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Android APIs
Added in API level 1
package

android.os

Provides basic operating system services, message passing, and inter-process communication on the device.

Interfaces

CancellationSignal.OnCancelListener Listens for cancellation. 
Handler.Callback Callback interface you can use when instantiating a Handler to avoid having to implement your own subclass of Handler. 
IBinder Base interface for a remotable object, the core part of a lightweight remote procedure call mechanism designed for high performance when performing in-process and cross-process calls. 
IBinder.DeathRecipient Interface for receiving a callback when the process hosting an IBinder has gone away. 
IInterface Base class for Binder interfaces. 
MessageQueue.IdleHandler Callback interface for discovering when a thread is going to block waiting for more messages. 
Parcelable Interface for classes whose instances can be written to and restored from a Parcel. 
Parcelable.ClassLoaderCreator<T> Specialization of Parcelable.Creator that allows you to receive the ClassLoader the object is being created in. 
Parcelable.Creator<T> Interface that must be implemented and provided as a public CREATOR field that generates instances of your Parcelable class from a Parcel. 
ParcelFileDescriptor.OnCloseListener Callback indicating that a ParcelFileDescriptor has been closed. 
RecoverySystem.ProgressListener Interface definition for a callback to be invoked regularly as verification proceeds. 

Classes

AsyncTask<Params, Progress, Result>

AsyncTask enables proper and easy use of the UI thread. 
BatteryManager The BatteryManager class contains strings and constants used for values in the ACTION_BATTERY_CHANGED Intent. 
Binder Base class for a remotable object, the core part of a lightweight remote procedure call mechanism defined by IBinder. 
Build Information about the current build, extracted from system properties. 
Build.VERSION Various version strings. 
Build.VERSION_CODES Enumeration of the currently known SDK version codes. 
Bundle A mapping from String values to various Parcelable types. 
CancellationSignal Provides the ability to cancel an operation in progress. 
ConditionVariable Class that implements the condition variable locking paradigm. 
CountDownTimer Schedule a countdown until a time in the future, with regular notifications on intervals along the way. 
Debug Provides various debugging methods for Android applications, including tracing and allocation counts. 
Debug.InstructionCount API for gathering and querying instruction counts. 
Debug.MemoryInfo This class is used to retrieved various statistics about the memory mappings for this process. 
DropBoxManager Enqueues chunks of data (from various sources -- application crashes, kernel log records, etc.). 
DropBoxManager.Entry A single entry retrieved from the drop box. 
Environment Provides access to environment variables. 
FileObserver Monitors files (using inotify) to fire an event after files are accessed or changed by by any process on the device (including this one). 
Handler A Handler allows you to send and process Message and Runnable objects associated with a thread's MessageQueue. 
HandlerThread Handy class for starting a new thread that has a looper. 
Looper Class used to run a message loop for a thread. 
MemoryFile MemoryFile is a wrapper for the Linux ashmem driver. 
Message Defines a message containing a description and arbitrary data object that can be sent to a Handler. 
MessageQueue Low-level class holding the list of messages to be dispatched by a Looper. 
Messenger Reference to a Handler, which others can use to send messages to it. 
Parcel Container for a message (data and object references) that can be sent through an IBinder. 
ParcelFileDescriptor The FileDescriptor returned by readFileDescriptor(), allowing you to close it when done with it. 
ParcelFileDescriptor.AutoCloseInputStream An InputStream you can create on a ParcelFileDescriptor, which will take care of calling ParcelFileDescriptor.close() for you when the stream is closed. 
ParcelFileDescriptor.AutoCloseOutputStream An OutputStream you can create on a ParcelFileDescriptor, which will take care of calling ParcelFileDescriptor.close() for you when the stream is closed. 
ParcelUuid This class is a Parcelable wrapper around UUID which is an immutable representation of a 128-bit universally unique identifier. 
PatternMatcher A simple pattern matcher, which is safe to use on untrusted data: it does not provide full reg-exp support, only simple globbing that can not be used maliciously. 
PowerManager This class gives you control of the power state of the device. 
PowerManager.WakeLock A wake lock is a mechanism to indicate that your application needs to have the device stay on. 
Process Tools for managing OS processes. 
RecoverySystem RecoverySystem contains methods for interacting with the Android recovery system (the separate partition that can be used to install system updates, wipe user data, etc.)  
RemoteCallbackList<E extends IInterface> Takes care of the grunt work of maintaining a list of remote interfaces, typically for the use of performing callbacks from a Service to its clients. 
ResultReceiver Generic interface for receiving a callback result from someone. 
StatFs Retrieve overall information about the space on a filesystem. 
StrictMode

StrictMode is a developer tool which detects things you might be doing by accident and brings them to your attention so you can fix them. 
StrictMode.ThreadPolicy StrictMode policy applied to a certain thread. 
StrictMode.ThreadPolicy.Builder Creates StrictMode.ThreadPolicy instances. 
StrictMode.VmPolicy StrictMode policy applied to all threads in the virtual machine's process. 
StrictMode.VmPolicy.Builder Creates StrictMode.VmPolicy instances. 
SystemClock Core timekeeping facilities. 
TokenWatcher Helper class that helps you use IBinder objects as reference counted tokens. 
Trace Writes trace events to the system trace buffer. 
UserHandle Representation of a user on the device. 
UserManager Manages users and user details on a multi-user system. 
Vibrator Class that operates the vibrator on the device. 
WorkSource Describes the source of some work that may be done by someone else. 

Enums

AsyncTask.Status Indicates the current status of the task. 

Exceptions

BadParcelableException The object you are calling has died, because its hosting process no longer exists. 
DeadObjectException The object you are calling has died, because its hosting process no longer exists. 
NetworkOnMainThreadException The exception that is thrown when an application attempts to perform a networking operation on its main thread. 
OperationCanceledException An exception type that is thrown when an operation in progress is canceled. 
ParcelFileDescriptor.FileDescriptorDetachedException Exception that indicates that the file descriptor was detached. 
ParcelFormatException The contents of a Parcel (usually during unmarshalling) does not contain the expected data. 
RemoteException Parent exception for all Binder remote-invocation errors  
TransactionTooLargeException The Binder transaction failed because it was too large.