1. What is Android?

1.1. Android Operation System

Android is an operating system based on Linux with a Java programming interface.
The Android Software Development Kit (Android SDK) provides all necessary tools to develop Android applications. This includes a compiler, debugger and a device emulator, as well as its own virtual machine to run Android programs.
Android is currently primarily developed by Google.
Android allows background processing, provides a rich user interface library, supports 2-D and 3-D graphics using the OpenGL libraries, access to the file system and provides an embedded SQLite database.
Android applications consist of different components and can re-use components of other applications. This leads to the concept of a task in Android; an application can re-use other Android components to archive a task.
For example you can write an application which use the Android Gallery application to pick a photo.

1.2. Google Play (Android Market)

Google offers the Google Play service in which programmers can offer their Android application to Android users. Google phones include the Google Play application which allows to install applications.
Google Play also offers an update service, e.g. if a programmer uploads a new version of his application to Google Play, this service will notify existing users that an update is available and allow to install it.
Google Play used to be called Android Market.

1.3. Security and permissions

During deployment on an Android device, the Android system will create a unique user and group ID for every Android application. Each application file is private to this generated user, e.g. other applications cannot access these files.
In addition each Android application will be started in its own process.
Therefore by means of the underlying Linux operating system, every Android application is isolated from other running applications.
If data should be shared, the application must do this explicitly, e.g. via a Service or a ContentProvider.
Android also contains a permission system. Android predefines permissions for certain tasks but every application can define additional permissions.
An Android application declare its required permissions in its AndroidManifest.xml configuration file.For example an application may declare that it requires Internet
Permissions have different levels. Some permissions are automatically granted by the Android system, some are automatically rejected.
In most cases the requested permissions will be presented to the user before installation of the application. The user needs to decided if these permissions should be given to the application.
If the user denies a permission required by the application, this application cannot be installed. The check of the permission is only performed during installation, permissions cannot be denied or granted after the installation.
While not all users pay attention to the required permissions during installation some some users do and they write bad reviews on Google Play.

2. Android components

The following gives a short overview of the most important Android components.

2.1. Activity

Activity represents the presentation layer of an Android application. A simplified description is that an Activity represents a screen in your Android application. This is slightly incorrect as Activities can be displayed as Dialogs or can be transparent.
An Android application can have several Activities.

2.2. Views and ViewGroups

Views are user interface widgets, e.g. buttons or text fields. The base class for all Views is android.view.View. Views often have attributes which can be used to change their appearance and behavior.
A ViewGroup is responsible for arranging other Views. A ViewGroup is also called layout manager. The base class for a layout manager is android.view.ViewGroups which extends View.
ViewGroups can be nestled to create complex layouts. You should not nestle ViewGroups too deeply as this has a negative impact on the performance.

2.3. Intents

Intents are asynchronous messages which allow the application to request functionality from other components of the Android system, e.g. from Services or Activities. An application can call a component directly (explicit Intent ) or ask the Android system to evaluate registered components for a certain Intent (implicit Intents ). For example the application could implement sharing of data via an Intent and all components which allow sharing of data would be available for the user to select. Applications register themselves to an Intent via an IntentFilter.
Intents allow to combine loosely coupled components to perform certain tasks.

2.4. Services

Services perform background tasks without providing a user interface. They can notify the user via the notification framework in Android.

2.5. ContentProvider

ContentProvider provides a structured interface to application data. Via a ContentProvider your application can share data with other applications. Android contains an SQLite database which is frequently used in conjunction with a ContentProvider. The SQLite database would store the data, which would be accessed via the ContentProvider.

2.6. BroadcastReceiver

BroadcastReceiver can be registered to receive system messages and Intents. A BroadcastReceiver will get notified by the Android system, if the specified situation happens. For example a BroadcastReceiver could get called once the Android system completed the boot process or if a phone call is received.

2.7. (HomeScreen) Widgets

Widgets are interactive components which are primarily used on the Android homescreen. They typically display some kind of data and allow the user to perform actions via them. For example a Widget could display a short summary of new emails and if the user selects an email, it could start the email application with the selected email.

2.8. Other

Android provide many more components but the list above describes the most important ones. Other Android components are "Live Folders" and "Live Wallpapers". Live Folders display data on the homescreen without launching the corresponding application.

3. Android Development Tools

3.1. What are the Android Development Tools?

Google provides the Android Development Tools (ADT) to develop Android applications with Eclipse. ADT is a set of components (plug-ins) which extend the Eclipse IDE with Android development capabilities.
ADT contains all required functionalities to create, compile, debug and deploy Android applications from the Eclipse IDE and from the command line. Other IDE's, e.g. IntellJ, are also reusing components of ADT.
ADT also provides an Android device emulator, so that Android applications can be tested without a real Android phone.

