Definitions : Android

Activity:

1. Activities form the lifeline of an Android application. 

2. An Activity is an application component that provides a screen with which users can interact in order to do something, such as dial the phone, take a photo, send an email, or view a map.
3. Activity class is a pre-defined class in Android and every application which has UI must inherit it to create window.
4. To be of use with Context.startActivity(), all activity classes must have a corresponding <activity> declaration in their package'sAndroidManifest.xml.

Resource:
1. A resource is a piece of our application that is not code - things like image files, audio files, XML files. (A layout is a resource.)
2. To access a resource in code, you use its resource ID.

AndroidManifest.xml:
1. This file is essential in every application.
2. It is declared in the root directory and contains information about the application that the Android system must know before the code can be executed.

Intent:
An intent is an object that a component can use to communicate with the OS.

Explicit Intent:
We use explicit intents to start activities within our application.

Implicit Intent:
When an activity in our application wants to start an activity in another application, we create an implicit intent.

Fragment:

1. Fragment is a part of user interface.

2. A Fragment is closely tied to the Activity it is in, and can not be used apart from one.

3. Interaction with fragments is done through FragmentManager.

Service:
1. A Service is an application component that can perform long running operations in the background & does not provide a user interface.
2. Starting a Service is almost exactly the same as starting an activity.
3. Once its done, the Service should stop itself.

Bound Service:
1. A bound service is the server in a client-server interface. A bound service allows components (such as activities) to bind to the service, send requests, receive responses, and even perform interprocess communication (IPC).
2. A bound service typically lives only while it serves another application component and does not run in the background indefinitely.
3. A client can bind to the service by calling bindService(). When it does, it must provide an implementation of ServiceConnection, which monitors the connection with the service. The bindService() method returns immediately without a value, but when the Android system creates the connection between the client and service, it calls onServiceConnected() on the ServiceConnection, to deliver the IBinder that the client can use to communicate with the service.
4. When creating a service that provides binding, you must provide an IBinder that provides the programming interface that clients can use to interact with the service.

Gradle:
1. Gradle is the official build system for Android Studio.

2. If you create a new project in Android studio, the Gradle build scripts are automatically created.

3. If you press the run button in Android Studio, it triggers the corresponding Gradle task for you and starts the application.

4. The build system automatically takes all the source files (.java or .xml), then applies the appropriate tool (e.g. takes java class files and converts them to dex files), and groups all of them into one compressed file, our beloved APK.

5. An apk file gets signed and pushed to the device using ADB(Android Debug Bridge) where it gets executed.

ADB (Android Debug Bridge):

1. Android Debug Bridge (adb) is a versatile (command line) tool that lets you communicate with an emulator instance or connected Android-powered device.

Ahead of time (AOT) compilation:

1. Android Runtime (ART) introduces the use of ahead-of-time (AOT) compilation by compiling entire applications into native machine code upon their installation.

2. In Android 5.0 "Lollipop", Dalvik was entirely replaced by ART.