How to develop your first android app ?

Building Your First App

This class teaches you how to build your first Android app. You’ll learn how to create an Android project and run a debuggable version of the app. You'll also learn some fundamentals of Android app design, including how to build a simple user interface and handle user input.

Before you start this class, be sure you have your development environment set up. You need to:

1. Download the Android SDK.
2. Install the ADT plugin for Eclipse (if you’ll use the Eclipse IDE).
3. Download the latest SDK tools and platforms using the SDK Manager.

If you haven't already done these tasks, start by downloading the Android SDK and following the install steps. Once you've finished the setup, you're ready to begin this class.

This class uses a tutorial format that incrementally builds a small Android app that teaches you some fundamental concepts about Android development, so it's important that you follow each step.

Creating an Android Project

An Android project contains all the files that comprise the source code for your Android app. The Android SDK tools make it easy to start a new Android project with a set of default project directories and files.

This lesson shows how to create a new project either using Eclipse (with the ADT plugin) or using the SDK tools from a command line.

Note: You should already have the Android SDK installed, and if you're using Eclipse, you should also have the ADT plugininstalled (version 20.0.0 or higher). If you don't have these, follow the guide to Installing the Android SDK before you start this lesson.

Create a Project with Eclipse

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1. In Eclipse, click New Android App Project  in the toolbar. (If you don’t see this button, then you have not installed the ADT plugin—see Installing the Eclipse Plugin.)


Figure 1. The New Android App Project wizard in Eclipse.
2. Fill in the form that appears:
   • Application Name is the app name that appears to users. For this project, use "My First App."
   • Project Name is the name of your project directory and the name visible in Eclipse.
   • Package Name is the package namespace for your app (following the same rules as packages in the Java programming language). Your package name must be unique across all packages installed on the Android system. For this reason, it's generally best if you use a name that begins with the reverse domain name of your organization or publisher entity. For this project, you can use something like "com.example.myfirstapp." However, you cannot publish your app on Google Play using the "com.example" namespace.
   • Build SDK is the platform version against which you will compile your app. By default, this is set to the latest version of Android available in your SDK. (It should be Android 4.1 or greater; if you don't have such a version available, you must install one using the SDK Manager). You can still build your app to support older versions, but setting the build target to the latest version allows you to enable new features and optimize your app for a great user experience on the latest devices.
   • Minimum Required SDK is the lowest version of Android that your app supports. To support as many devices as possible, you should set this to the lowest version available that allows your app to provide its core feature set. If any feature of your app is possible only on newer versions of Android and it's not critical to the app's core feature set, you can enable the feature only when running on the versions that support it.
     Leave this set to the default value for this project.
   Click Next.
3. The following screen provides tools to help you create a launcher icon for your app.
   You can customize an icon in several ways and the tool generates an icon for all screen densities. Before you publish your app, you should be sure your icon meets the specifications defined in the Iconography design guide.
   Click Next.
4. Now you can select an activity template from which to begin building your app.
   For this project, select BlankActivity and click Next.
5. Leave all the details for the activity in their default state and click Finish.

Your Android project is now set up with some default files and you’re ready to begin building the app.

Running Your App

If you followed the previous lesson to create an Android project, it includes a default set of "Hello World" source files that allow you to immediately run the app.

How you run your app depends on two things: whether you have a real Android-powered device and whether you’re using Eclipse. This lesson shows you how to install and run your app on a real device and on the Android emulator, and in both cases with either Eclipse or the command line tools.

Before you run your app, you should be aware of a few directories and files in the Android project:

AndroidManifest.xml

The manifest file describes the fundamental characteristics of the app and defines each of its components. You'll learn about various declarations in this file as you read more training classes.

src/

Directory for your app's main source files. By default, it includes an Activity class that runs when your app is launched using the app icon.

res/

Contains several sub-directories for app resources. Here are just a few:

drawable-hdpi/

Directory for drawable objects (such as bitmaps) that are designed for high-density (hdpi) screens. Other drawable directories contain assets designed for other screen densities.

layout/

Directory for files that define your app's user interface.

values/

Directory for other various XML files that contain a collection of resources, such as string and color definitions.

When you build and run the default Android app, the default Activity class starts and loads a layout file that says "Hello World." The result is nothing exciting, but it's important that you understand how to run your app before you start developing.

Run on a Real Device

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If you have a real Android-powered device, here's how you can install and run your app:

1. Plug in your device to your development machine with a USB cable. If you’re developing on Windows, you might need to install the appropriate USB driver for your device. For help installing drivers, see the OEM USB Drivers document.
2. Ensure that USB debugging is enabled in the device Settings (open Settings and navitage to Applications > Development on most devices, or click Developer options on Android 4.0 and higher).

To run the app from Eclipse, open one of your project's files and click Run  from the toolbar. Eclipse installs the app on your connected device and starts it.

Or to run your app from a command line:

1. Change directories to the root of your Android project and execute:

   ant debug

2. Make sure the Android SDK platform-tools/ directory is included in your PATH environment variable, then execute:

   adb install bin/MyFirstApp-debug.apk

3. On your device, locate MyFirstActivity and open it.

That's how you build and run your Android app on a device! To start developing, continue to the next lesson.

Run on the Emulator

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Whether you’re using Eclipse or the command line, to run your app on the emulator you need to first create anAndroid Virtual Device (AVD). An AVD is a device configuration for the Android emulator that allows you to model different devices.

Figure 1. The AVD Manager showing a few virtual devices.

To create an AVD:

1. Launch the Android Virtual Device Manager:
   1. In Eclipse, click Android Virtual Device Manager from the toolbar.
   2. From the command line, change directories to<sdk>/tools/ and execute:

      android avd

2. In the Android Virtual Device Manager panel, click New.
3. Fill in the details for the AVD. Give it a name, a platform target, an SD card size, and a skin (HVGA is default).
4. Click Create AVD.
5. Select the new AVD from the Android Virtual Device Manager and click Start.
6. After the emulator boots up, unlock the emulator screen.

To run the app from Eclipse, open one of your project's files and click Run  from the toolbar. Eclipse installs the app on your AVD and starts it.

Or to run your app from the command line:

1. Change directories to the root of your Android project and execute:

   ant debug

2. Make sure the Android SDK platform-tools/ directory is included in your PATH environment variable, then execute:

   adb install bin/MyFirstApp-debug.apk

3. On the emulator, locate MyFirstActivity and open it.

That's how you build and run your Android app on the emulator!

Source: http://developer.android.com/training/basics/firstapp/index.html