3.2. Dalvik Virtual Machine

The Android system uses a special virtual machine, i.e. the Dalvik Virtual Machine to run Java based applications. Dalvik uses an own bytecode format which is different from Java bytecode.
Therefore you cannot directly run Java class files on Android, they need to get converted in the Dalvik bytecode format.

3.3. How to develop Android Applications

Android applications are primarily written in the Java programming language. The Java source files are converted to Java class files by the Java compiler.
Android provides a tool called "dx"" which converts Java class files into a dex (Dalvik Executable) file. All class files of one application are placed in one compressed .dex file. During this conversion process redundant information in the class files are optimized in the .dex file. For example if the same String is found in different class files, the .dex file contains only once reference of this String.
These dex files are therefore much smaller in size than the corresponding class files.
The .dex file and the resources of an Android project, e.g. the images and XML files, are packed into an .apk (Android Package) file. The program aapt (Android Asset Packaging Tool) performs this packaging.
The resulting .apk file contains all necessary data to run the Android application and can be deployed to an Android device via the "adb" tool.
The Android Development Tools (ADT) allows that all these steps are performed transparently to the user; either within Eclipse or via the command line.
If you use the ADT tooling you press a button or run a script and the whole Android application (.apk file) will be created and deployed.

3.4. Resource editors

The ADT allows the developer to define certain artifacts, e.g. Strings and layout files, in two ways: via a rich editor, and directly via XML. This is done via multi-page editors in Eclipse. In these editors you can switch between both representations by clicking on the tab on the lower part of the screen.
For example if you open the "res/layout/main.xml" file in the Package Explorer, you can switch between the two representations as depicted in the following screenshot.

3.5. Logging

Android uses the android.util.Log class for logging with the Log.i(), Log.w() and Log.e() and Log.wtf() methods for logging. This list is sorted by severence.
The first parameter of these method is the category and the second is the message.
Typically you create a Constants class in your Android application and provide your log flag as a public static final field.

    
package de.vogella.android.first;

public class Constants {
 public static final String LOG = "de.vogella.android.first";
}

   

Android advises that a deployed application should not contain logging code. The Android development tools provide the BuildConfig.DEBUG flag for this purpose. This flag will be automatically set to false, if you export the Android application for deployment. During development it will be set to true, therefore allow you to see your logging statements during development.
The following example show how to write an error log message. This message is visible in the "LogCat" view in Eclipse.

    
if (BuildConfig.DEBUG) {
 Log.e(Constants.LOG, "onCreate called");
}
   

4. Android Application Architecture

4.1. AndroidManifest.xml

The components and settings of an Android application are described in the file AndroidManifest.xml. For example all Activities and Services of the application must be declared in this file.
It must also contain the required permissions for the application. For example if the application requires network access it must be specified here.

    
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
      package="de.vogella.android.temperature"
      android:versionCode="1"
      android:versionName="1.0">
    <application android:icon="@drawable/icon" android:label="@string/app_name">
        <activity android:name=".Convert"
                  android:label="@string/app_name">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>

    </application>
    <uses-sdk android:minSdkVersion="9" />

</manifest> 
   

The package attribute defines the base package for the Java objects referred to in this file. If a Java object lies within a different package, it must be declared with the full qualified package name.
Google Play requires that every Android application uses its own unique package. Therefore it is a good habit to use your reverse domain name as package name. This will avoid collisions with other Android applications.
android:versionName and android:versionCode specify the version of your application. versionName is what the user sees and can be any String.
versionCode must be an integer. The Android Market determine based on the versionCode, if it should perform an update of the applications for the existing installations. You typically start with "1" and increase this value by one, if you roll-out a new version of your application.
The tag <activity> defines an Activity, in this example pointing to the Convert class in the de.vogella.android.temperature package. An intent filter is registered for this class which defines that this Activity is started once the application starts (action android:name="android.intent.action.MAIN" ). The category definition category android:name="android.intent.category.LAUNCHER" defines that this application is added to the application directory on the Android device.
The @string/app_name value refers to resource files which contain the actual value of the application name. The usage of resource file makes it easy to provide different resources, e.g. strings, colors, icons, for different devices and makes it easy to translate applications.
The "uses-sdk" part of the "AndroidManifest.xml" file defines the minimal SDK version for which your application is valid. This will prevent your application being installed on devices with older SDK versions.

4.2. R.java and Resources

The " gen " directory in an Android project contains generated values. R.java is a generated class which contains references to certain resources of the project.
These resources must be defined in the "res" directory and can be XML files, icons or pictures. You can for example define values, menus, layouts or animations via XML files.
If you create a new resource, the corresponding reference is automatically created in R.java via the Eclipse ADT tools. These references are static int values and define ID's for the resources.
The Android system provides methods to access the corresponding resource via these ID's.
For example to access a String with the R.string.yourString ID, you would use the getString(R.string.yourString)) method.
R.java is automatically created by the Eclipse development environment, manual changes are not necessary and will be overridden by the tooling.

