Tutorial

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.

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.

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.

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.

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.

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.

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

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.

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.

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.

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.

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.

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.

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.

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.

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");
}
			

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.

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.

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();
}
			

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.

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.

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:

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>
			

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.

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.

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.

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.

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.

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.

The following step is optional.

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.

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.

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.

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.

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".

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

Several users report that they get the following errors:

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

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.

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.

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.

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

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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>
			

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>
			

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>
			

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;
	}
}
			

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.

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.

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.

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.

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.

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;
			

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.

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);
}
			

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.

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.

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".

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.

A layout manager is a subclass of ViewGroup and is responsible for the layout of itself and its child Views. Android supports different default layout managers.

As of Android 4.0 the most relevant layout manager are LinearLayout, FrameLayout, RelativeLayout and GridLayout.

All layouts allow the developer to define attributes. Children can also define attributes which may be evaluated by their parent layout.

AbsoluteLayoutLayout is deprecated and TableLayout can be implemented more effectively via GridLayout

LinearLayout puts all its child elements into a single column or row depending on the android:orientation attribute. Possible values for this attribute are horizontal and vertical, horizontal is the default value.

LinearLayout can be nested to achieve more complex layouts.

RelativeLayout allow to position the widget relative to each other. This allows for complex layouts.

A simple usage for RelativeLayout is if you want to center a single component. Just add one component to the RelativeLayout and set the android:layout_centerInParent attribute to true.

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

    <ProgressBar
        android:id="@+id/progressBar1"
        style="?android:attr/progressBarStyleLarge"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerInParent="true"
         />

</RelativeLayout>
			

GridLayout was introduced with Android 4.0. This layout allows you to organize a view into a Grid. GridLayout separates its drawing area into: rows, columns, and cells.

You can specify how many columns you want for define for each View in which row and column it should be placed and how many columns and rows it should use. If not specified GridLayout uses defaults, e.g. one column, one row and the position of a View depends on the order of the declaration of the Views.

ScrollViews can be used to contain one view that might be to big to fit on one screen. If the view is to big the ScrollView will display a scroll bar to scroll the context. Of course this view can be a layout which can then contain other elements.

A instance of a Drawable resource is a general concept for a graphic which can be drawn. The simplest case is a bitmap file, which would be represented in Android via a BitmapDrawable class. Bitmaps are typically stored in one of the "res/drawable" folders. The Android project creation wizard creates several of these folders per default, you can provide different sized files for different resolutions of Android devices. If you only provide

In additional to graphical files, Android supports XML drawables and 9-patch graphics. XML drawables are used to describe shapes (color, border, gradient), State and Transitions and more.

9-patch graphics are used to define which part of a graphic should be increased if the View which uses this graphic is larger then the graphic.

Shape Drawables are XML files which allow to define a geometric object with colors, borders and gradients which can get assigned to Views. The advantage of using XML Shape Drawables is that they automatically adjust to the correct size.

The following listing shows an example of a Shape Drawable.

				
<?xml version="1.0" encoding="UTF-8"?>
<shape
	xmlns:android="http://schemas.android.com/apk/res/android"
	android:shape="rectangle">
	<stroke
		android:width="2dp"
		android:color="#FFFFFFFF" />
	<gradient
		android:endColor="#DDBBBBBB"
		android:startColor="#DD777777"
		android:angle="90" />
	<corners
		android:bottomRightRadius="7dp"
		android:bottomLeftRadius="7dp"
		android:topLeftRadius="7dp"
		android:topRightRadius="7dp" />
</shape>
			

You could for example assign that drawable to the background property of your layout.

				
<?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="@drawable/myshape"
    android:orientation="vertical" >

    <EditText
        android:id="@+id/editText1"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
	 >
    </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:text="@string/calc" 
        android:onClick="myClickHandler">
    </Button>

</LinearLayout>
			

State drawables allow to define states. For each state a different drawable can get assigned to the View. For example the following defines different drawables for a button depending on its state.

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

	<item android:drawable="@drawable/button_pressed"
		android:state_pressed="true" />
	<item android:drawable="@drawable/button_checked"
		android:state_checked="true" />
	<item android:drawable="@drawable/button_default" />

</selector>
			

Transition Drawables allow to define transitions which can be triggered in the coding.

