// Developer code
setFeatures(Features.A, Features.B, Features.C);

Defensive copies

Both Java and Kotlin implementations of vararg parameters compile to the same array-backed bytecode and as a result may be called from Java code with a mutable array. API designers are strongly encouraged to create a defensive shallow copy of the array parameter in cases where it will be persisted to a field or anonymous inner class.

public void setValues(SomeObject... values) {
   this.values = Arrays.copyOf(values, values.length);
}

Note that creating a defensive copy does not provide any protection against concurrent modification between the initial method call and the creation of the copy, not does it protect aginst mutation of the objects contained in the array.

Provide correct semantics with collection type parameters / returned types {#type-semantics .numbered}

List<Foo> is default option, but consider other types to provide additional meaning:

  • Use Set<Foo>, if your API is indifferent to the order of elements and it doesn’t allow duplicates or duplicates are meaningless.

  • Collection<Foo>, if your API is indifferent to the order and allows duplicates.

Note: Don’t forget that Java Collections are mutable by default, so consider defensive copying for your return and parameter types. Another option for the return type is Collection.unmodifiable*.

Kotlin conversion functions {#kotlin-conversion-functions .numbered}

Kotlin frequently uses .toFoo() and .asFoo() to obtain an object of a different type from an existing object where Foo is the name of the conversion's return type. This is consistent with the familiar JDK Object.toString(). Kotlin takes this further by using it for primitive conversions such as 25.toFloat().

The distinction between conversions named .toFoo() and .asFoo() is significant:

Use .toFoo() when creating a new, independent object {.numbered}

Like .toString(), a “to” conversion returns a new, independent object. If the original object is modified later, the new object will not reflect those changes. Similarly, if the new object is modified later, the old object will not reflect those changes.

fun Foo.toBundle(): Bundle = Bundle().apply {
    putInt(FOO_VALUE_KEY, value)
}

Use .asFoo() when creating a dependent wrapper, decorated object, or cast {.numbered}

Casting in Kotlin is performed using the as keyword. It reflects a change in interface but not a change in identity. When used as a prefix in an extension function, .asFoo() decorates the receiver. A mutation in the original receiver object will be reflected in the object returned by asFoo(). A mutation in the new Foo object may be reflected in the original object.

fun <T> Flow<T>.asLiveData(): LiveData<T> = liveData {
    collect {
        emit(it)
    }
}

Conversion functions should be written as extensions {.numbered}

Writing conversion functions outside of both the receiver and the result class definitions reduces coupling between types. An ideal conversion needs only public API access to the original object. This proves by example that a developer can write analogous conversions to their own preferred types as well.

Listeners and Callbacks {#callbacks .numbered}

These are the rules around the classes and methods used for listener/callback mechanisms.

Use Listener for single-method interfaces, Callback otherwise {#callbacks-listener .numbered}

Use Listener when there is a single callback method and there will never be any others. It should be an interface whose name is the same as the callback method plus Listener.

public interface OnFooListener {
  void onFoo();
}

Methods involving listeners should specify the whole name.

public void setOnFooListener(@Nullable OnFooListener listener)

Use Callback when there are multiple callback methods, when there are associated constants, when it is desirable to provide default behavior, or when the callback could plausibly grow to add new methods or overloads.

In cases where Java 8 default methods are supported, callback classes may be defined as interfaces; otherwise, the callback should be an abstract class. In all cases, the class should have a generic name signifying the overall purpose of the callbacks but not naming one of them in particular.

// Java 7
public abstract class FooCallback {
  public abstract void onFooStarted();
  public abstract void onFooStopped();
}

// Java 8 with defaults
public interface MostlyOptionalCallback {
  void onImportantAction();
  default void onOptionalInformation() {
    // Empty stub, this method is optional.
  }
}

Methods involving callbacks should specify the whole name.

public void registerFooCallback(@NonNull FooCallback callback)

When registering callbacks that have no explicit threading expectations (pretty much anywhere outside the UI toolkit), it is strongly encouraged to include an Executor parameter as part of registration to allow the developer to specify the thread upon which the callbacks will be invoked.

public void registerFooCallback(
    @NonNull @CallbackExecutor Executor executor,
    @NonNull FooCallback callback)

NOTE Developers must provide a valid Executor. The new @CallbackExecutor annotation will add automatic documentation to tell developers about common default options. Also note that the callback argument is required to be last to enable idiomatic usage from Kotlin.