4.3. Assets

While the res directory contains structured values which are known to the Android platform, the assets directory can be used to store any kind of data. You access this data via the AssetsManager which you can access the getAssets() method.
AssetsManager allows to read an assets as InputStream with the open() method.

    
// Get the AssetManager
AssetManager manager = getAssets();

// Read a Bitmap from Assets
try {
 InputStream open = manager.open("logo.png");
 Bitmap bitmap = BitmapFactory.decodeStream(open);
 // Assign the bitmap to an ImageView in this layout
 ImageView view = (ImageView) findViewById(R.id.imageView1);
 view.setImageBitmap(bitmap);
} catch (IOException e) {
 e.printStackTrace();
}
   

4.4. Activities and Layouts

The user interface for Activities is defined via layouts. The layout defines the included Views (widgets) and their properties.
A layout can be defined via Java code or via XML. In most cases the layout is defined as an XML file.
XML based layouts are defined via a resource file in the /res/layout folder. This file specifies the ViewGroups, Views, their relationship and their attributes for this specific layout.
If a View needs to be accessed via Java code, you have to give the View a unique ID via the android:id attribute. To assign a new ID to a View use @+id/yourvalue. The following shows an example in which a Button gets the "button1" ID assigned.

    
<Button
 android:id="@+id/button1"
 android:layout_width="wrap_content"
 android:layout_height="wrap_content"
 android:text="Show Preferences" >
</Button>
   

By conversion this will create and assign a new yourvalue ID to the corresponding View. In your Java code you can later access a View via the method findViewById(R.id.yourvalue).
Defining layouts via XML is usually the preferred way as this separates the programming logic from the layout definition. It also allows the definition of different layouts for different devices. You can also mix both approaches.

4.5. Reference to resources in XML files

In your XML files, for example your layout files, you can refer to other resources via the @ sign.
For example, if you want to refer to a color which is defined in a XML resource, you can refer to it via @color/your_id. Or if you defined a "hello" string in an XML resource, you could access it via @string/hello.

4.6. Activities and Lifecycle

The Android system controls the lifecycle of your application. At any time the Android system may stop or destroy your application, e.g. because of an incoming call. The Android system defines a lifecycle for Activities via predefined methods. The most important methods are:

• onSaveInstanceState() - called if the Activity is stopped. Used to save data so that the Activity can restore its states if re-started
• onPause() - always called if the Activity ends, can be used to release resource or save data
• onResume() - called if the Activity is re-started, can be used to initialize fields

4.7. Configuration Change

An Activity will also be restarted, if a so called "configuration change" happens. A configuration change happens if an event is triggered which may be relevant for the application. For example if the user changes the orientation of the device (vertically or horizontally). Android assumes that an Activity might want to use different resources for these orientations and restarts the Activity.
In the emulator you can simulate the change of the orientation via CNTR+F11.
You can avoid a restart of your application for certain configuration changes via the configChanges attribute on your Activity definition in your AndroidManifest.xml. The following Activity will not be restarted in case of orientation changes or position of the physical keyboard (hidden / visible).

    
<activity android:name=".ProgressTestActivity"
     android:label="@string/app_name"
     android:configChanges="orientation|keyboardHidden|keyboard">
</activity>
   

4.8. Context

The class android.content.Context provides the connections to the Android system. It is the interface to global information about the application environment. Context also provides access to Android Services, e.g. the Location Service. Activities and Services extend the Context class and can therefore be used as Context.

5. Installation

5.1. Eclipse

The following assumes that you have already Java and Eclipse installed and know how to use Eclipse. For an introduction into Eclipse please see the following tutorial: Eclipse IDE Tutorial.
The tutorial above also describes how to install new components into Eclipse. This is required to install the Android Development Tools. You find the necessary steps described in the following section of the tutorial: Eclipse Update Manager.
The author of this text has also published a Kindle book on the usage of the Eclipse IDE, which can be found here: Eclipse IDE Book for Kindle.

5.2. Pre-requisites for using a 64bit Linux

The Android SDK is 32bit, therefore on a 64bit Linux system you need to have the package ia32-libs installed. For Ubuntu you can do this via the following command.

    
apt-get install ia32-libs
   

Please check your distribution documentation, if you are using a different flavor of Linux.

5.3. Install ADT Plug-ins and Android SDK

Use the Eclipse update manager to install all available components for the Android Development Tools (ADT) from the URL https://dl-ssl.google.com/android/eclipse/.
After the new Android development components are installed, you will be prompted to install the Android SDK. You can use the following wizard or go to the next section to learn how to do it manually.

