andriod's hello world

Create the Project

Creating the project is as simple as can be. An Eclipse plugin is available making Android development a snap.

You'll need to have a development computer with the Eclipse IDE installed (see System and Software Requirements), and you'll need to install the Android Eclipse Plugin (ADT). Once you have those ready, come back here.

First, here's a high-level summary of how to build "Hello, World!":

1. Create a new "Android Project" via the File > New > Project menu.
2. Fill out the project details in the New Android Project dialog.
3. Edit the auto-generated source code template to display some output.

That's it! Next, let's go through each step above in detail.

1. Create a new Android Project

   From Eclipse, select the File > New > Project menu item. If the Android Plugin for Eclipse has been successfully installed, the resulting dialog should have a folder labeled "Android" which should contain a single entry: "Android Project".

   Once you've selected "Android Project", click the Next button.

2. Fill out the project details

   The next screen allows you to enter the relevant details for your project. Here's an example:

   Here's what each field on this screen means:

   Project Name	This is the name of the directory or folder on your computer that you want to contain the project.
   Package Name	This is the package namespace (following the same rules as for packages in the Java programming language) that you want all your source code to reside under. This also sets the package name under which the stub Activity will be generated. The package name you use in your application must be unique across all packages installed on the system; for this reason, it's very important to use a standard domain-style package for your applications. In the example above, we used the package domain "com.android"; you should use a different one appropriate to your organization.
   Activity Name	This is the name for the class stub that will be generated by the plugin. This will be a subclass of Android's Activity class. An Activity is simply a class that can run and do work. It can create a UI if it chooses, but it doesn't need to.
   Application Name	This is the human-readable title for your application.

   The checkbox for toggling "Use default location" allows you to change the location on disk where the project's files will be generated and stored.

3. Edit the auto-generated source code

   After the plugin runs, you'll have a class named HelloAndroid that looks like this:

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

   The next step is to start modifying it!

Construct the UI

Once you've got the project set up, the obvious next step is to get some text up there on the screen. Here's the finished product — next we'll dissect it line by line:

package com.android.hello;
import android.app.Activity;
import android.os.Bundle;
import android.widget.TextView;
public class HelloAndroid extends Activity {
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle icicle) {
super.onCreate(icicle);
TextView tv = new TextView(this);
tv.setText("Hello, Android");
setContentView(tv);
}
}

In Android, user interfaces are composed of hierarchies of classes called Views. A View is simply a drawable object, such as a radio button, an animation, or (in our case) a text label. The specific name for the View subclass that handles text is simply TextView.

Here's how you construct a TextView:

TextView tv = new TextView(this);

The argument to TextView's constructor is an Android Context instance. The Context is simply a handle to the system; it provides services like resolving resources, obtaining access to databases and preferences, and so on. The Activity class inherits from Context. Since our HelloAndroid class is a subclass of Activity, it is also a Context, and so we can pass the 'this' reference to the TextView.

Once we've constructed the TextView, we need to tell it what to display:

tv.setText("Hello, Android");

Nothing too surprising there.

At this point, we've constructed a TextView and told it what text to display. The final step is to connect this TextView with the on-screen display, like so:

setContentView(tv);

The setContentView() method on Activity indicates to the system which View should be associated with the Activity's UI. If an Activity doesn't call this method, no UI is present at all and the system will display a blank screen. For our purposes, all we want is to display some text, so we pass it the TextView we just created.

There it is — "Hello, World" in Android! The next step, of course, is to see it running.

Run the Code: Hello, Android

The Eclipse plugin makes it very easy to run your applications. Begin by selecting the Run > Open Run Dialog menu entry; you should see a dialog like this:

Next, highlight the "Android Application" entry, and then click the icon in the top left corner (the one depicting a sheet of paper with a plus sign in the corner) or simply double-click the "Android Application" entry. You should have a new launcher entry named "New_configuration".

Change the name to something expressive, like "Hello, Android", and then pick your project by clicking the Browse button. (If you have more than one Android project open in Eclipse, be sure to pick the right one.) The plugin will automatically scan your project for Activity subclasses, and add each one it finds to the drop-down list under the "Activity:" label. Since your "Hello, Android" project only has one, it will be the default, and you can simply continue.

Press the "Apply" button. Here's an example:

That's it — you're done! Press the Run button, and the Android Emulator should start. Once it's booted up your application will appear. When all is said and done, you should see something like this:

That's "Hello, World" in Android. Pretty straightforward, eh? The next sections of the tutorial offer more detailed information that you may find valuable as you learn more about Android.

Upgrading the UI to an XML Layout

The "Hello, World" example you just completed uses what we call "programmatic" UI layout. This means that you construct and build your application's UI directly in source code. If you've done much UI programming, you're probably familiar with how brittle that approach can sometimes be: small changes in layout can result in big source-code headaches. It's also very easy to forget to properly connect Views together, which can result in errors in your layout and wasted time debugging your code.

