NOTE
TextView takes a handle to the current context as an argument. Pass this to the
TextView to associate it with the current context. If you follow the hierarchy
through the SDK, HelloWorldText extends Activity, which extends
ApplicationContext, which in turn extends Context. This is how you can
pass this to your TextView.
The preceding line creates an instance of TextView named HelloWorldTextView
and then instantiates HelloWorldTextView, by setting it to a new TextView. The new
TextView is passed the context of this to be fully instantiated.
Now that the TextView is defined, you can add your text to it. The following line of
code assigns the text “Hello World!” to the TextView:
HelloWorldTextView.setText(“Hello World!”);
This line lets you set the text of your TextView. setText( ) lets you assign a string to the
TextView.

Your TextView has been created and now contains the message that you want to
display. However, simply passing “Hello World!” to the TextView does not display
anything to the screen. As discussed previously, you need to set the ContentView to

display something to the screen. You have to use the following code to set TextView
to the context and display it to the screen:
setContentView(HelloWorldTextView);
Examining this line, you can see that you pass to setContentView your TextView. The
preceding three lines of code are what it takes to make your Hello World! application.
You created a TextView, assigned your text to it, and set it to the screen. All things
considered, this is not very complicated at all.
The full contents of your HelloWorldText.java file should look like the following:
package android_programmers_guide.HelloWorldText;

import android.app.Activity;
import android.os.Bundle;
import android.widget.TextView;
public class HelloWorldText extends Activity {
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle icicle) {
super.onCreate(icicle);
/**Hello World JFD */
/**BEGIN           */
/**Create TextView */
TextView HelloWorldTextView = new TextView(this);
/**Set text to Hello World */
HelloWorldTextView.setText(“Hello World!”);

/**Set ContentView to TextView */
setContentView(HelloWorldTextView);
/**END             */
}
}
Now compile and run your new Hello World! application in the Android Emulator.
Choose Run | Run or press CTRL-F11 to launch the application in the Android Emulator. The following illustration depicts the results of your Hello World! application.

You have just created your first full Android Activity. This small project demonstrated
a fairly common execution of a Hello World! application. You set a TextView to the
Activity’s ContentView and displayed the “Hello World!” message to a cell phone screen
in the Android Emulator. The following section looks at a slightly different way of
implementing Hello World!, using an image.

Hello World! Again
In this section, you will create another Hello World! application for Android. However, this time you will program the UI in code rather than by using the xml file—and you will actually do most of the work. The first step here is to remove the TextView code that is in main.xml. The following section of code represents the TextView. Removing it essentially makes your application an empty shell.
<TextView
android:layout_width=”fill_parent”
android:layout_height=”wrap_content”
android:text=”Hello World, HelloWorldText”
/>

After you have removed the TextView code, your main.xml file should look like this:
<?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”
>
</LinearLayout>
Now that you have a clean main.xml file, and thus a clean application shell, you can
begin to add the code that will display “Hello World!” on the screen. Start by opening the
HelloWorldText.java file and removing the following line:
setContentView(R.layout.main);

NOTE
You still need to set a ContentView for your new application; however, you are going
to implement it slightly differently from how it is implemented here, so it is best to just
remove the entire statement for now.

The new project you just established contains the code to create a Hello World!
application on its own. However, that is not very engaging, nor does it teach you very
much about programming an Android application. You need to dissect the project and
see exactly how the project displayed the “Hello World!” message.
What happened when you created the new Android project is that the Android plugin
modified main.xml. This is a perfect example of one way to modify the UI in Android.
The following lines of code are added to main.xml by the Android SDK when the project
is created:
<TextView
android:layout_width=”fill_parent”
android:layout_height=”wrap_content”
android:text=”Hello World, HelloWorldText”
/>

The next line in the file is the one that really does some perceptible action:
setContentView(R.layout.main);
The method setContentView( ) sets the Activity’s content to the specified resource.
In this case, we are using the main.xml file from the layout directory via the pointer in
the R.java file. The main.xml file, right now, contains nothing more than the size of the
HelloWorldText screen and a TextView. The TextView is derived from View and is used
to display text in an Android environment. Reviewing the contents of main.xml, you can
see that it contains the following line:
android:text=”Hello World, HelloWorldText”

package android_programmers_guide.HelloWorldText;
import android.app.Activity;
import android.os.Bundle;

public class HelloWorldText extends Activity {
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle icicle) {
super.onCreate(icicle);
setContentView(R.layout.main);
}
}
The three lines at the top of the file are the standard preprocessor directives—that is,
as in most programming languages, statements that are directives to the compiler to run
before the application process. In this case, you have the definition and inclusion of your
package android_programmers_guide.HelloWorldText.
The next two lines import specific packages from the Android SDK via android.jar:
import android.app.Activity;
and

import android.os.Bundle;
These lines tell the project to include all the code from the imported packages before all
the code in your application. These two lines are critical for your base Android application
and should not be removed..