5.4. Manual installation of the Android SDK

After the installation of the ADT the Eclipse tooling allows to download the Android SDK automatically. Alternatively you can also manually download the Android SDK from the Android SDK download page.
The download contains a zip file, which you can extract to any place in your file system, e.g. on my Linux system I placed it under "/home/vogella/android-sdks". Avoid using spaces in the path name, otherwise you may experience problems with the usage of the Android SDK.
You also have to define the location of the Android SDK in the Eclipse Preferences. In Eclipse open the Preferences dialog via the menu Windows → Preferences. Select Android and enter the installation path of the Android SDK.

5.5. Install a specific Android version

The Android SDK Manager allows you to install specific versions of Android. Select Window → Android SDK Manager from the Eclipse menu.

The dialog allows you to install new packages and also allows you to delete them.
Select "Available packages" and open the "Third Party Add-ons". Select the Google API 15 (Android 4.0.3) version of the SDK and press "Install".

Press the "Install" button and confirm the license for all packages. After the installation completes, restart Eclipse.

5.6. Android Source Code

The following step is optional.

5.6.1. As of Android 4.0

During Android development it is very useful to have the Android source code available.
As of Android 4.0 the Android development tools provides also the source code. You can download it via the Android SDK Manager by selecting the "Sources for Android SDK".
The sources are downloaded to the source directory located in "path_to_android_sdk/sources/android-xx". xx is the API level of Android, e.g. 15 for the Android 4.0.3 version.
To connect the sources with the android.jar file in your Android project, right click on your android.jar in the Eclipse Package Explorer and select Properties → Java Source Attachment. Type in the source directory name and press OK.
Afterwards you can browse through the source code.

5.6.2. Prior to Android 4.0

For earlier versions Haris Peco maintains plugins, which provide the Android Source code code. Use the Eclipse update manager to install the Android Source plugin from the following update site: "http://adt-addons.googlecode.com/svn/trunk/source/com.android.ide.eclipse.source.update".
More details can be found on the project website.

6. Android virtual device - Emulator

6.1. What is the Android Emulator?

The Android Development Tools (ADT) includes an emulator to run an Android system. The emulator behaves like a real Android device (in most cases) and allows you to test your application without having a real device.
You can configure the version of the Android system you would like to run, the size of the SD card, the screen resolution and other relevant settings. You can define several of them with different configurations.
These devices are called "Android Virtual Device" (AVD) and you can start several in parallel. Starting a new emulator is very slow, because the file system of the new AVD needs to get prepared.
The ADT allows to deploy and run your Android program on the AVD.

6.2. Google vs. Android AVD

During the creation of an AVD you decide if you want an Android device or a Google device.
An AVD created for Android will contain the programs from the Android Open Source Project. An AVD created for the Google API's will also contain several Google applications, most notable the Google Maps application.
If you want to use functionality which is only provided via the Google API's, e.g. Cloud2DeviceMessaging or Google Maps you must run this application on an AVD with Google API's.

6.3. Emulator Shortcuts

The following shortcuts are useful for working with the emulator.
Alt+Enter Maximizes the emulator. Nice for demos.
Ctrl+F11 changes the orientation of the emulator.
F8 Turns network on / off.

6.4. Performance

The graphics of the emulator are rendered in software, which is very slow. To have a responsive emulator use a small resolution for your emulator, as for example HVGA.
Also if you have sufficient memory on your computer, add at least 1 GB of memory to your emulator. This is the value "Device ram size" during the creation of the AVD.
Also set the flag "Enabled" for Snapshots. This will save the state of the emulator and will let it start much faster.

6.5. Hardware button

Android 4.0 introduced that devices do not have to have hardware button anymore. If you want to create such an AVD, add the "Hardware Back/Home keys" property to the device configuration and set it to "false".

7. Tutorial: Create and run Android Virtual Device

To define an Android Virtual Device (ADV) open the "AVD Manager" via Windows → AVD Manager and press "New".

Enter the following.

We can also select the box "Enabled" for Snapshots. This will make the second start of the virtual device much faster.
At the end press the button "Create AVD". This will create the AVD configuration and display it under the "Virtual devices".
To test if your setup is correct, select your device and press "Start".
After (a long time) your AVD starts. You are able to use it via the mouse and via the virtual keyboard of the emulator.

8. Error handling and typical problems

Things are not always working as they should. This section gives an overview over typical problems and how to solve them.

8.1. Workspace

Never use a workspace in Eclipse which has spaces in the directory path name, this may lead to several strange problems.

8.2. Clean Project

Several users report that they get the following errors:

1. Project ... is missing required source folder: 'gen'
2. The project could not be built until build path errors are resolved.
3. Unable to open class file R.java.

To solve any of these errors, go to the project menu and select Project → Clean.

8.3. Problems with Android Debug Bridge (adb)

The communication with the emulator or your Android device might have problems. This communication is handled by the Android Debug Bridge (adb).
Eclipse allows to reset the adb in case this causes problems. Select therefore the DDMS perspective via Window → Open Perspective → Other → DDMS
To restart the adb, select the "Reset adb" in the Device View.

