Introduction to Android development

The open source appliance platform

Android is a complete operating environment based upon the Linux® V2.6 kernel. Initially, the deployment target for Android was the mobile-phone arena, including smart phones and lower-cost flip-phone devices. However, Android's full range of computing services and rich functional support have the potential to extend beyond the mobile-phone market. Android can be useful for other platforms and applications. In this article, get an introduction to the Android platform and learn how to code a basic Android application.

Frank Ableson, Software designer

After his college basketball career came to an end without a multiyear contract to play for the L.A. Lakers, Frank Ableson shifted his focus to computer software design. He enjoys solving complex problems, particularly in the areas of communications and hardware interfacing. When not working, he can be found spending time with his wife Nikki and their children. You can reach Frank at frank@cfgsolutions.com.



12 May 2009

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Introduction

The BlackBerry and iPhone, which have appealing and high-volume mobile platforms, are addressing opposite ends of a spectrum. The BlackBerry is rock-solid for the enterprise business user. For a consumer device, it's hard to compete with the iPhone for ease of use and the "cool factor." Android, a young and yet-unproven platform, has the potential to play at both ends of the mobile-phone spectrum and perhaps even bridge the gulf between work and play.

Today, many network-based or network-capable appliances run a flavor of the Linux kernel. It's a solid platform: cost-effective to deploy and support and readily accepted as a good design approach for deployment. The UI for such devices is often HTML-based and viewable with a PC or Mac browser. But not every appliance needs to be controlled by a general computing device. Consider a conventional appliance, such as a stove, microwave or bread maker. What if your household appliances were controlled by Android and boasted a color touch screen? With an Android UI on the stove-top, the author might even be able to cook something.

In this article, learn about the Android platform and how it can be used for mobile and nonmobile applications. Install the Android SDK and build a simple application. Download the source code for the example application in this article.


A brief history of Android

The Android platform is the product of the Open Handset Alliance, a group of organizations collaborating to build a better mobile phone. The group, led by Google, includes mobile operators, device handset manufacturers, component manufacturers, software solution and platform providers, and marketing companies. From a software development standpoint, Android sits smack in the middle of the open source world.

The first Android-capable handset on the market was the G1 device manufactured by HTC and provisioned on T-Mobile. The device became available after almost a year of speculation, where the only software development tools available were some incrementally improving SDK releases. As the G1 release date neared, the Android team released SDK V1.0 and applications began surfacing for the new platform.

To spur innovation, Google sponsored two rounds of "Android Developer Challenges," where millions of dollars were given to top contest submissions. A few months after the G1, the Android Market was released, allowing users to browse and download applications directly to their phones. Over about 18 months, a new mobile platform entered the public arena.


The Android platform

With Android's breadth of capabilities, it would be easy to confuse it with a desktop operating system. Android is a layered environment built upon a foundation of the Linux kernel, and it includes rich functions. The UI subsystem includes:

  • Windows
  • Views
  • Widgets for displaying common elements such as edit boxes, lists, and drop-down lists

Android includes an embeddable browser built upon WebKit, the same open source browser engine powering the iPhone's Mobile Safari browser.

Android boasts a healthy array of connectivity options, including WiFi, Bluetooth, and wireless data over a cellular connection (for example, GPRS, EDGE, and 3G). A popular technique in Android applications is to link to Google Maps to display an address directly within an application. Support for location-based services (such as GPS) and accelerometers is also available in the Android software stack, though not all Android devices are equipped with the required hardware. There is also camera support.

Historically, two areas where mobile applications have struggled to keep pace with their desktop counterparts are graphics/media, and data storage methods. Android addresses the graphics challenge with built-in support for 2-D and 3-D graphics, including the OpenGL library. The data-storage burden is eased because the Android platform includes the popular open source SQLite database. Figure 1 shows a simplified view of the Android software layers.

Figure 1. Android software layers
The Android software layers

Application architecture

As mentioned, Android runs atop a Linux kernel. Android applications are written in the Java programming language, and they run within a virtual machine (VM). It's important to note that the VM is not a JVM as you might expect, but is the Dalvik Virtual Machine, an open source technology. Each Android application runs within an instance of the Dalvik VM, which in turn resides within a Linux-kernel managed process, as shown below.