That's why Android provides an alternate UI construction model: XML-based layout files. The easiest way to explain this concept is to show an example. Here's an XML layout file that is identical in behavior to the programmatically-constructed example you just completed:

<?xml version="1.0" encoding="utf-8"?>
<TextView xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:text="Hello, Android"/>

The general structure of an Android XML layout file is simple. It's a tree of tags, where each tag is the name of a View class. In this example, it's a very simple tree of one element, a TextView. You can use the name of any class that extends View as a tag name in your XML layouts, including custom View classes you define in your own code. This structure makes it very easy to quickly build up UIs, using a much simpler structure and syntax than you would in source code. This model is inspired by the web development model, where you can separate the presentation of your application (its UI) from the application logic used to fetch and fill in data.

In this example, there are also four XML attributes. Here's a summary of what they mean:

Attribute	Meaning
xmlns:android	This is an XML namespace declaration that tells the Android tools that you are going to refer to common attributes defined in the Android namespace. The outermost tag in every Android layout file must have this attribute.
android:layout_width	This attribute defines how much of the available width on the screen this View should consume. In this case, it's our only View so we want it to take up the entire screen, which is what a value of "fill_parent" means.
android:layout_height	This is just like android:layout_width, except that it refers to available screen height.
android:text	This sets the text that the TextView should contain. In this example, it's our usual "Hello, Android" message.

So, that's what the XML layout looks like, but where do you put it? Under the res/ directory in your project. The "res" is short for "resources" and that directory contains all the non-code assets that your application requires. This includes things like images, localized strings, and XML layout files.

The Eclipse plugin creates one of these XML files for you. In our example above, we simply never used it. In the Package Explorer, expand the folder res/layout, and edit the file main.xml. Replace its contents with the text above and save your changes.

Now open the file named R.java in your source code folder in the Package Explorer. You'll see that it now looks something like this:

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;
};
};

A project's R.java file is an index into all the resources defined in the file. You use this class in your source code as a sort of short-hand way to refer to resources you've included in your project. This is particularly powerful with the code-completion features of IDEs like Eclipse because it lets you quickly and interactively locate the specific reference you're looking for.

The important thing to notice for now is the inner class named "layout", and its member field "main". The Eclipse plugin noticed that you added a new XML layout file and then regenerated this R.java file. As you add other resources to your projects you'll see R.java change to keep up.

The last thing you need to do is modify your HelloAndroid source code to use the new XML version of your UI, instead of the hard-coded version. Here's what your new class will look like. As you can see, the source code becomes much simpler:

package com.android.hello;
import android.app.Activity;
import android.os.Bundle;
public class HelloAndroid extends Activity {
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle icicle) {
super.onCreate(icicle);
setContentView(R.layout.main);
}
}

When you make this change, don't just copy-and-paste it in. Try out the code-completion feature on that R class. You'll probably find that it helps a lot.

Now that you've made this change, go ahead and re-run your application — all you need to do is click the green Run arrow icon, or select Run > Run History > Hello, Android from the menu. You should see.... well, exactly the same thing you saw before! After all, the point was to show that the two different layout approaches produce identical results.

There's a lot more to creating these XML layouts, but that's as far as we'll go here. Read the Implementing a User Interface documentation for more information on the power of this approach.

Debugging Your Project

The Android Plugin for Eclipse also has excellent integration with the Eclipse debugger. To demonstrate this, let's introduce a bug into our code. Change your HelloAndroid source code to look like this:

package com.android.hello;
import android.app.Activity;
import android.os.Bundle;
public class HelloAndroid extends Activity {
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle icicle) {
super.onCreate(icicle);
Object o = null;
o.toString();
setContentView(R.layout.main);
}
}

This change simply introduces a NullPointerException into your code. If you run your application again, you'll eventually see this:

Press "Force Quit" to terminate the application and close the emulator window.

To find out more about the error, set a breakpoint in your source code on the line Object o = null; (double-click on the marker bar next to the source code line). Then select Run > Debug History > Hello, Android from the menu to enter debug mode. Your app will restart in the emulator, but this time it will suspend when it reaches the breakpoint you set. You can then step through the code in Eclipse's Debug Perspective, just as you would for any other application.

Creating the Project without Eclipse

If you don't use Eclipse (such as if you prefer another IDE, or simply use text editors and command line tools) then the Eclipse plugin can't help you. Don't worry though — you don't lose any functionality just because you don't use Eclipse.

The Android Plugin for Eclipse is really just a wrapper around a set of tools included with the Android SDK. (These tools, like the emulator, aapt, adb, ddms, and others are documented elsewhere.) Thus, it's possible to wrap those tools with another tool, such as an 'ant' build file.

The Android SDK includes a Python script named "activityCreator.py" that can be used to create all the source code and directory stubs for your project, as well as an ant-compatible build.xml file. This allows you to build your project from the command line, or integrate it with the IDE of your choice.

For example, to create a HelloAndroid project similar to the one we just created via Eclipse, you'd use this command:

activityCreator.py --out HelloAndroid com.android.hello.HelloAndroid

To build the project, you'd then run the command 'ant'. When that command successfully completes, you'll be left with a file named HelloAndroid.apk under the 'bin' directory. That .apk file is an Android Package, and can be installed and run in your emulator using the 'adb' tool.

For more information on how to use these tools, please read the documentation cited above.