8.4. LogCat

The "LogCat" View shows you the log messages of your Android device and helps you to analyze problems. For example Java exceptions in your program would be shown here. To open this view, select Window → Show View → Other → Android → LogCat.

8.5. Android editor not opened

Android provides nice editors to edit Android resource files, unfortunately these editor are not always automatically used due to bugs in the ADT. If that happens, you can open this editor manually. Right-click on your menu file and select Open with → Android Menu Editor.

8.6. Emulator does not start

If your emulator does not start, make sure that the android-sdk version is in a path without any spaces in the path name.

8.7. Timeout during deployment

If you face timeout issues during deployment you can increase the default timeout in the Eclipse preferences. Select Window → Preferences → Android → DDMS and increase the "ADB connection timeout (in ms)" value.

8.8. Installation failed due to insufficient storage

Sometimes the emulator will refuse to install an application with the error message: INSTALL_FAILED_INSUFFICIENT_STORAGE.
An Android virtual device provides per default only 64M for the storaging Android applications. You can clean your installed application by re-starting the emulator and selecting the "Wipe user data" flag.
Alternatively you can set the data partition size. If you press edit on the AVD, you can set the "Ideal size of data partition" property via the "New" button.

8.9. Debug Certificate expired

If you get the error message "Debug Certificate expired" switch to the folder which contains the Android AVD, e.g. ".android" under Linux and delete the "debug.keystore" file. This file is only valid for a year and if not present, Eclipse will regenerate the password.

8.10. Error message for @override

The @override annotation was introduced in Java 1.6. If you receive an error message for @override, change the Java compiler level to Java 1.6. To do this, right-click on the project, select Properties → Java Compiler → Compiler compliance level and select "1.6" in the drop-down box.

8.11. Missing Imports

Java requires that classes which are not part of the standard Java Language are either fully qualified or declared via imports.
If you see an error message with "XX cannot be resolved to a variable", right-click in your Editor and select Source → Organize Imports to important required packages.

8.12. Eclipse Tips

To work more efficiently with Eclipse, select Window → Preferences → Java → Editor → Save Actions and select that the source code should be formated and that the imports should be organized at every save action.

9. Conventions for the tutorials

9.1.  API version, package and application name

The tutorials of this document have been developed and tested with Android 4.0.4, API Level 15. Please use this version for all tutorials in this book. Higher versions of the API level should also work. A lower version of the Android API might also work, but if you face issues, try the recommended version.
The base package for the projects is always the same as the project name, e.g. if you are asked to create a project "de.vogella.android.example.test", then the corresponding package name is "de.vogella.android.example.test".
The application name, which must be entered on the Android project generation wizard, will not be predefined. Choose a name you like.

9.2.  Warning messages for Strings

The Android development tools show warnings, if you use hard-coded strings, for example in layout files. For real applications you should use String resource files. To simplify the creation of the examples, we use Strings directly. Please ignore the corresponding warnings.

10. Tutorial: Your first Android project

10.1. Install the demo application

This application is also available on the Android Marketplace under Android Temperature converter .
Alternatively you can also scan the following barcode with your Android smartphone to install it via the Google Play application.

10.2. Create Project

Select File → New → Other → Android → Android Project and create the Android project "de.vogella.android.temperature". Enter the following.

Press "Finish". This should create the following directory structure.

10.3. Modifying resources

As described in the Android Development Tools (ADT) chapter, ADT provides specialized editors for resources files, e.g. layout files. These editors allow to switch between the XML representation of the file and a richer user interface via tabs on the bottom of the editor.
The following description uses the rich user interface to build layout files. For validation purposes, the resulting XML is also included in the description.

10.4. Create attributes

Android allows you to create static attributes, e.g. Strings or colors. These attributes can for example be used in your XML layout files or referred to via Java source code.
Select the file "res/values/string.xml" and press the Add button. Select "Color" and enter "myColor" as the name and "#3399CC" as the value.

Add the following "String" attributes. String attributes allow the developer to translate the application at a later point.

Table 1. String Attributes

Name	Value
celsius	to Celsius
fahrenheit	to Fahrenheit
calc	Calculate

Switch to the XML representation and validate that the values are correct.

    
<?xml version="1.0" encoding="utf-8"?>
<resources>
 <string name="hello">Hello World, Convert!</string>
 <string name="app_name">Temperature Converter</string>
 <color name="myColor">#3399CC</color>
 <string name="myClickHandler">myClickHandler</string>
 <string name="celsius">to Celsius</string>
 <string name="fahrenheit">to Fahrenheit</string>
 <string name="calc">Calculate</string>
</resources>
   

10.5. Add Views

Select "res/layout/main.xml" and open the Android editor via a double-click. This editor allows you to create the layout via drag and drop or via the XML source code. You can switch between both representations via the tabs at the bottom of the editor. For changing the position and grouping elements you can use the Eclipse "Outline" view.
The following shows a screenshot of the "Palette" view from which you can drag and drop new user interface components into your layout. Please note that the "Palette" view changes frequently so your view might be a bit different.

