The Android Software Development Kit supports Windows (XP and Vista), Linux (tested on Ubuntu Dapper Drake, but any recent Linux distro should work), and Mac OS X (10.4.8 or later, Intel platform only) as host development environments. Installation of the SDK is substantially the same for any of the operating systems, and most of this description applies equally to all of them. Where the procedure differs, we will clearly tell you what to do for each environment:

Congratulations—you have installed a complete Android development environment without spending a penny. As you’ll see in this and subsequent chapters, the environment includes a very sophisticated set of tools to make Android programming easier, including:

There isn’t much functionality in this program. We just want to display some text on the Android emulator window that says “Hello Android!” (see Figure 2-1).

Figure 2-1. “Hello Android” screenshot

Several components are needed to build an Android application. Fortunately, the Eclipse IDE with the Android plug-in automates a lot of the work needed to create and maintain these components. We will start by using the IDE to create a project for our application. Start up Eclipse and select “File → New → Project...” from the menu bar (be sure to select “Project...”, not “Java Project”). You’ll see a list of project types, similar to the menu in Figure 2-2.

Figure 2-2. Eclipse New Project menu

Select “Android Project” and click “Next” to get the “New Android Project” dialog box (Figure 2-3).

Figure 2-3. Eclipse New Android Project dialog

We’ll use “HelloWorld” as the name for both the Project and the Application. You don’t need to change the button or checkbox selections, and we’ll use the package name com.oreilly.helloworld as shown.

Every Android application has to have at least one Activity (an executable that usually has a user interface), so let’s say we’re going to include an Activity called HelloWorldActivity, as shown in the dialog box. Click “Finish,” and the Android Software Development Kit does a number of things for you, to make your life easier as a developer. In Figure 2-4, I’ve expanded the tree in the Package Explorer window to show some of the files and directories that the Android SDK created.

The Android SDK created a HelloWorld directory in the default Eclipse workspace for your project. It also created subdirectories for your source files (.src), references to the Android Library, assets, resources (.res), and a manifest file (AndroidManifest.xml). In each of the subdirectories it created another level of subdirectories as appropriate. Let’s take a quick look at them:

In the Eclipse Package Explorer window, double-click on HelloWorldActivity.java. This opens the source file of that name in the center window, ready for editing:

package com.oreilly.helloworld;

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

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

Looking quickly at the template code that the Android SDK has provided for us, we can note several things:

Again in the Eclipse Package Explorer window, double-click on main.xml and you will see the default layout screen in the center window. There are two tabs at the bottom of the panel that say “Layout” and “main.xml”. Click on the one that says “main.xml” to bring up the code version:

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

Again, let’s look at the key features of this template code:

Save the Project either from the Eclipse File menu (File → Save) or by clicking on the disk icon in the menu bar.

Believe it or not, we’re done. We don’t have to write a single line of Java to create this application.

From the Eclipse menu bar, select Run → Run. A “Run As” dialog box will pop up. Select “Android Application” from the list, which displays the dialog shown in Figure 2-5.

Figure 2-5. Eclipse Application Type selection

A command window will pop up, followed quickly by an emulator window that looks just like a mobile phone. The emulated phone will then go through its boot sequence, which takes a few minutes (relax; if anything goes wrong, it will tell you). After a few minutes you should see the screen shown in Figure 2-6.

Figure 2-6. First try at HelloAndroid

Notice anything different between that screen image and the one we showed in Figure 2-1? The application prints out “Hello Android!”, or whatever you wrote into the android:text line earlier, but it also displays the title of the application as “Hello World”. Let’s change the title to match our creative change to the application text.

In the Package Explorer in the left panel of the Eclipse workbench, reopen the strings.xml file (the one where we found the String hello before). This will open the file in the editing window. The intent of this file is to give you a place to define strings that will be used by your application, without actually embedding them in the Java source code. The other string that’s defined here is app_name. To make things consistent, change the definition of app_name to HelloAndroid, as shown in Figure 2-7.

Figure 2-7. HelloWorld String editing

Now when we run the application, we get a screen that looks just like what we set out to do, as shown previously in Figure 2-1.

Congratulations! You’ve just created your first Android program by doing nothing more than changing the text in one line of code. There are much greater challenges ahead.