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 officially guided by the Open Handset Alliance. In reality Google leads the project.

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.

An Android application consists out of the following parts:

Android provide much 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.

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. A misbehaving application cannot easily harm other Android 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 also define its own permissions.

An application must declare in its configuration file (AndroidManifest.xml) that it requires certain permissions.

Depending on the details of the defined permission, the Android system will during installation either automatically grant the permission, reject it or ask the user if he grants these permissions to the application.

If for example the application declares that is requires Internet access then the user need to confirm this during installation.

This is called "user driven security". The user decides to grant a permission or to deny it. If the user does not want to give all permissions 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.

Typically not all users check the permissions in detail but some users do and if there is something strange with them, they will write bad reviews on the corresponding Android markets.

Google provides the Android Development Tools (ADT) to develop Android applications with Eclipse. ADT is a set of plug-in which extended the Eclipse IDE with Android development capabilities.

ADT contains all required functionality 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 special bytecode which is different from Java bytecode.

Therefore you cannot run standard Java bytecode on Android.

Android applications are primary written in the Java programming language. The Java source files are converted to Java class files by the Java compiler.

Android provides a tool 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 in different class file is found, the .dex file is stored only once and reference this String in the corresponding classes.

.dex files are therefore much smaller in size then 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) perform 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 transparent 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.

An Android application is described in the file AndroidManifest.xml. This file must declare all Activities, Services, BroadcastReceivers and ContentProvider of the application. It must also contain the required permissions for the application. For example if the application requires network access it must be specified here. AndroidManifest.xml can be thought as the deployment descriptor for an Android application.

				
<?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 following Java elements. It also must be unique as the Android Marketplace only allows application for a specific package once. Therefore a good habit is to use your reverse domain name as a package to avoid collisions with other developers.

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 and the Android Market uses this to determine if you provided a newer version to trigger the update on devices which have your application installed. 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 class "de.vogella.android.temperature.Convert". 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 refer to resource files which contain the actual values. This 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" 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 directory gen in an Android project contains generated values. R.java is a generated class which contains references to resources of the res folder in the project. These resources are defined in the res directory and can be values, menus, layouts, icons or pictures or animations. For example a resource can be an image or an XML file which defines strings.

If you create a new resource, the corresponding reference is automatically created in R.java. The references are static int values, the Android system provides methods to access the corresponding resource. For example to access a String with the reference id R.string.yourString use the method getString(R.string.yourString));. R.java is automatically maintained by the Eclipse development environment, manual changes are not necessary.

While the directory res contains structured values which are known to the Android platform the directory assets can be used to store any kind of data. In Java you can access this data via the AssetsManager and the method getAssets().

In your XML files, e.g. your layout files you can refer to other resources via the @ sign. For example if you want to refer to a color you defined as resources you can refer to it via @color/your_id or if you have defined a "hello" string as resource you can access it via @string/hello .

The user interface for Activities is defined via layouts. At runtime, layouts are instances of android.view.ViewGroups . The layout defines the UI elements, their properties and their arrangement.

UI elements are based on the class android.view.View . ViewGroup is a subclass of the class View and a layout can contain UI components ( Views ) or other layouts ( ViewGroups ). You should not nestle ViewGroups too deeply as this has a negative impact on performance.

A layout can be defined via Java code or via XML. You typically uses Java code to generate the layout if you don't know the content until runtime; for example if your layout depends on content which you read from the Internet.

XML based layouts are defined via a resource file in the folder /res/layout . This file specifies the ViewGroups , Views , their relationship and their attributes for a specific layout. If a UI element needs to be accessed via Java code you have to give the UI element an unique id via the android:id attribute. To assign a new id to an UI element use @+id/yourvalue . By conversion this will create and assign a new id yourvalue to the corresponding UI element. In your Java code you can later access these UI elements 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.

The operating system controls the life cycle 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 life cycle for activities via pre-defined methods. The most important methods are:

The activity will also be restarted if a so called "configuration change" happens. A configuration change for example happens if the user changes the orientation of the device (vertical or horizontal). The activity is in this case restarted to enable the Android platform to load different resources for these configuration, e.g. layouts for vertical or horizontal mode. 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. theLocation Service. As Activities and Services extend the class Context you can directly access the context via this.

The Android SDK is 32bit, therefore on an 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/. If you are not familiar with the Eclipse update manager the usage is described in Eclipse update manager.

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.