You will now create your new layout.
Right-click on the existing text object “Hello World, Hello!” in the layout. Select "Delete" from the popup menu to remove the text object. Then, from the “Palette” view, select Text Fields and locate "Plain Text". Drag this onto the layout to create a text input field. All object types in the section "Text Fields” derive from the class "EditText", they just specify via an additional attribute which text type can be used.
Afterwards select the Palette section "Form Widgets" and drag a “RadioGroup” object onto the layout. The number of radio buttons added to the radio button group depends on your version of Eclipse. Make sure there are two radio buttons by deleting or adding radio buttons to the group.
From the Palette section Form Widgets, drag a Button object onto the layout.
The result should look like the following.

Switch to "main.xml" and verify that your XML looks like the following.

    
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical" >

    <EditText
        android:id="@+id/editText1"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="EditText" >
    </EditText>

    <RadioGroup
        android:id="@+id/radioGroup1"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" >

        <RadioButton
            android:id="@+id/radio0"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:checked="true"
            android:text="RadioButton" >
        </RadioButton>

        <RadioButton
            android:id="@+id/radio1"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="RadioButton" >
        </RadioButton>
    </RadioGroup>

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Button" >
    </Button>

</LinearLayout>
   

10.6. Edit View properties

If you select a user interface component (an instance of View ), you can change its properties via the Eclipse "Properties" view. Most of the properties can be changed via the right mouse menu. You can also edit properties of fields directly in XML. Changing properties in the XML file is much faster, if you know what you want to change. But the right mouse functionality is nice, if you are searching for a certain property.
Open your file "main.xml". The EditText control shows currently a default text. We want to delete this initial text in the XML code. Switch to the XML tab called "main.xml" and delete the android:text="EditText" property from the EditText part. Switch back to the "Graphical Layout" tab and check that the text is removed.
Use the right mouse click on the first radio button to assign the "celsius" String attribute to its "text" property. Assign the "fahrenheit" string attribute to the second radio button.

From now on, I assume you are able to use the properties menu on user interface components. You can always either edit the XML file or modify the properties via right mouse click.
Set the property "Checked" to true for the first RadioButton.
Assign "calc" to the text property of your button and assign "myClickHandler" to the "onClick" property.
Set the "Input type" property to "numberSigned" and "numberDecimal" on your EditText.
All your user interface components are contained in a LinearLayout. We want to assign a background color to this LinearLayout. Right-click on an empty space in Graphical Layout mode, then select Other Properties → All by Name → Background. Select “Color” and then select "myColor" "in the list which is displayed.

Switch to the "main.xml" tab and verify that the XML is correct.

    
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:background="@color/myColor"
    android:orientation="vertical" >

    <EditText
        android:id="@+id/editText1"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:inputType="numberDecimal|numberSigned" >
    </EditText>

    <RadioGroup
        android:id="@+id/radioGroup1"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" >

        <RadioButton
            android:id="@+id/radio0"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:checked="true"
            android:text="@string/celsius" >
        </RadioButton>

        <RadioButton
            android:id="@+id/radio1"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@string/fahrenheit" >
        </RadioButton>
    </RadioGroup>

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:onClick="myClickHandler"
        android:text="@string/calc" >
    </Button>

</LinearLayout>
   

10.7. Change the Activity source code

During the generation of your new Android project you specified that an Activity called ConvertActivity should be created. The project wizard created the corresponding Java class.
Change your code in ConvertActivity.java to the following. Note that the myClickHandler will be called based on the OnClick property of your button.

    
package de.vogella.android.temperature;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;
import android.widget.RadioButton;
import android.widget.Toast;

public class ConvertActivity extends Activity {
 private EditText text;

 @Override
 public void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.main);
  text = (EditText) findViewById(R.id.editText1);

 }

 // This method is called at button click because we assigned the name to the
 // "On Click property" of the button
 public void myClickHandler(View view) {
  switch (view.getId()) {
  case R.id.button1:
   RadioButton celsiusButton = (RadioButton) findViewById(R.id.radio0);
   RadioButton fahrenheitButton = (RadioButton) findViewById(R.id.radio1);
   if (text.getText().length() == 0) {
    Toast.makeText(this, "Please enter a valid number",
      Toast.LENGTH_LONG).show();
    return;
   }

   float inputValue = Float.parseFloat(text.getText().toString());
   if (celsiusButton.isChecked()) {
    text.setText(String
      .valueOf(convertFahrenheitToCelsius(inputValue)));
    celsiusButton.setChecked(false);
    fahrenheitButton.setChecked(true);
   } else {
    text.setText(String
      .valueOf(convertCelsiusToFahrenheit(inputValue)));
    fahrenheitButton.setChecked(false);
    celsiusButton.setChecked(true);
   }
   break;
  }
 }

