Android has gone through quite a number of updates since its first release. Table 1-1 shows the various versions of Android and their codenames.

As Android is open source and freely available to manufacturers for customization, there are no fixed hardware and software configurations. However, Android itself supports the following features:

In order to understand how Android works, take a look at Figure 1-1, which shows the various layers that make up the Android operating system (OS).

Figure 1-1. FIGURE 1-1

The Android OS is roughly divided into five sections in four main layers:

Android devices come in all shapes and sizes. As of late November 2010, the Android OS can be seen powering the following types of devices:

Figure 1-2. FIGURE 1-2

Chances are good that you own at least one of the preceding devices. Figure 1-2 shows (clockwise) the Samsung Galaxy S, the HTC Desire HD, and the LG Optimus One smartphones.

Another popular category of devices that manufacturers are rushing out is the tablet. Tablet sizes typically start at seven inches, measured diagonally. Figure 1-3 shows the Samsung Galaxy Tab and the Dell Streak, which is a five-inch phone tablet.

Besides smartphones and tablets, Android is also beginning to appear in dedicated devices, such as e-book readers. Figure 1-4 shows the Barnes and Noble's NOOKcolor, which is a color e-Book reader running the Android OS.

Figure 1-3. FIGURE 1-3

Figure 1-4. FIGURE 1-4

In addition to these popular mobile devices, Android is also slowly finding its way into your living room. People of Lava, a Swedish company, has developed an Android-based TV, call the Scandinavia Android TV (see Figure 1-5).

Google has also ventured into a proprietary smart TV platform based on Android and co-developed with companies such as Intel, Sony, and Logitech. Figure 1-6 shows Sony's Google TV.

Figure 1-5. FIGURE 1-5

Figure 1-6. FIGURE 1-6

As mentioned earlier, one of the main factors determining the success of a smartphone platform is the applications that support it. It is clear from the success of the iPhone that applications play a very vital role in determining whether a new platform swims or sinks. In addition, making these applications accessible to the general user is extremely important.

As such, in August 2008, Google announced the Android Market, an online application store for Android devices, and made it available to users in October 2008. Using the Market application that is preinstalled on their Android device, users can simply download third-party applications directly onto their devices. Both paid and free applications are supported on the Android Market, though paid applications are available only to users in certain countries due to legal issues.

Similarly, in some countries, users can buy paid applications from the Android Market, but developers cannot sell in that country. As an example, at the time of writing, users in India can buy apps from the Android Market, but developers in India cannot sell apps on the Android Market. The reverse may also be true; for example, users in South Korea cannot buy apps, but developers in South Korea can sell apps on the Android Market.

Chapter 11 discusses more about the Android Market and how you can sell your own applications in it.

The first step towards developing any applications is obtaining the integrated development environment (IDE). In the case of Android, the recommended IDE is Eclipse, a multi-language software development environment featuring an extensible plug-in system. It can be used to develop various types of applications, using languages such as Java, Ada, C, C++, COBOL, Python, etc.

Figure 1-7. FIGURE 1-7

For Android development, you should download the Eclipse IDE for Java EE Developers (www.eclipse.org/downloads/packages/eclipse-ide-java-ee-developers/heliossr1). Six editions are available: Windows (32 and 64-bit), Mac OS X (Cocoa 32 and 64), and Linux (32 and 64-bit). Simply select the relevant one for your operating system. All the examples in this book were tested using the 32-bit version of Eclipse for Windows.

Once the Eclipse IDE is downloaded, unzip its content (the eclipse folder) into a folder, say C:Android. Figure 1-7 shows the content of the eclipse folder.

The next important piece of software you need to download is, of course, the Android SDK. The Android SDK contains a debugger, libraries, an emulator, documentation, sample code, and tutorials.

You can download the Android SDK from http://developer.android.com/sdk/index.html.

Once the SDK is downloaded, unzip its content (the android-sdk-windows folder) into the C:Android folder, or whatever name you have given to the folder you just created.

The Android Development Tools (ADT) plug-in for Eclipse is an extension to the Eclipse IDE that supports the creation and debugging of Android applications. Using the ADT, you will be able to do the following in Eclipse:

To install the ADT, first launch Eclipse by double-clicking on the eclipse.exe file located in the eclipse folder.

When Eclipse is first started, you will be prompted for a folder to use as your workspace. In Eclipse, a workspace is a folder where you store all your projects. Take the default suggested and click OK.

Once Eclipse is up and running, select the Help

In the Install window that appears, type http://dl-ssl.google.com/android/eclipse in the text box (see Figure 1-9) and click Add....

After a while, you will see the Developer Tools item appear in the middle of the window (see Figure 1-10). Expand it, and it will reveal its content: Android DDMS, Android Development Tools, and Android Hierarchy Viewer. Check all of them and click Next.

Figure 1-8. FIGURE 1-8

Figure 1-9. FIGURE 1-9

Figure 1-10. FIGURE 1-10

When you see the installation details, as shown in Figure 1-11, click Next.

Figure 1-11. FIGURE 1-11

You will be asked to review the licenses for the tools. Check the option to accept the license agreements (see Figure 1-12). Click Finish to continue.

Figure 1-12. FIGURE 1-12

Eclipse will now proceed to download the tools from the Internet and install them (see Figure 1-13). This will take some time, so be patient.