/* AUTO-GENERATED FILE.  DO NOT MODIFY.
*
* This class was automatically generated by the
* aapt tool from the resource data it found.  It
* should not be modified by hand.
*/
package testPackage.HelloWorldText;
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=0x7f040000;
}
}

NOTE
The comment section of the R.java file provides an explanation of the origin of the file. It states that the file was created by the aapt tool. In Chapter 6, when you create a command-line–only version of the Hello World! application, you will use command-line tools to create all of the auto-generated files.

There are also three directories in the project root—res, assets, and src—each of which has a distinct purpose. These directories play an integral part in the operation of your application.

res Directory

The res directory is where your in project resources are held and compiled into your application. When you create a new Android project, the res directory contains three subdirectories: drawable, layout, and values. You will use the drawable and layout directories in many of your projects to hold and display images and layouts respectively, whereas the values directory holds string globals that can be used throughout your

application.

NOTE

A reference to the res directory and its contents is contained by the R.java file, located in the src directory. This file is covered in much more detail later in the chapter. The drawable directory contains actual image files that your application can use and reference. The layout directory holds an XML file, main.xml, that is referenced by your application when building its interface. In most of the applications in this book, you will

be editing the main.xml file included in the layout directory. This will allow you to insert Views into the application’s visual layout and display them. An unaltered main.xml file contains the following code:

<?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=”Hello World, HelloWorldText”

/>

</LinearLayout>

The last directory under res, values, holds an XML file named strings. The strings.xml file is used to hold global string values that can be referenced by your application.

assets Directory

The assets directory is used to hold raw asset files. The files contained in the assets directory can include audio files for streaming and animation assets. we will not use any audio assets in the applications for this book because the beta audio drivers for the Android Emulator are not yet optimized.

src Directory

The src directory contains all the source files for your project. When your project is first created, it will contain two files, R.java and <activity>.java (in this example,

HelloWorldText.java), described next.

NOTE

<activity>.java is always named according to your Activity name.

The AndroidManifest.xml file is where your global settings are made. If you are an ASP.NET developer, you can think of AndroidManifest.xml as Web.config and Global.asax rolled into one. (If you are not an ASP.NET developer, this means that AndroidManifest.xml is a place for storing settings.) AndroidManifest.xml will include such settings as application permissions, Activities, and intent filters.

The standard AndroidManifest.xml file should contain the following information:

<?xml version=”1.0″ encoding=”utf-8″?>
<manifest xmlns:android=http://schemas.android.com/apk/res/android
package=”testPackage.HelloWorldText”>
<application android:icon=”@drawable/icon”>
<activity android:label=”@string/app_name”>
<intent-filter>
<action android:value=”android.intent.action.MAIN” />
<category android:value=”android.intent.category.LAUNCHER”
/>
</intent-filter>
</activity>
</application>
</manifest>
As you create future applications, you will be adding information to this file. Notice that the package name you supplied is listed here, as well as the action that your Activity will handle.

Referenced Libraries

A list of the Referenced Libraries is also included in the root of the project. Typically, for a beginner project, you should see only one library here. Expand the Referenced Libraries branch and examine its contents, the libraries that are currently referenced by your application project. Given that this is a new Android project, you will see one library in your project’s references, android.jar, the Android SDK. (If you are familiar with the Java SDK, android.java is analogous to Java’s rt.java file, containing many of the Java APIs found in rt.java.) The Android plugin ensures that this file is the only library referenced by your application. The application needs to reference the SDK to gain access to all the classes contained in the SDK libraries, such as your Views, Controls, and even the Google API.

CAUTION
Eclipse enables you to add other user-defined libraries and external classes to your project’s references. However, unless you are sure that those external references will work with your Android application (and thus on the Android platform), you should think twice before you add them.