 // Converts to celsius
 private float convertFahrenheitToCelsius(float fahrenheit) {
  return ((fahrenheit - 32) * 5 / 9);
 }

 // Converts to fahrenheit
 private float convertCelsiusToFahrenheit(float celsius) {
  return ((celsius * 9) / 5) + 32;
 }
}
   

10.8. Start Project

To start the Android Application, select your project, right click on it, and select Run-As → Android Application. If an emulator is not yet running, it will be started. Be patient, the emulator starts up very slowly.
You should get the following result.

Type in a number, select your conversion and press the button. The result should be displayed and the other option should get selected.

11. Starting an installed application

After you run your application on the virtual device, you can start it again on the device. If you press the "Home" button you can select your application.

12. OptionMenu and ActionBar

12.1. ActionBar

The ActionBar is located at the top of the Activity that may display the Activity title, navigation modes, and other interactive items.
The following picture show the ActionBar of a typical Google Application with interactive items and a nagivation bar.

12.2. OptionsMenu

The application can also open a menu which shows actions via a popup menu. This OptionsMenu is only available if the phone has a hardware "Options" button. Even if the hardware button is available, it is recommended to use the ActionBar, which is available for phones as of Android 4.0.
The following picture highlights the hardware button and the resulting menu as popup.

One of the reasons why the ActionBar is superior to the OptionsMenu, if that it is clearly visible, while the OptionsMenu is only shown on request and the user may not recognize that options are available.

12.3. Creating the menu

The OptionsMenu and the ActionBar is filled by the onCreateOptionsMenu() method of your Activity.
In the onCreateOptionsMenu() method you can create the menu entries. You can add menu entries via code or via the inflation of an existing XML resources.
The MenuInflator class allows to inflate menu entries defined in XML to the menu. MenuInflator can get accessed via the getMenuInflator() method in your Activity.
The onCreateOptionsMenu() method is only called once. If you want to influence the menu later you have to use the onPrepareOptionsMenu() method. onPrepareOptionsMenu() is not called for entries in the ActionBar for these entries you have to use the invalidateOptionsMenu() method.

12.4. Reacting to menu entry selection

If a menu entry is selected then then onOptionsItemSelected() method is called. As parameter you receive the menu entry which was selected so that you can react differently to different menu entries.

12.5. Using the home icon

The ActionBar also shows an icon of your application. You can also add an action to this icon. If you select this icon the onOptionsItemSelected() method will be called with the value android.R.id.home. The recommendation is to return to the main Activity in your program.

    
// If home icon is clicked return to main Activity
case android.R.id.home:
 Intent intent = new Intent(this, OverviewActivity.class);
 intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
 startActivity(intent);
 break;
   

12.6. ActionBar tabs

It is also possible to add tabs to the ActionBar which can be used for navigation. Typically Fragments are used for this purpose. We demonstrate this in the Fragments chapter.

12.7. Custom Views in the ActionBar

You can also add a custom View to the ActionBar. The following code snippet demonstrates that.

    
@Override
public void onCreate(Bundle savedInstanceState) {
 super.onCreate(savedInstanceState);
 setContentView(R.layout.main);
 
 ActionBar actionBar = getActionBar();
 // add the custom view to the action bar
 actionBar.setCustomView(R.layout.actionbar_view);
 actionBar.setDisplayOptions(ActionBar.DISPLAY_SHOW_CUSTOM
  | ActionBar.DISPLAY_SHOW_HOME);
}
   

12.8. Contextual action mode

A contextual action mode activates a temporary ActionBar that overlays the application ActionBar for the duration of a particular sub-task.
The contextual action mode is typically activated by selecting an item or by long clicking on it.
To implemented this, call the startActionMode() method on a View or on your Activity. This method gets an ActionMode.Callback object which is responsible for the lifecycle of the contextual ActionBar.

12.9. Context menus

You can also assign a context menu to a View. A context menu is also activated if the user "long presses" the view.
If possible the contextual action mode should be preferred over a context menu.
A context menu for a view is registered via the registerForContextMenu(view) method. The onCreateContextMenu() method is called every time a context menu is activated as the context menu is discarded after its usage. The Android platform may also add options to your View, e.g. EditText provides context options to select text, etc.

13. Tutorial: ActionBar

13.1. Project

This chapter will demonstrate how to create items in the ActionBar and react to the selection of the user.
Create a project called "de.vogella.android.socialapp" with the Activity called "OverviewActivity".