As an exception to our usual guidelines about optional parameters, it is ok to provide an overload omitting the Executor even though it is not the final argument in the parameter list. If the Executor is not provided, the callback should be invoked on the main thread using Looper.getMainLooper() and this should be documented on the associated overloaded method.

/**
 * ...
 * Note that the callback will be executed on the main thread using
 * {@link Looper.getMainLooper()}. To specify the execution thread, use
 * {@link registerFooCallback(Executor, FooCallback)}.
 * ...
 */
public void registerFooCallback(
    @NonNull FooCallback callback)

public void registerFooCallback(
    @NonNull @CallbackExecutor Executor executor,
    @NonNull FooCallback callback)

Executor implementation gotchas: Note that the following is a valid executor!

public class SynchronousExecutor implements Executor {
    @Override
    public void execute(Runnable r) {
        r.run();
    }
}

This means that when implementing APIs that take this form, your incoming binder object implementation on the app process side must call Binder.clearCallingIdentity() before invoking the app’s callback on the app-supplied Executor. This way any application code that uses Binder identity (e.g. Binder.getCallingUid()) for permission checks correctly attributes the code running to the application and not to the system process calling into the app. If users of your API want the UID / PID information of the caller then this should be an explicit part of your API surface, rather than implicit based on where the Executor they supplied ran.

The above should be supported by your API. In performance-critical cases apps may need to run code either immediately or synchronously with feedback from your API. Accepting an Executor permits this. Defensively creating an additional HandlerThread or similar to trampoline from defeats this desirable use case.

If an app is going to run expensive code somewhere in their own process, let them. The workarounds that app developers will find to overcome your restrictions will be much harder to support in the long term.

Exception for single callback: when the nature of the events being reported calls for only supporting a single callback instance, use the following style:

public void setFooCallback(
    @NonNull @CallbackExecutor Executor executor,
    @NonNull FooCallback callback)

public void clearFooCallback()

Symmetry in Registration {#callbacks-symmetry .numbered}

If there is a way to add or register something, there should also be a way to remove/unregister it. The method

registerThing(Thing)

should have a matching

unregisterThing(Thing)

Register/unregister callback, add/remove listener {#callbacks-accessors .numbered}

Callback objects should be added via a register method and removed via an unregister method. Listener objects should be added via an add method and removed via a remove method.

For example:

public void registerFooCallback(
    @NonNull @CallbackExecutor Executor executor,
    @NonNull FooCallback callback)
public void unregisterFooCallback()

public void addOnDoThingListener(@NonNull @CallbackExecutor Executor executor,
    @NonNull OnDoThingListener)
public void removeOnDoThingListener(@NonNull OnDoThingListener)

// For cases supporting only a single callback
public void setFooCallback(@NonNull @CallbackExecutor Executor executor,
    @NonNull FooCallback callback)
public void clearFooCallback()

// For cases supporting only a single listener
public void setOnDoThingListener(@NonNull @CallbackExecutor Executor executor,
    @NonNull OnDoThingListener)
public void clearOnDoThingListener()

Do not add getFooListener() methods. This is a tempting escape hatch for cases where developers may want to chain an existing listener together with their own replacement, but it is brittle and makes the current state difficult to reason about for component developers. For example,

  • Developer A calls setFooListener(a)
  • Developer B calls setFooListener(new B(getFooListener()))
  • Developer A wishes to remove its listener a and has no way to do so without knowledge of B’s type, and B having been built to allow such modifications of its wrapped listener.

Use language consistent with behavior {#callbacks-naming .numbered}

Callback methods regarding actions should be named consistent to whether the action has been performed yet.

For example, if the method is called after a click action has been performed:

public void onClicked()

However, if the method is responsible for performing the click action:

public boolean onClick()

Prefer functional interfaces over creating new single abstract method (SAM) types {#callbacks-sam .numbered}

API level 24 added the java.util.function.* (reference docs) types, which offer generic SAM interfaces such as Consumer<T> that are suitable for use as callback lambdas. In many cases, creating new SAM interfaces provides little value in terms of type safety or communicating intent while unnecessarily expanding the Android API surface area.