				
<?xml version="1.0" encoding="utf-8"?>
<transition xmlns:android="http://schemas.android.com/apk/res/android">
	<item android:drawable="@drawable/first_image" />
	<item android:drawable="@drawable/second_image" />
</transition>
			

				
final ImageView image = (ImageView) findViewById(R.id.image);
final ToggleButton button = (ToggleButton) findViewById(R.id.button);
button.setOnClickListener(new OnClickListener() {
	@Override
	public void onClick(final View v) {
		TransitionDrawable drawable = (TransitionDrawable) image.getDrawable();
		if (button.isChecked()) {
			drawable.startTransition(500);
		} else {
			drawable.reverseTransition(500);
		}
	}
});
			

9 Patch drawables are drawables which have a one pixel additional border. On the top and left you define the area which should grow if the drawable is to small for the View.

On the right and bottom side you define where a text should be placed if this Drawable is used on a View which can write text on it, e.g. a Button.

The ADT supplies the "draw9patch" program in the "android-sdk/tools" installation folder, which makes it easy to create 9 patch drawables.

Styles in Android allow to define the look and feel of Android application in external files. Styles can get assigned to Views.

You can define styles in XML and assign them to these elements. This way you only have to set common attributes once and can later change the look in one central place.

To define a style, save an XML file in the /res/values" directory of your project. The root node of the XML file must be <resources> .

The following "styles.xml" XML file would be created in the "/res/xml" folder.

				
<?xml version="1.0" encoding="utf-8"?>
<resources>
    <style name="text">
        <item name="android:padding">4dip</item>
        <item name="android:textAppearance">?android:attr/textAppearanceLarge</item>
        <item name="android:textColor">#000000</item>
    </style>
    <style name="layout">
        <item name="android:background">#C0C0C0</item>
    </style>
</resources>
			

You assign the style attribute to your elements, for example to the text elements via style=”@style/text”.

You can also refer to individual attributes of Android styles. For example the following will define button with the Android 4.0 button style.

				
<?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" >

    <LinearLayout
        style="?android:attr/buttonBarStyle"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal" >

        <Button
            android:id="@+id/Button01"
            style="?android:attr/buttonBarButtonStyle"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="Show" />

        <Button
            android:id="@+id/Button02"
            style="?android:attr/buttonBarButtonStyle"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="Change" />
    </LinearLayout>

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

        <requestFocus />
    </EditText>

</LinearLayout>
			

A theme is a style applied to an entire Activity or application, rather than an individual View (as in the example above).

The next example show how to define your own Theme while extending a platform theme.

				
<?xml version="1.0" encoding="utf-8"?>
<resources>

    <style name="MyTheme" parent="android:Theme.Light">
        <item name="android:windowNoTitle">true</item>
        <item name="android:windowBackground">@color/translucent_red</item>
        <item name="android:listViewStyle">@style/MyListView</item>
    </style>

    <style name="MyListView" parent="@android:style/Widget.ListView">
        <item name="android:listSelector">@drawable/ic_menu_home</item>
    </style>

</resources>
			

How to design your Android application is described on the Android Design Pages, which can be found here http://developer.android.com/design/index.html .

This page also contains several downloadable resources, e.g. an Icon set for the ActionBar.

Android devices are different in terms of resolution and in terms of density. Therefore it is recommended never to use fixed sized dimensions.

The unit of measurement which should be used is "dp" (which is the same as "dip" but shorter). dp refers to the base line of an Android device, e.g. 320x480 with 160dpi (dots per inch), which was the size of the first Android device (G1). This size is also known as mdpi (medium dots per inch).

If you specify the size in "dp", Android will automatically scale it, depending on the device. On a mdpi device "dp" will be equal to pixel but it will be smaller on a ldpi (approx. 120dip) and larger on a hdip (approx. 240dpi) device, so that it always occupies the same physical space.