Figure 2. Dalvik VM
Dalvik VM

An Android application consists of one or more of the following classifications:

Activities
An application that has a visible UI is implemented with an activity. When a user selects an application from the home screen or application launcher, an activity is started.
Services
A service should be used for any application that needs to persist for a long time, such as a network monitor or update-checking application.
Content providers
You can think of content providers as a database server. A content provider's job is to manage access to persisted data, such as a SQLite database. If your application is very simple, you might not necessarily create a content provider. If you're building a larger application, or one that makes data available to multiple activities or applications, a content provider is the means of accessing your data.
Broadcast receivers
An Android application may be launched to process a element of data or respond to an event, such as the receipt of a text message.

An Android application, along with a file called AndroidManifest.xml, is deployed to a device. AndroidManifest.xml contains the necessary configuration information to properly install it to the device. It includes the required class names and types of events the application is able to process, and the required permissions the application needs to run. For example, if an application requires access to the network — to download a file, for example — this permission must be explicitly stated in the manifest file. Many applications may have this specific permission enabled. Such declarative security helps reduce the likelihood that a rogue application can cause damage on your device.

The next section discusses the development environment required to build an Android application.


Required tools

The easiest way to start developing Android applications is to download the Android SDK and the Eclipse IDE (see Resources). Android development can take place on Microsoft® Windows®, Mac OS X, or Linux.

This article assumes you are using the Eclipse IDE and the Android Developer Tools plug-in for Eclipse. Android applications are written in the Java language, but compiled and executed in the Dalvik VM (a non-Java virtual machine). Coding in the Java language within Eclipse is very intuitive; Eclipse provides a rich Java environment, including context-sensitive help and code suggestion hints. Once your Java code is compiled cleanly, the Android Developer Tools make sure the application is packaged properly, including the AndroidManifest.xml file.

It's possible to develop Android applications without Eclipse and the Android Developer Tools plug-in, but you would need to know your way around the Android SDK.

The Android SDK is distributed as a ZIP file that unpacks to a directory on your hard drive. Since there have been several SDK updates, it is recommended that you keep your development environment well organized so you can easily switch between SDK installations. The SDK includes:

android.jar
Java archive file containing all of the Android SDK classes necessary to build your application.
documention.html and docs directory
The SDK documentation is provided locally and on the Web. It's largely in the form of JavaDocs, making it easy to navigate the many packages in the SDK. The documentation also includes a high-level Development Guide and links to the broader Android community.
Samples directory
The samples subdirectory contains full source code for a variety of applications, including ApiDemo, which exercises many APIs. The sample application is a great place to explore when starting Android application development.
Tools directory
Contains all of the command-line tools to build Android applications. The most commonly employed and useful tool is the adb utility (Android Debug Bridge).
usb_driver
Directory containing the necessary drivers to connect the development environment to an Android-enabled device, such as the G1 or the Android Dev 1 unlocked development phone. These files are only required for developers using the Windows platform.

Android applications may be run on a real device or on the Android Emulator, which ships with the Android SDK. Figure 3 shows the Android Emulator's home screen.

Figure 3. Android Emulator
The Android Emulator

Android Debug Bridge

The adb utility supports several optional command-line arguments that provide powerful features, such as copying files to and from the device. The shell command-line argument lets you connect to the phone itself and issue rudimentary shell commands. Figure 4 shows the adb shell command against a real device connected to a Windows laptop with a USB cable.

Figure 4. Using the adb shell command
Using the adb shell command

Within this shell environment, you can:

  • Display the network configuration that shows multiple network connections. Note the multiple network connections:
    • lo is the local or loopback connection.
    • tiwlan0 is the WiFi connection with an address provisioned by a local DHCP server.
  • Display the contents of the PATH environment variable.
  • Execute the su command to become the super-user.
  • Change the directory to /data/app, where user applications are stored.
  • Do a directory listing where you see a single application. Android application files are actually archive files that are viewable with WinZip or equivalent. The extension is apk.
  • Issue a ping command to see if Google.com is available.

From this same command-prompt environment, you can also interact with SQLite databases, start programs, and many other system-level tasks. This is fairly remarkable function, considering you're connected to a telephone.

In the next section, you'll create a simple Android application.