See an example of platform-defined functional interfaces -- Runnable, Supplier, and Consumer -- in the CTS theme test’s ConditionCheck class and associated usage.

Consider using these generic interfaces, rather than creating new ones:

  • Runnable (nilary function to void)
  • Supplier<R> (nilary function to R)
  • Consumer<T> (unary function T to void)
  • Function<T,R> (unary function from T to R)
  • Predicate<T> (unary function T to boolean)
  • many more available in reference docs

Placement of SAM parameters

SAM parameters should be placed last to enable idiomatic usage from Kotlin, even if the method is being overloaded with additional parameters.

public void schedule(Runnable runnable)

public void schedule(int delay, Runnable runnable)

Docs [D] {#docs .numbered}

These are rules about the public docs (JavaDocs) for APIs.

All public APIs must be documented {#docs-obviously .numbered}

All public APIs must have sufficient javadocs to explaining how a developer would use the API. Assume the developer found it via auto-complete or while browsing through API reference docs and has a minimal amount of context from the adjacent API surface (ex. on the same class).

Methods

Method parameters and return values must be documented using @param and @return docs annotations, respectively. The javadoc body should be formatted as though it is preceded by “This method...”.

In cases where a method takes no parameters, has no special considerations, and simply returns what the method name says it does, the @return may be omitted and docs may be written similar to:

/**
 * Returns the priority of the thread.
 */
@IntRange(from = 1, to = 10)
public int getPriority() { ... }

Always use links in JavaDocs {#docs-links .numbered}

Docs should link to other docs for related constants, methods, etc. Use Javadoc tags (e.g., @see and {@link foo}), not just plain-text words.

For:

public static final int FOO = 0;
public static final int BAR = 1;

Follow:

/**
 * Sets value to one of FOO or <code>BAR</code>.
 *
 * @param value the value being set, one of FOO or BAR
 */
public void setValue(int value) { ... }
/**
 * Sets value to one of {@link #FOO} or {@link #BAR}.
 *
 * @param value the value being set
 */
public void setValue(@ValueType int value) { ... }

Note that using an IntDef annotation such as @ValueType on a parameter will automatically generate documentation specifying the allowed types. See the guidance on annotations for more information on IntDef.

Run update-api or docs target when adding JavaDocs {#docs-update-api .numbered}

This rule is particularly important when adding @link or @see tags, and make sure the output looks as expected. It is common to see ERROR output in JavaDocs from bad links. Either the update-api or docs Make target will perform this check, but the docs target might be quicker if you are simply changing javadocs and do not otherwise need to run the update-api target.

Use {@code foo} to distinguish Java values {#docs-code-font .numbered}

Java values like true, false, and null should be wrapped with {@code ...} to distinguish them from documentation text.

@param and @return summaries should be a single sentence fragment {#docs-param-return .numbered}

Parameter and return value summaries should start with a lowercase character and contain only a single sentence fragment. If you have additional information that extends beyond a single sentence, move it to the method javadoc body.

/**
 * @param e The element to be appended to the list. This must not be
 *       null. If the list contains no entries, this element will
 *       be added at the beginning.
 * @return This method returns true on success.
 */
/**
 * @param e element to be appended to this list, must be non-{@code null}
 * @return {@code true} on success, {@code false} otherwise
 */

Docs annotations need explanations {#docs-annotations .numbered}

Annotations @hide and @removed should include documentation as to why they are hidden from public API. Use of @deprecated annotation must include instructions on how to replace usages of the deprecated API.

Use @throws to document exceptions {#docs-throws .numbered}

If a method throws an exception indicating a preventable error, for example IllegalArgumentException or IllegalStateException, the exception must be documented with an explanation of why the exception is thrown. The thrown exception should also indicate why it was thrown.

/**
 * ...
 * @throws IllegalArgumentException if the template requests a value that
 *         was not provided, or if more than 9 values are provided
 */
public static CharSequence expandTemplate(CharSequence template,
                                          CharSequence... values) {
    if (values.length > 9) {
        throw new IllegalArgumentException("max of 9 values are supported");
    }
    ...

