Introduction to Android development

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

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

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.

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

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

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.


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

 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.