You can use "dp" in your resources, e.g. layout files. The Android SDK expects that you specify everything in pixels. You can therefore use the following formulator to calculate the right amount of pixels for a dimension specified in dp.

				
public int convertToDp(int input) {
	// Get the screen's density scale
	final float scale = getResources().getDisplayMetrics().density;
	// Convert the dps to pixels, based on density scale
	return (int) (input * scale + 0.5f);
}
			

Android also allows to use resource qualifiers to specify that certain resources should only be used for certain resolutions or configurations. For example your can provide a special "main.xml" layout file for a device in landscape mode, if you place such a file in the "res/layout-land" folder.

The different resource qualifiers are described on the following webpage: Providing Resources

Fragment components allow you to organize your application code so that it is easier to support different sized devices.

Fragments are components with their own lifecycle and their own user interface. They can be defined via layout files or via coding.

Fragments always run in the context of an Activity. If an Activity is stopped its Fragments will also be stopped; if an Activity is destroyed its Fragments will also get destroyed.

If a Fragment component is defined in an XML layout file, the android:name attribute points to the Fragments class.

The base class for Fragments is android.app.Fragment. For special purposes you can also use more special classes, like ListFragment or DialogFragment.

The onCreateView() method is called by Android once the Fragment should create its user interface. Here you can inflate an layout. The onStart() method is called once the Fragment gets visible.

Fragments can be dynamically added and removed from an Activity via Fragment transactions. This will add the action to the history stack of the Activity, i.e. this will allow to revert the Fragment changes in the Activity via the back button.

Fragments make it easy to re-use components in different layouts, e.g. you can build single-pane layouts for handsets (phones) and multi-pane layouts for tablets.

This is not limited to tablets; for example you can use Fragments also to support different layout for landscape and portrait orientation. But as tablets offer significantly more space you typically include more views into the layout and Fragments makes that easier.

The typical example is a list of items in an activity. On a tablet you see the details immediately on the same screen on the right hand side if you click on item. On a handset you jump to a new detail screen. The following discussion will assume that you have two Fragments (main and detail) but you can also have more. We will also have one main activity and one detailed activity. On a tablet the main activity contains both Fragments in its layout, on a handheld it only contains the main fragment.

To check for an fragment you can use the FragmentManager.

				
DetailFragment fragment = (DetailFragment) getFragmentManager().
 	findFragmentById(R.id.detail_frag);
if (fragment==null || ! fragment.isInLayout()) {
	// start new Activity
	}
else {
	fragment.update(...);
}

			

To create different layouts with Fragments you can:

The second approach is the most flexible and in general preferable way of using Fragments. In this case the main activity checks if the detail fragment is available in the layout. If the detailed fragment is there, the main activity tells the fragment that is should update itself. If the detail fragment is not available the main activity starts the detailed activity.

It is good practice that Fragments do not manipulate each other. For this purpose a Fragment typically implements an interface to get new data from its host Activity.

The following tutorial demonstrates how to use Fragments. The entry Activity (called MainActivity of our application ) will use different layouts for portrait and for landscape mode.

In portrait mode MainActivity will show one Fragment with a list of names. If the user touches an item in the list, a second Activity called DetailActivity will start and show the selected text.

In landscape mode MainActivity will show two Fragments. The first is again the Fragments which shows the list of names. The second Fragment shows the text of the current selected item. This is similar to the portrait mode, but the whole information will be shown on one screen.

Create a new project de.vogella.android.fragments with an Activity called MainActivity.

Create or change the following layout files in the "res/layout/" folder.

First create the following file called "details.xml". This layout will be used by the DetailFragment.

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

    <TextView
        android:id="@+id/detailsText"
        android:layout_width="wrap_content"
        android:layout_height="match_parent"
        android:layout_gravity="center_horizontal|center_vertical"
        android:layout_marginTop="20dip"
        android:text="Large Text"
        android:textAppearance="?android:attr/textAppearanceLarge"
        android:textSize="30dip" />

</LinearLayout>
			

Change the existing "main.xml" file. This layout will be used by MainActivity in landscape mode and shows two Fragments.