If the method invokes asynchronous code that may throw exceptions, please consider how the developer will find out about and respond to such exceptions. Typically this involves forwarding the exception to a callback and documenting the exceptions thrown on the method that receives them. Asynchronous exceptions should not be documented with @throws unless they are actually re-thrown from the annotated method.

End the first sentence of docs with a period {#docs-period .numbered}

The doclava tool parses docs simplistically, ending the synopsis doc (the first sentence, used in the quick description at the top of the class docs) as soon as it sees a period (.) followed by a space. There are two problems that this causes:

  • If a short doc is not ended with a period, and if that member has inherited docs that are picked up by the tool, then the synopsis also picks up those inherited docs. See, for example, actionBarTabStyle in the R.attr docs, which has the description of the dimension added into the synopsis.
  • Avoid “e.g.” in the first sentence for the same reason, because doclava will end the synopsis docs after “g.”. See, for example, TEXT_ALIGNMENT_CENTER in View.java. Note that Metalava will automatically correct this error by inserting a non-breaking space after the period; however, please don’t make this mistake in the first place.

Format docs to be rendered in HTML {#docs-html .numbered}

Javadocs will be rendered in HTML, so format them accordingly:

  • Line breaks should use an explicit <p> tag. Do not add a closing </p> tag.

  • Do not use ASCII to render lists or tables.

  • Lists should use <ul> or <ol> for unordered and ordered, respectively. Each item should begin with a <li> tag, but does not need a closing </li> tag. A closing </ul> or </ol> tag is required after the last item.

  • Tables should use <table>, <tr> for rows, <th> for headers, and <td> for cells. All table tags require matching closing tags. You may use class="deprecated" on any tag to denote deprecation.

  • To create inline code font, use {@code foo}.

  • To create code blocks, use <pre>.

  • All text inside a <pre> block is parsed by the browser, so be careful with brackets <>. You can escape them with &lt; and &gt; HTML entities.

  • Alternatively, you can leave raw brackets <> in your code snippet if you wrap the offending sections in {@code foo}. For example:

    <pre>{@code <manifest>}</pre>
    

Follow the API reference style guide {#docs-style-guide .numbered}

To ensure consistency in the style for class summaries, method descriptions, parameter descriptions, etc., follow the recommendations in the official Java language guidelines at How to Write Doc Comments for the Javadoc Tool.

Android Framework-specific rules [FW] {#android-framework-specific-rules .numbered}

These rules are about APIs, patterns, and data structures that are specific to APIs and functionality built into the Android framework (Bundles, Parcelables, etc.).

Intent builders should use the create*Intent() pattern {#intent-builder-createintent .numbered}

Creators for intents should use methods named createFooIntent().

Use Bundles instead of creating new general-purpose data structures {#prefer-bundle .numbered}

Instead of creating a new type/class to hold various args or various types, consider simply using a Bundle instead.

Parcelable implementations must have public CREATOR field {#parcelable-creator .numbered}

If a class implements Parcelable, then its CREATOR field must also be public API.

Use CharSequence for UI strings {#charsequence-ui-string .numbered}

When a string will be used in the UI, use CharSequence to allow for Spannables.

If it’s just a key, or some other non-user-visible label or value, String is fine.

Avoid using Enums {#avoid-enum .numbered}

Enums are expensive, both in code and memory footprint.

public enum State {
  CONNECTING, CONNECTED, DISCONNECTED
}
public static final int STATE_CONNECTING = 1;
public static final int STATE_CONNECTED = 2;
public static final int STATE_DISCONNECTED = 3;

Note that using the @IntDef annotation, which is supported by Android Studio and Gradle 1.3+, will give your code build-time type safety (when lint errors are enabled), while retaining the size and performance benefits of using int variables.

Follow Android package layering hierarchy {#package-layering .numbered}

The android.* package hierarchy has an implicit ordering, where lower-level packages cannot depend on higher-level packages.

Avoid referring to Google, other companies, and their products {#7-avoid-referring-to-google-other-companies-and-their-products .numbered}

The Android platform is an open-source project and aims to be vendor neutral. The API should be generic and equally usable by system integrators or applications with the requisite permissions.

