Android is a Linux-based operating system designed primarily for mobile devices such as smartphones and tablets. The latest versions of Android are based on Linux kernel Version 3.x (Version 2.6 for versions prior to Android 4.0).

Android is delivered with a set of native libraries written in C/C++, which provide various types of services. These libraries predominantly come from the open source community.

Android apps run within the Dalvik Virtual Machine (Dalvik VM), which is similar to a Java VM but has been optimized for devices with limited memory and processing capacity.

Android apps are initially compiled to the Java byte code using the Java compiler, but they have an additional compilation step that transforms the Java byte code to the Dalvik byte code, suitable to run within the Dalvik VM.

Dalvik is delivered with the Android core libraries. These libraries do not align with a specific Java platform (JSE, JEE, or JME) but rather act as a hybrid platform most closely aligned with JSE, minus the user interface-focused components AWT and Swing. The Android Application Framework (AAF) provides an alternate means of creating user interfaces.

The Application Framework is the part of the Android platform, most familiar to developers. It is delivered as a set of Java libraries and allows you to build user interfaces, interact with device capabilities such as the camera or location services, load and work with various types of application resources, and perform many more useful tasks.

At the top of the stack sits the humble application, the component that actually delivers value to the user. Android comes with a set of applications that provide base functionality such as managing contacts, using the phone, checking email, and browsing the Web. The key to Android's success is the vast array of third-party applications that can be installed, which allow users to do things such as stream live sports' events, edit a movie captured on the phone, interact with friends through their favorite social media site, and much more.

Identifying the version of the Android platform can be somewhat confusing; there is a version number, API level, and nickname, and these are sometimes used interchangeably.

The version number represents a release of the platform. Sometimes, a new release is created to deliver new capabilities, while sometimes it is created to fix bugs.

The API level represents a set of capabilities. As the API level increases, new capabilities are delivered to the developer.

The following table lists the versions of the platform in the reverse chronological order:

One of the most fundamental parts of an Android application is an activity. An activity provides a single function that a user can perform with an application such as list contacts, enter new contact, and display location(s) on a map. A single application is composed of many activities.

A user interacts with an activity through one or more Views, which are described later in this chapter. If you are familiar with the Model-View-Controller pattern, you would have noticed that activities fulfill the role of the Controller.

The life cycle of an activity

Activities have a well-defined life cycle that can be described in terms of states, transitions, and events. The following diagram provides a graphical view of the life cycle of an activity:

Services are components that run in the background to perform long-running operations with no direct user interface. Services may load data into a cache, play music, or perform some other type of processing, while a user interacts with other activities uninterrupted.

Content providers manage access to a central repository of data such as contacts. A content provider is a part of an application, which usually provides a user interface to manage its data. A standard interface is also provided, which allows other applications to access its repository.

Broadcast receivers are components that perform some type of processing in response to system-wide broadcasts. Broadcasts are generally initiated by the system for events such as low battery, taking a picture, or turning on Bluetooth. Applications may also choose to send broadcasts; a content provider might send a broadcast when data, such as a contact, has been updated. While broadcast receivers do not have a user interface, they may indirectly cause updates to a status.

Everything that you see in an Android app is a View; buttons, labels, text boxes, and radio buttons are all examples of Views. Views are organized in a hierarchy using various types of ViewGroups. A ViewGroup is a special kind of View which is used to arrange (layout) other Views on the screen.

Common layouts

The Application Framework has a number of subclasses of ViewGroup, each of which provides a unique and useful way of organizing content.

The previous diagram depicts a few of the more common layouts, each of which can be used for specific needs.

Layout	Description	Scenario
Linear layout	Organizes its children into a single horizontal or vertical row and creates a scrollbar when required.	Use when a widget positions flow horizontally or vertically.
Relative layout	Organizes child objects relative to each other or to the parent.	Use when widget positions can best be described in relationship to another widget (to the left of) or the boundary area of the parent (right side, centered).
Table layout	Organizes its children into rows and columns.	Use when widget positions would naturally fit into rows and columns. Great when multiple columns of entry and labels are needed.

For complex layout scenarios, Android allows layouts to be nested. Deeply nested layouts can impact performance and should be avoided if possible.

Adapter layouts

For layouts that are driven by a dynamic data source, the Application Framework has a set of classes derived from AdapterView.

The previous diagram depicts two of the most common adapter layouts.

• List View: Organizes content from the data source into a scrolling single column list

• Grid View: Organizes content from the data source into a grid of columns and rows

To create a UI using a declarative method, Android provides an XML vocabulary with tags that define the various types of elements that can compose a View. The concept behind Android XML layout files is very similar to the way HTML tags are used to define web pages or Microsoft's XAML tags are used to define WPF (Windows Presentation Foundation) user interfaces. The following example shows a simple View using a linear layout and containing a search entry field and search button:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
  android:orientation="vertical"
  android:layout_width="fill_parent"
  android:layout_height="fill_parent">
  <TextView
    android:text="Enter Search Criteria"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:id="@+id/searchCriteriaTextView" />
    <Button
      android:text="Search"
      android:layout_width="fill_parent"
      android:layout_height="wrap_content"
      android:id="@+id/searchButton" />
</LinearLayout>

android:id="@+id/searchButton"

setContentView(R.layout.main);

Intents are messages that can be sent to the various types of components in an Android App in order to request some type of action to be performed. Intents may be used to accomplish any of the following:

Creating an Android application involves more than simply writing code. A rich mobile app requires things such as images, audio files, animations, menus, and style, just to name a few. The Application Framework provides APIs that can be used to load and utilize the various types of resources with your Android apps.

public final class R {
  public static final class attr {
  }
  public static final class drawable {
    public static final int icon=0x7f020000;
  }
  public static final class id {
    public static final int myButton=0x7f050000;
    public static final int searchButton=0x7f050002;
    public static final int searchCriteriaTextView=0x7f050001;
  }
  public static final class layout {
    public static final int main=0x7f030000;
    public static final int search=0x7f030001;
  }
  public static final class string {
    public static final int app_name=0x7f040001;
    public static final int hello=0x7f040000;
  }
}