Coding a basic application

This section provides a whirlwind tour of building an Android application. The example application is about as simple as you can imagine: a modified "Hello Android" application. You'll add a minor modification to make the screen background color all white so you can use the phone as a flashlight. Not very original, but it will be useful as an example. Download the full source code.

To create an application in Eclipse, select File > New > Android project, which starts the New Android Project wizard.

Figure 5. New Android project wizard
The new Android project wizard

Next, you create a simple application with a single activity, along with a UI layout stored in main.xml. The layout contains a text element you're going to modify to say Android FlashLight. The simple layout is shown below.

Listing 1. Flashlight layout
<?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"
    android:background="@color/all_white">
<TextView  
    android:layout_width="fill_parent" 
    android:layout_height="wrap_content" 
    android:text="@string/hello" android:textColor="@color/all_black" 
   android:gravity="center_horizontal"/>
</LinearLayout>

Create a couple of color resources in strings.xml.

Listing 2. Color in strings.xml
<?xml version="1.0" encoding="utf-8"?>
<resources>
    <string name="hello">Android FlashLight</string>
    <string name="app_name">FlashLight</string>
    <color name="all_white">#FFFFFF</color>
    <color name="all_black">#000000</color>
</resources>

The main screen layout has a background color defined as all_white. In the strings.xml file, you see that all_white is defined as an RGB triplet value of #FFFFFF, or all white.

The layout contains a single TextView, which is really just a piece of static text; it is not editable. The text is set to be black and is centered horizontally with the gravity attribute.

The application has a Java source file called FlashLight.java, as shown below.

Listing 3. Flashlight.java
package com.msi.flashlight;
import android.app.Activity;
import android.os.Bundle;
public class FlashLight extends Activity {
    /** Called when the activity is first created. */
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
    }
}

The code is boiler-plate directly from the New Project wizard:

  • It is part of a Java package called com.msi.flashlight.
  • It has two imports:
    • One for the activity class
    • One for the bundle class
  • When this activity is initiated, the onCreate method is invoked, passing in a savedInstanceState. Don't be concerned with this bundle for our purposes; it is used when an activity is suspended and then resumed.
  • The onCreate method is an override of the activity class method of the same name. It calls the super class's onCreate method.
  • A call to setContentView() associates the UI layout defined in the file main.xml. Anything in main.xml and strings.xml gets automatically mapped to constants defined in the R.java source file. Never edit this file directly, as it is changed upon every build.

Running the application presents a white screen with black text.

Figure 6. White screen of flashlight
White screen of flashlight

The AndroidManifest.xml file setup for the FlashLight application is shown below.

Listing 4. AndroidManifest.xml for FlashLight
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
      package="com.msi.flashlight"
      android:versionCode="1"
      android:versionName="1.0.0">
    <application android:icon="@drawable/icon" android:label="@string/app_name">
        <activity android:name=".FlashLight"
                  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>
</manifest>

This file was created automatically by the Android Developer Tools plug-in for Eclipse. You didn't have to do anything.

Of course, the application is not terribly magnificent. But it could come in handy if you want to do some reading without disturbing your sleeping spouse, or if you need to find your way to the fuse box in the basement during a power outage.


Summary

In this article, you learned about Android at a very high level and built a small application. Hopefully, the example got you excited enough to explore more of the Android platform. Android promises to be a market-moving open source platform that will be useful well beyond cell phones.


Download

DescriptionNameSize
FlashLight source codeos-android-devel-FlashLight.zip22KB

Resources

Learn

  • The Open Handset Alliance is a group of 47 technology and mobile companies who have come together to accelerate innovation in mobile and offer consumers a richer, less expensive, and better mobile experience. Together, they have developed Android, the first complete, open, and free mobile platform.
  • The Android developers site offers documentation, downloads, blogs, and more.
  • Learn more about the Dalvik Virtual Machine.
  • Check out the tutorials hosted on YouTube that discuss the internals of the Dalvik VM.
  • Unlocking Android: A Developer's Guide provides concise, hands-on instruction for the Android operating system and development tools.
  • To listen to interesting interviews and discussions for software developers, check out developerWorks podcasts.
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  • Follow developerWorks on Twitter.
  • Check out upcoming conferences, trade shows, webcasts, and other Events around the world that are of interest to IBM open source developers.
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