Parcelable implementations should be final {#parcelable-final .numbered}

Parcelable classes defined by the platform are always loaded from framework.jar, so it’s invalid for an app to try overriding a Parcelable implementation. If the sending app extends a Parcelable, the receiving app won’t have the sender’s custom implementation to unpack with. Note about backward compatibility: if your class historically wasn’t final, but didn’t have a publicly available constructor, you still can mark it final.

Methods calling into system process should rethrow RemoteException as RuntimeException {#rethrow-remoteexception .numbered}

RemoteException is typically thrown by internal AIDL, and indicates that the system process has died, or the app is trying to send too much data. In both cases, public API should rethrow as a RuntimeException to ensure that apps don’t accidentally persist security or policy decisions.

If you know the other side of a Binder call is the system process, this simple boilerplate code is the best-practice:

try {
    ...
} catch (RemoteException e) {
    throw e.rethrowFromSystemServer();
}

Implement copy constructor instead of clone() {#avoid-clone .numbered}

Use of the Java clone() method is strongly discouraged due to the lack of API guarantees provided by the Object class and difficulties inherent in extending classes that use clone(). Instead, use a copy constructor that takes an object of the same type.

/**
 * Constructs a shallow copy of {@code other}.
 */
public Foo(Foo other)

Classes that rely on a Builder for construction should consider adding a Builder copy constructor to allow modifications to the copy.

public class Foo {
    public static final class Builder {
        /**
         * Constructs a Foo builder using data from {@code other}.
         */
        public Builder(Foo other)

Use ParcelFileDescriptor over FileDescriptor. {#prefer-parcelfiledescriptor .numbered}

The java.io.FileDescriptor object has a poor definition of ownership, which can result in obscure use-after-close bugs. Instead, APIs should return or accept ParcelFileDescriptor instances. Legacy code can convert between PFD and FD if needed using dup() or getFileDescriptor().

Avoid using odd-sized numerical values. {#avoid-short-byte .numbered}

Avoid using “short” or “byte” values directly, since they often limit how you might be able to evolve the API in the future.

Avoid using BitSet. {#avoid-bitset .numbered}

java.util.BitSet is great for implementation but not for public API. It's mutable, requires an allocation for high-frequency method calls, and does not provide semantic meaning for what each bit represents.

For high-performance scenarios, use an int or long with @IntDef. For low-performance scenarios, consider a Set<EnumType>. For raw binary data, use byte[].

Prefer android.net.Uri. {#prefer-android-uri .numbered}

android.net.Uri is the preferred encapsulation for URIs in Android APIs.

Avoid java.net.URI, because it is overly strict in parsing URIs, and never use java.net.URL, because its definition of equality is severely broken.

Hide annotations marked as @IntDef, @LongDef or @StringDef {#no-public-typedefs .numbered}

Annotations marked as @IntDef, @LongDef or @StringDef denote a set of valid constants that can be passed to an API. However, when they are exported as API themselves, the compiler inlines the constants and only the (now useless) values remain in the annotation's API stub.

The annotations are exported via a seperate mechanism, and must thus be marked @hide. Note that the API linter enforces this.

Do not expose Binder interfaces as public APIs directly {#no-public-binder .numbered}

Keep AIDL interfaces as implementation details. Generated AIDL classes do not generally meet the API style guide requirements to begin with (for example, they cannot use overloading), and additionally this significantly constrains future implementation improvements. This applies to both generated interfaces and generated parcelable data classes.

Instead, add a public API layer on top of the AIDL interface, even if it initially is just a shallow wrapper.

If the Binder interface is an implementation detail, it can be changed freely in the future, with the public layer allowing for the required backward compatibility to be maintained. For example, you may find you need to add new arguments to the internal calls, or optimize IPC traffic via batching/streaming, using shared memory, or similar. None of these can easily be done if your AIDL interface is also the public API.