				
<?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="horizontal" >

    <fragment
        android:id="@+id/listFragment"
        android:layout_width="150dip"
        android:layout_height="match_parent"
        android:layout_marginTop="?android:attr/actionBarSize"
        class="de.vogella.android.fragments.ListFragment" ></fragment>

    <fragment
        android:id="@+id/detailFragment"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        class="de.vogella.android.fragments.DetailFragment" >
        <!-- Preview: layout=@layout/details -->
    </fragment>

</LinearLayout>
			

Create now the Fragment classes. Create the ListFragment class.

				
package de.vogella.android.fragments;

import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.ListView;

public class ListFragment extends android.app.ListFragment {
	@Override
	public void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);

	}

	@Override
	public void onActivityCreated(Bundle savedInstanceState) {
		super.onActivityCreated(savedInstanceState);
		String[] values = new String[] { "Android", "iPhone", "WindowsMobile",
				"Blackberry", "WebOS", "Ubuntu", "Windows7", "Max OS X",
				"Linux", "OS/2" };
		ArrayAdapter<String> adapter = new ArrayAdapter<String>(getActivity(),
				android.R.layout.simple_list_item_1, values);
		setListAdapter(adapter);
	}

	@Override
	public void onListItemClick(ListView l, View v, int position, long id) {
		String item = (String) getListAdapter().getItem(position);
		DetailFragment fragment = (DetailFragment) getFragmentManager()
				.findFragmentById(R.id.detailFragment);
		if (fragment != null && fragment.isInLayout()) {
			fragment.setText(item);
		} else {
			Intent intent = new Intent(getActivity().getApplicationContext(),
					DetailActivity.class);
			intent.putExtra("value", item);
			startActivity(intent);

		}

	}
}

			

Create the DetailFragment class.

				
package de.vogella.android.fragments;

import android.app.Fragment;
import android.os.Bundle;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

public class DetailFragment extends Fragment {
	@Override
	public void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		Log.e("Test", "hello");
	}

	@Override
	public void onActivityCreated(Bundle savedInstanceState) {
		super.onActivityCreated(savedInstanceState);

	}

	@Override
	public View onCreateView(LayoutInflater inflater, ViewGroup container,
			Bundle savedInstanceState) {
		View view = inflater.inflate(R.layout.details, container, false);
		return view;
	}

	public void setText(String item) {
		TextView view = (TextView) getView().findViewById(R.id.detailsText);
		view.setText(item);
	}
}

			

We want that Android uses a different main.xml file in portrait model then in landscape mode.

For this reason create the "res/layout-port" folder.

In portrait mode Android will check the "layout-port" folder for fitting layout files. Only if we would not have a "main.xml" file in "layout-port", Android would check the "layout" folder.

Therefore create the following "main.xml" layout file in "res/layout-port".

				
<?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="horizontal" >

    <fragment
        android:id="@+id/listFragment"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_marginTop="?android:attr/actionBarSize"
        class="de.vogella.android.fragments.ListFragment" />
</LinearLayout>
			

Also create the "details_activity_layout.xml" layout file. This layout will be used in the DetailActivity which is only used in portrait mode. Please note that we could have create this file also in the "layout" folder, but as it is only used in portrait mode it is best practise to place it into this folder.

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

    <fragment
        android:id="@+id/detailFragment"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        class="de.vogella.android.fragments.DetailFragment" />

</LinearLayout>
			

Create a new Activity called DetailActivity with the following class.

				
package de.vogella.android.fragments;

import android.app.Activity;
import android.content.res.Configuration;
import android.os.Bundle;
import android.widget.TextView;

public class DetailActivity extends Activity {
	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);

		// Need to check if Activity has been switched to landscape mode
		// If yes, finished and go back to the start Activity
		if (getResources().getConfiguration().orientation == Configuration.ORIENTATION_LANDSCAPE) {
			finish();
			return;
		}

		setContentView(R.layout.details_activity_layout);
		Bundle extras = getIntent().getExtras();
		if (extras != null) {
			String s = extras.getString("value");
			TextView view = (TextView) findViewById(R.id.detailsText);
			view.setText(s);
		}
	}
}

			

MainActivity will remain unmodified.

				
package de.vogella.android.fragments;

import android.app.Activity;
import android.os.Bundle;

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

Run your example. If you run the application in portrait mode you should see only one Fragment. Use Ctrl+F11 to switch the orientation. In horizontal mode you should see two Fragments. If you select an item in portrait mode a new Activity should get started with the selected item. In horizontal mode your second Fragment should display the select item.

