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Android: A Programmer s Guide
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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 mainxml This is a perfect example of one way to modify the UI in Android The following lines of code are added to mainxml by the Android SDK when the project is created:
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<TextView android:layout_width="fill_parent" android:layout_height="wrap_content" android:text="Hello World, HelloWorldText" />
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While I have discussed the existence of this TextView in the xml, I have not yet discussed why it works without any corresponding code I mentioned earlier in this book that there are two ways to design a UI for Android: through the code, and through the mainxml file The preceding code sample creates a TextView in xml and sets the text to Hello World, HelloWorldText Edit this line of the mainxml file to read as follows:
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android:text="This is the text of an Android TextView!"
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Rerun the project, and your results should appear as they do in this illustration Take some time and experiment with the xml TextView Then you can move on to another way of creating a Hello World! application
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Application: Hello World!
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Hello World! Again
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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 mainxml The following section of code represents the TextView Removing it essentially makes your application an empty shell
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<TextView android:layout_width="fill_parent" android:layout_height="wrap_content" android:text="Hello World, HelloWorldText" />
After you have removed the TextView code, your mainxml file should look like this:
< xml version="10" encoding="utf-8" > <LinearLayout xmlns:android=http://schemasandroidcom/apk/res/android android:orientation="vertical" android:layout_width="fill_parent" android:layout_height="fill_parent" > </LinearLayout>
Now that you have a clean mainxml 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 HelloWorldTextjava file and removing the following line:
setContentView(Rlayoutmain);
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
This line uses setContentView( ) to draw the mainxml file to the screen Since you will not be using mainxml to define your TextView, you will not be setting it to your view Instead, you will be building the TextView in code
Android: A Programmer s Guide
Your next step is to import the package TextView from androidwidget This will give you access to the TextView and let you create your own instance of it Place this code near the top of your current HelloWorldTextjava file, where the existing import statements are
import androidwidgetTextView;
Now, create an instance of TextView By creating the TextView instance, you can use it to display text to the screen without directly modifying mainxml Place the following code after the onCreate( ) statement is fired:
TextView HelloWorldTextView = new TextView(this);
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:
HelloWorldTextViewsetText("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
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Application: Hello World!
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 HelloWorldTextjava file should look like the following:
package android_programmers_guideHelloWorldText; import androidappActivity; import androidosBundle; import androidwidgetTextView; public class HelloWorldText extends Activity { /** Called when the activity is first created */ @Override public void onCreate(Bundle icicle) { superonCreate(icicle); /**Hello World JFD */ /**BEGIN */ /**Create TextView */ TextView HelloWorldTextView = new TextView(this); /**Set text to Hello World */ HelloWorldTextViewsetText("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
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