For example, instead of exposing FooService as a public API directly:

// BAD: Public API generated from IFooService.aidl
public class IFooService {
   public void doFoo(String foo);
}

instead wrap the Binder interface inside a manager or other class:

/**
 * @hide
 */
public class IFooService {
   public void doFoo(String foo);
}

public IFooManager {
   public void doFoo(String foo) {
      mFooService.doFoo(foo);
   }
}

and if later a new argument is needed for this call, the internal interface can be kept simple and convenient overloads added to the public API. And the wrapping layer can be used to handle other backwards-compatibility concerns as the implementation evolves, as well:

/**
 * @hide
 */
public class IFooService {
   public void doFoo(String foo, int flags);
}

public IFooManager {
   public void doFoo(String foo) {
      if (mAppTargetSdkLevel &lt; 26) {
         useOldFooLogic(); // Apps targeting API before 26 are broken otherwise
         mFooService.doFoo(foo, FLAG_THAT_ONE_WEIRD_HACK);
      } else {
         mFooService.doFoo(foo, 0);
      }
   }

   public void doFoo(String foo, int flags) {
      mFooService.doFoo(foo, flags);
   }
}

For Binder interfaces that are not part of the Android platform (for example, a service interface exported by Google Play Services for applications to use), the requirement for a stable, published, and versioned IPC interface means that it is much harder to evolve the interface itself. However, it is still worthwhile to have a wrapper layer around it, to match other API guidelines and to make it easier to use the same public API for a new version of the IPC interface, if that ever becomes necessary.

Note that generated AIDL interface definitions in libraries cannot currently be hidden from clients (b/135686385) and appear as public API no matter what. Additional code rewriting / manually written Binder interfaces may be needed until the generated code can be properly hidden.

Do not add new setting provider keys {#no-settings-provider .numbered}

Do not expose new keys from [Settings.Global] (https://developer.android.com/reference/android/provider/Settings.Global), [Settings.System] (https://developer.android.com/reference/android/provider/Settings.System), and/or [Settings.Secure] (https://developer.android.com/reference/android/provider/Settings.Secure).

Instead, add a proper getter and setter Java API in a relevant class, which is typically a “manager” class. Add a listener mechanism or a broadcast to notify clients of changes as needed.

SettingsProvider settings have a number of problems compared to getters/setters:

  • No type safety.
  • No unified way to provide a default value.
  • No proper way to customize permissions.
    • For example, it's not possible to protect your setting with a custom permission.
  • No proper way to add custom logic properly.
    • For example, it‘s not poissble to change setting A’s value depending on setting B's value.

Example: [Settings.Secure.LOCATION_MODE] (https://developer.android.com/reference/android/provider/Settings.Secure#LOCATION_MODE) has existed for a long time, but the location team has deprecated it for a proper Java API [LocationManager.isLocationEnabled()] (https://developer.android.com/reference/android/location/LocationManager#isLocationEnabled()) and the [MODE_CHANGED_ACTION] (https://developer.android.com/reference/android/location/LocationManager#MODE_CHANGED_ACTION) broadcast, which gave the team a lot more flexibility, and the semantics of the APIs are a lot clearer now.

Services {#services .numbered}

Handling Intents in system-bound developer services {#services-intents .numbered}

Services that are intended to be extended by the developer and bound by the system, for example abstract services like NotificationListenerService, may respond to an Intent action from the system. Such services should meet the following criteria:

  1. Define a SERVICE_INTERFACE string constant on the class containing the fully-qualified class name of the service. This constant must be annotated with @SdkConstant(SdkConstant.SdkConstantType.SERVICE_ACTION).
  2. Document on the class that a developer must add an <intent-filter> to their AndroidManifest.xml in order to receive Intents from the platform.
  3. Strongly consider adding a system-level permission to prevent rogue apps from sending Intents to developer services.

Kotlin-Java interop

See the official Android Kotlin-Java interop guide for a full list of guidelines. Select guidelines have been copied to this guide to improve discoverability.

Companion objects

Kotlin uses companion object to expose static members. In some cases, these will show up from Java on an inner class named Companion rather than on the containing class.

To maximize compatibility with Java, annotate companion objects' non-const fields with @JvmField and public functions with @JvmStatic to expose them directly on the containing class.

companion object {
  @JvmField val BIG_INTEGER_ONE = BigInteger.ONE
  @JvmStatic fun fromPointF(pointf: PointF) {
    /* ... */
  }
}

Evolution of Android platform APIs {#evolution .numbered}

Policies around what types of changes may be made to existing Android APIs and how those changes should be implemented to maximize compatibility with existing apps and codebases.

