Hello, World In this document 1. Create an AVD 2. Create the Project 3. Construct the UI 4. Run the Code 5. Upgrade the UI to an XML Layout 6. Debug Your Project 7. Creating the Project Without Eclipse As a developer, you know that the first impression of a development framework is how easy it is to write "Hello, World." Well, on Android, it's pretty easy. It's particularly easy if you're using Eclipse as your IDE, because we've provided a great plugin that handles your project creation and management to greatly speed-up your development cycles. If you're not using Eclipse, that's okay. Familiarize yourself with Developing in Other IDEs. You can then return to this tutorial and ignore anything about Eclipse. Before you start, you should already have the very latest SDK installed, and if you're using Eclipse, you should have installed the ADT plugin as well. If you have not installed these, see Installing the Android SDK and return here when you've completed the installation. Create an AVD To learn more about how to use AVDs and the options available to you, refer to the Android Virtual Devices document. In this tutorial, you will run your application in the Android Emulator. Before you can launch the emulator, you must create an Android Virtual Device (AVD). An AVD defines the system image and device settings used by the emulator. To create an AVD, use the "android" tool provided in the Android SDK. Open a command prompt or terminal, navigate to the tools/ directory in the SDK package and execute: android create avd --target 2 --name my_avd The tool now asks if you would like to create a custom hardware profile. For the time being, press Return to skip it ("no" is the default response). That's it. This configures an AVD named "my_avd" that uses the Android 1.5 platform. The AVD is now ready for use in the emulator. In the above command, the --target option is required and specifies the deployment target to run on the emulator. The --name option is also required and defines the name for the new AVD. Create a New Android ProjectAfter you've created an AVD, the next step is to start a new Android project in Eclipse. 1. From Eclipse, select File > New > Project. If the ADT Plugin for Eclipse has been successfully installed, the resulting dialog should have a folder labeled "Android" which should contain "Android Project". (After you create one or more Android projects, an entry for "Android XML File" will also be available.) 2. Select "Android Project" and click Next. 3. Fill in the project details with the following values: o Project name: HelloAndroid o Application name: Hello, Android o Package name: com.example.helloandroid (or your own private namespace) o Create Activity: HelloAndroid o Min SDK Version: 2Click Finish. Here is a description of each field: Project Name This is the Eclipse Project name — the name of the directory that will contain the project files. Application Name This is the human-readable title for your application — the name that will appear on the Android device. 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.Your package name must be unique across all packages installed on the Android system; for this reason, it's very important to use a standard domain-style package for your applications. The example above uses the "com.example" namespace, which is a namespace reserved for example documentation — when you develop your own applications, you should use a namespace that's appropriate to your organization or entity. Create Activity 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. As the checkbox suggests, this is optional, but an Activity is almost always used as the basis for an application. Min SDK Version This value specifies the minimum API Level required by your application. If the API Level entered here matches the API Level provided by one of the available targets, then that Build Target will be automatically selected (in this case, entering "2" as the API Level will select the Android 1.1 target). With each new version of the Android system image and Android SDK, there have likely been additions or changes made to the APIs. When this occurs, a new API Level is assigned to the system image to regulate which applications are allowed to be run. If an application requires an API Level that is higher than the level supported by the device, then the application will not be installed. Other fields: The checkbox for "Use default location" allows you to change the location on disk where the project's files will be generated and stored. "Build Target" is the platform target that your application will be compiled against (this should be selected automatically, based on your Min SDK Version). Notice that the "Build Target" you've selected uses the Android 1.1 platform. This means that your application will be compiled against the Android 1.1 platform library. If you recall, the AVD created above runs on the Android 1.5 platform. These don't have to match; Android applications are forward-compatible, so an application built against the 1.1 platform library will run normally on the 1.5 platform. The reverse is not true. Your Android project is now ready. It should be visible in the Package Explorer on the left. Open the HelloAndroid.java file, located inside HelloAndroid > src > com.example.helloandroid). It should look like this: package com.example.helloandroid; 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 savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main); } } Notice that the class is based on the Activity class. An Activity is a single application entity that is used to perform actions. An application may have many separate activities, but the user interacts with them one at a time. The onCreate() method will be called by the Android system when your Activity starts — it iswhere you should perform all initialization and UI setup. An