Figure 1-13. FIGURE 1-13

Once the ADT is installed, you will be prompted to restart Eclipse. After doing so, go to Window

Figure 1-14. FIGURE 1-14

In the Preferences window that appears, select Android. You will see an error message saying that the SDK has not been set up (see Figure 1-15). Click OK to dismiss it.

Figure 1-15. FIGURE 1-15

Enter the location of the Android SDK folder. In this example, it would be C:Androidandroid-sdk-windows. Click OK.

The next step is to create AVD to be used for testing your Android applications. AVD stands for Android Virtual Devices. An AVD is an emulator instance that enables you to model an actual device. Each AVD consists of a hardware profile, a mapping to a system image, as well as emulated storage, such as a secure digital (SD) card.

You can create as many AVDs as you want in order to test your applications with several different configurations. This testing is important to confirm the behavior of your application when it is run on different devices with varying capabilities.

To create an AVD, go to Windows

Select the Available packages option in the left pane and expand the package name shown in the right pane. Figure 1-16 shows the various packages available for you to create AVDs to emulate the different versions of an Android device.

Figure 1-16. FIGURE 1-16

Check the relevant tools, documentation, and platforms you need for your project.

Once you have selected the items you want, click the Install Selected button to download them. Because it takes a while to download from Google's server, it is a good idea to download only whatever you need immediately, and download the rest when you have more time.

Each version of the Android OS is identified by an API level number. For example, Android 2.3 is level 9 (API 9), while Android 2.2 is level 8 (API 8), and so on. For each level, two platforms are available. For example, level 9 offers the following:

The key difference between the two is that the Google APIs platform contains the Google Maps library. Therefore, if the application you are writing requires Google Maps, you need to create an AVD using the Google APIs platform (more on this in Chapter 9, "Location Based Services."

Click the Virtual Devices item in the left pane of the window. Then click the New... button located in the right pane of the window.

In the Create new Android Virtual Device (AVD) window, enter the items as shown in Figure 1-17. Click the Create AVD button when you are done.

Figure 1-17. FIGURE 1-17

In this case, you have created an AVD (put simply, an Android emulator) that emulates an Android device running version 2.3 of the OS. In addition to what you have created, you also have the option to emulate the device with an SD card and different screen densities and resolutions.

It is preferable to create a few AVDs with different API levels so that your application can be tested on different devices. The example shown in Figure 1-18 shows the many AVDs created to test your applications on a wide variety of different Android platforms.

Figure 1-18. FIGURE 1-18

With all the tools and the SDK downloaded and installed, it is now time to start your engine! As in all programming books, the first example uses the ubiquitous Hello World application. This will enable you to have a detailed look at the various components that make up an Android project.

So, without any further ado, let's dive straight in!

codefile HelloWorld.zip available for download at Wrox.com

Now that you have created your first Hello World Android application, it is time to dissect the innards of the Android project and examine all the parts that make everything work.

Figure 1-30. FIGURE 1-30

First, note the various files that make up an Android project in the Package Explorer in Eclipse (see Figure 1-30).

The various folders and their files are as follows:

The main.xml file defines the user interface for your activity. Observe the following in bold:

<TextView
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:text="@string/hello" />

The @string in this case refers to the strings.xml file located in the res/values folder. Hence, @string/hello refers to the hello string defined in the strings.xml file, which is "Hello World, MainActivity!":

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <string name="hello">Hello World, MainActivity!</string>
    <string name="app_name">HelloWorld</string>
</resources>

It is recommended that you store all the string constants in your application in this strings.xml file and reference these strings using the @string identifier. That way, if you ever need to localize your application to another language, all you need to do is replace the strings stored in the strings.xml file with the targeted language and recompile your application.

Observe the content of the AndroidManifest.xml file:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
      package="net.learn2develop.HelloWorld"
      android:versionCode="1"
      android:versionName="1.0">
    <application android:icon="@drawable/icon" android:label="@string/app_name">
        <activity android:name=".MainActivity"
                  android:label="@string/app_name">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>
    <uses-sdk android:minSdkVersion="9" />
</manifest>

The AndroidManifest.xml file contains detailed information about the application:

As you add more files and folders to your project, Eclipse will automatically generate the content of R.java, which at the moment contains the following:

package net.learn2develop.HelloWorld;

public final class R {
    public static final class attr {
    }
    public static final class drawable {
        public static final int icon=0x7f020000;
    }
    public static final class layout {
        public static final int main=0x7f030000;
    }
    public static final class string {
        public static final int app_name=0x7f040001;
        public static final int hello=0x7f040000;
    }
}

You are not supposed to modify the content of the R.java file; Eclipse automatically generates the content for you when you modify your project.

Finally, the code that connects the activity to the UI (main.xml) is the setContentView() method, which is in the MainActivity.java file:

package net.learn2develop.HelloWorld;

import android.app.Activity;
import android.os.Bundle;

public class MainActivity extends Activity {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
    }
}

Here, R.layout.main refers to the main.xml file located in the res/layout folder. As you add additional XML files to the res/layout folder, the filenames will automatically be generated in the R.java file. The onCreate() method is one of many methods that are fired when an activity is loaded. Chapter 2 discusses the life cycle of an activity in more detail.