Binary-breaking changes {#evolution-binary-compat .numbered}

Binary-breaking changes are not allowed in public API and will generally raise errors when running make update-api. There may, however, be edge cases that are not caught by Metalava’s API check. When in doubt, refer to the Eclipse Foundation’s Evolving Java-based APIs wiki entry for a detailed explanation of what types of API changes are compatible. Binary-breaking changes in non-public (ex. system) APIs should follow the Deprecate/replace cycle.

Source-breaking changes {#evolution-source-compat .numbered}

Source-breaking changes are discouraged even if they are not binary-breaking. One example of a binary-compatible but source-breaking change is adding a generic to an existing class, which is binary-compatible but may introduce compilation errors due to inheritance or ambiguous references. Source-breaking changes will not raise errors when running make update-api, so care must be taken to understand the impact of changes to existing API signatures.

Changes to private APIs (@SystemApi, @TestApi) {#evolution-private-apis .numbered}

APIs annotated with @TestApi may be changed at any time.

APIs annotated with @SystemApi must be preserved for three years. Removal or refactoring of a system API must occur on the following schedule:

  • API y - Added
  • API y+1 - Deprecation
    • Mark the code as @Deprecated
    • Add replacements, and link to the replacement in the javadoc for the deprecated code using the @deprecated tag.
    • Mid-development-cycle, file bugs against internal users telling them API is going away, giving them a chance to ensure replacement APIs are adequate.
  • API y+2 - Soft removal
    • Mark code as @removed
    • Optionally, throw or no-op for apps that target the current sdk level for the release
  • API y+3 - Hard removal
    • Code is completely removed from source tree

Deprecation {#evolution-deprecation .numbered}

Deprecation is considered an API change and may occur in a major (e.g. letter) release. Use the @Deprecated source annotation and @deprecated <summary> docs annotation together when deprecating APIs. Your summary must include a migration strategy, which may link to a replacement API or explain why the API should not be used.

/**
 * Simple version of ...
 *
 * @deprecated Use the {@link androidx.fragment.app.DialogFragment}
 *             class with {@link androidx.fragment.app.FragmentManager}
 *             instead.
 */
@Deprecated
public final void showDialog(int id)

APIs defined in XML and exposed in Java, including attributes and styleable properties exposed in the android.R class, must also be deprecated with a summary.

<!-- Attribute whether the accessibility service ...
     {@deprecated Not used by the framework}
 -->
<attr name="canRequestEnhancedWebAccessibility" format="boolean" />

Changes to deprecated APIs {#evolution-deprecation-changes .numbered}

The behavior of deprecated APIs must be maintained, which means test implementations must remain the same and tests must continue to pass after the API has been deprecated. If the API does not have tests, tests should be added.

Deprecated API surfaces should not be expanded in future releases. Lint correctness annotations (ex. @Nullable) may be added to an existing deprecated API, but new APIs should not be added to deprecated classes or interfaces.

New APIs should not be added as deprecated. APIs that were added and subsequently deprecated within a pre-release cycle -- thus would initially enter the public API surface as deprecated -- should be removed before API finalization.

Soft removal {#evolution-soft-removal .numbered}

Soft removal is a source-breaking change and should be avoided in public APIs unless explicitly approved by API Council. For system APIs, soft removals must be preceded by deprecation for the duration of a major release. Remove all docs references to the APIs and use the @removed <summary> docs annotation when soft-removing APIs. Your summary must include the reason for removal and may include a migration strategy as explained in Deprecation.

/**
 * Ringer volume. This is ...
 *
 * @removed Not functional since API 2.
 */
public static final String VOLUME_RING = ...

Certain categories of API must not be soft removed.

Abstract methods

Abstract methods on classes that may be extended by developers must not be soft removed. Doing so will make it impossible for developers to successfully extend the class across all SDK levels.

In rare cases where it was never and will never be possible for developers to extend a class, abstract methods may still be soft removed.

Hard removal {#evolution-hard-removal .numbered}

Hard removal is a binary-breaking change and should never occur in public APIs. For system APIs, hard removals must be preceded by soft removal for the duration of a major release. Remove the entire implementation when hard-removing APIs.