13.2. Add a menu XML resource

Select your project, right click on it and select New → Other → Android → Android XML File to create a new XML resource.
Select the option "Menu", enter as File "mainmenu.xml" and press the button "Finish".

This will create a new file "mainmenu.xml" in the folder "res/menu" of your project. Open this file and select the "Layout" tab of the Android editor.
Press Add and select "Item". Maintain a entry similar to the following screenshot. Via the "ifRoom" attribute you define that the menu entry is displayed in the ActionBar if there is sufficient space available.

Add a similar entry to the menu with the ID set to "@+id/menuitem2", the Title set to "Test". Also set the "ifRoom" flag.
The resulting XML will look like the following.

    
<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android" >

    <item
        android:id="@+id/menuitem1"
        android:showAsAction="ifRoom"
        android:title="Prefs">
    </item>
    <item
        android:id="@+id/menuitem2"
        android:showAsAction="ifRoom"
        android:title="Test">
    </item>

</menu>
   

Change your Activity class "OverviewActivity" to the following.

    
package de.vogella.android.socialapp;

import android.app.Activity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.MenuItem;
import android.widget.Toast;

public class OverviewActivity extends Activity {
 @Override
 public void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.main);
 }

 @Override
 public boolean onCreateOptionsMenu(Menu menu) {
  MenuInflater inflater = getMenuInflater();
  inflater.inflate(R.menu.mainmenu, menu);
  return true;
 }

 @Override
 public boolean onOptionsItemSelected(MenuItem item) {
  switch (item.getItemId()) {
  case R.id.menuitem1:
   Toast.makeText(this, "Menu Item 1 selected", Toast.LENGTH_SHORT)
     .show();
   break;
  case R.id.menuitem2:
   Toast.makeText(this, "Menu item 2 selected", Toast.LENGTH_SHORT)
     .show();
   break;

  default:
   break;
  }

  return true;
 }
}

   

Run your application. As there is enough space in the ActionBar otherwise you may see the Overflow menu or you have to use the "Option" menu button on your phone. If you select one item, you should see a small info message.

14. Tutorial: Using the contextual action mode

Add a EditText element your your "main.xml" layout file.

   
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical" >

    <EditText
        android:id="@+id/myView"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:ems="10" >

        <requestFocus />
    </EditText>

</LinearLayout>
  

Create a new menu XML resource with the file name "contextual.xml"

   
<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android" >
    <item
        android:id="@+id/toast"
        android:title="Toast">
    </item>

</menu>
  

Change your Activity to the following.

   
package de.vogella.android.socialapp;

import android.app.Activity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.MenuItem;
import android.widget.Toast;

public class OverviewActivity extends Activity {
 protected Object mActionMode;
 @Override
 public void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.main);
  // Define the contextual action mode
  View view = findViewById(R.id.myView);
  view.setOnLongClickListener(new View.OnLongClickListener() {
   // Called when the user long-clicks on someView
   public boolean onLongClick(View view) {
    if (mActionMode != null) {
     return false;
    }

    // Start the CAB using the ActionMode.Callback defined above
    mActionMode = OverviewActivity.this
      .startActionMode(mActionModeCallback);
    view.setSelected(true);
    return true;
   }
  });
 }

 @Override
 public boolean onCreateOptionsMenu(Menu menu) {
  MenuInflater inflater = getMenuInflater();
  inflater.inflate(R.menu.mainmenu, menu);
  return true;
 }

 @Override
 public boolean onOptionsItemSelected(MenuItem item) {
  Toast.makeText(this, "Just a test", Toast.LENGTH_SHORT).show();
  return true;
 }

 private ActionMode.Callback mActionModeCallback = new ActionMode.Callback() {

  // Called when the action mode is created; startActionMode() was called
  public boolean onCreateActionMode(ActionMode mode, Menu menu) {
   // Inflate a menu resource providing context menu items
   MenuInflater inflater = mode.getMenuInflater();
   // Assumes that you have "contexual.xml" menu resources
   inflater.inflate(R.menu.contextual, menu);
   return true;
  }

  // Called each time the action mode is shown. Always called after
  // onCreateActionMode, but
  // may be called multiple times if the mode is invalidated.
  public boolean onPrepareActionMode(ActionMode mode, Menu menu) {
   return false; // Return false if nothing is done
  }

  // Called when the user selects a contextual menu item
  public boolean onActionItemClicked(ActionMode mode, MenuItem item) {
   switch (item.getItemId()) {
   case R.id.toast:
    Toast.makeText(OverviewActivity.this, "Selected menu",
      Toast.LENGTH_LONG).show();
    mode.finish(); // Action picked, so close the CAB
    return true;
   default:
    return false;
   }
  }

  // Called when the user exits the action mode
  public void onDestroyActionMode(ActionMode mode) {
   mActionMode = null;
  }
 };

}
  

If you run this example and long press the EditText widget, your contextual ActionBar is displayed.