Eclipse provides a perspective for interacting with your Android (virtual) device and your Android application program. Select Window → Open Perspective → Other → DDMS to open this perspective. It includes several Views which can also be used independently and allows for example the application to place calls and send SMS to the device. It also allows the application to set the current geo position and allows you to perform a performance trace of your application.

You can see the log (including System.out.print() statements) via the LogCat view.

The file explorer allows to see the files on the Android simulator.

You can access your Android emulator also via the console. Open a shell, switch to your "android-sdk" installation directory into the folder "tools". Start the shell via the following command "adb shell".

				
adb shell

			

You can also copy a file from and to your device via the following commands.

				
// Assume the gesture file exists on your Android device
adb pull /sdcard/gestures ~/test
// Now copy it back
adb push ~/test/gesture /sdcard/gestures2 
			

This will connect you to your device and give you Linux command line access to the underlying file system, e.g. ls, rm, mkdir, etc. The application data is stored in the directory "/data/data/package_of_your_app".

If you have several devices running you can issue commands to one individual device.

				
# Lists all devices
adb devices
#Result
List of devices attached
emulator-5554 attached
emulator-5555 attached
# Issue a command to a specific device
adb -s emulator-5554 shell

			

You can uninstall an android application via the shell. Switch the data/app directory (cd /data/app) and simply delete your android application.

You can also uninstall an app via adb with the package name.

				
adb uninstall <packagename>
			

Alternatively to adb you can also use telnet to connect to the device. This allows you to simulate certain things, e.g. incoming call, change the network "stability", set your current geocodes, etc. Use "telnet localhost 5554" to connect to your simulated device. To exit the console session, use the command "quit" or "exit".

For example to change the power settings of your phone, to receive an sms and to get an incoming call make the following.

				
# connects to device
telnet localhost 5554
# set the power level
power status full
power status charging
# make a call to the device
gsm call 012041293123
# send a sms to the device
sms send 12345 Will be home soon
# set the geo location
geo fix 48 51
			

For more information on the emulator console please see Emulator Console manual

In general there are you restrictions how to deploy an Android application to your device. You can use USB, email yourself the application or use one of the many Android markets to install the application. The following describes the most common ones.

Turn on "USB Debugging" on your device in the settings. Select in the settings of your device Applications → Development, then enable USB debugging.

You may also need to install the a driver for your mobile phone. Linux and Mac OS usually work out of the box while an Windows OS typically requires

For details please see Developing on a Device . Please note that the Android version you are developing for must be the installed version on your phone.

To select your phone, select the "Run Configurations", select "Manual" selection and select your device.

Android application must be signed before they can get installed on an Android device. During development Eclipse signs your application automatically with a debug key.

If you want to install your application without the Eclipse IDE you can right click on it and select Android Tools → Export Signed Application Package.

This wizard allows to use an existing key or to create a new one.

Please note that you need to use the same signature key in Google Play (Google Market) to update your application. If you loose the key you will NOT be able to update your application ever again.

Make sure to backup your key.

Android allow to install applications also directly. Just click on a link which points to an .apk file, e.g. in an email attachment or on a webpage. Android will prompt you if you want to install this application.

This requires a setting on the Android device which allows the installation of non-market application. Typically this setting can be found under the "Security" settings.

Google Play requires a one time fee, currently 25 Dollar. After that the developer can directly upload his application and the required icons, under Google Play Publishing .

Google performs some automatic scanning of applications, but no approval process is in place. All application, which do not contain malware, will be published. Usually a few minutes after upload, the application is available.

Source Code of Examples

Android 2 (German Book)

Android ListView and ListActivity

Android SQlite Database

Android Widgets

Android Live Wallpaper

Android Services

Android Location API and Google Maps

Android Intents

Android and Networking

Android Homepage

Android Developer Homepage

Android Issues / Bugs

Android Google Groups

Android Live Folder

Eclipse RCP Training (German) Eclipse RCP Training with Lars Vogel

Android Tutorial Introduction to Android Programming

GWT Tutorial Program in Java and compile to JavaScript and HTML

Eclipse RCP Tutorial Create native applications in Java

JUnit Tutorial Test your application

Git Tutorial Put everything you have under distributed version control system