The previous step downloads the Android SDK automatically for you. You can also 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. I placed it under "c:\android-sdk-windows". Avoid using spaces in the path name otherwise you may experience problems later.

You also have to define the location of the Android SDK in the Eclipse Preferences. In Eclipse open the Preferences dialog via 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 14 (Android 4.0) 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) include 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 devices with different configurations.

Via the emulator you select which device should be started, you can also start several in parallel. These devices are called "Android Virtual Device" (AVD).

The ADT allow 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 an 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.

Obviously you can use the emulator via the keyboard on the right side of the emulator. But there are also some nice shortcuts which are useful.

Alt+Enter Maximizes the emulator. Nice for demos.

Ctrl+F11 changes the orientation of the emulator.

F8 Turns network on / off.

Try to use a smaller resolution for your emulator as for example HVGA. The emulator gets slower the more pixels its needs to render as it is using software rendering.

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 let it start much faster.

Several users report that 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 help you analyze problems. For example Java exceptions in your program would be shown here. To open this view, select Window → Show View → Other → Android → LogCat.

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

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 be either fully qualified or declared via imports.

If you see 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 formatted and that the imports should be organized at every save.

This app is also available on the Android Marketplace. Search for "vogella" for find this example.

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.

While "res" contains structured values which are known to the Android platform the directory "assets" can be used to store any kind of data. In Java you can access this data via the AssetsManager and the method getAssets().

The Android SDK allows the developer to define certain artifacts, e.g. strings and UI's, in two ways: via a rich editor, and directly via XML.

The following description tries to use the rich UI but for validation the resulting XML is also displayed. You can switch between both things by clicking on the tab on the lower part of the screen. For example in the Package Explorer select "res/layout/main.xml".

Android allows you to create attributes for resources, e.g. for strings or colors. These attributes can be used in your UI definition via XML or in your Java source code.

Select the file "res/values/string.xml" and press "Add". Select "Color" and enter "myColor" as the name and "#3399CC" as the value.

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

Switch to the XML representation and validate the values.

				
<?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 UI 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 outline view.

The following shows a screenshot of the Palette view from which you can drag and drop new UI elements into your layout. Please note that the "Palette" view changes frequently so your view might be a bit different.

Right-click on the text object “Hello World, Hello!” in the layout. Select Delete on 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.

Now 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:orientation="vertical" android:layout_width="fill_parent"
	android:layout_height="fill_parent">
	<EditText android:layout_height="wrap_content" android:id="@+id/editText1"
		android:layout_width="match_parent" android:text="EditText"></EditText>
	<RadioGroup android:layout_height="wrap_content" android:id="@+id/radioGroup1"
		android:layout_width="match_parent">
		<RadioButton android:text="RadioButton"
			android:layout_width="wrap_content" android:id="@+id/radio0"
			android:layout_height="wrap_content" android:checked="true"></RadioButton>
		<RadioButton android:text="RadioButton"
			android:layout_width="wrap_content" android:id="@+id/radio1"
			android:layout_height="wrap_content"></RadioButton>
	</RadioGroup>
	<Button android:text="Button" android:id="@+id/button1"
		android:layout_width="wrap_content" android:layout_height="wrap_content"></Button>
</LinearLayout>

			

If you select a UI element you can change its properties via the properties view. Most of the properties can be changed via the right mouse menu. You can also edit properties of fields directy in XML. Typically you change properties directly in the XML file as this is much faster. But the right mouse functionality is nice if you are searching for a certain property.

Open your file "main.xml" We will delete the initial text for the EditText field in XML. 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 and "fahrenheit" string attribute to the second radio button.

From now on I assume you are able to use the properties menu on the UI elements. You can 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 other UI controls 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 “myColor” in the list.

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

				
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
	android:orientation="vertical" android:layout_width="fill_parent"
	android:layout_height="fill_parent" android:background="@color/myColor">
	<EditText android:layout_height="wrap_content" android:id="@+id/editText1"
		android:layout_width="match_parent" android:inputType="numberDecimal|numberSigned"></EditText>
	<RadioGroup android:layout_height="wrap_content" android:id="@+id/radioGroup1"
		android:layout_width="match_parent">
		<RadioButton android:layout_width="wrap_content"
			android:id="@+id/radio0" android:layout_height="wrap_content"
			android:text="@string/celsius" android:checked="true"></RadioButton>
		<RadioButton android:layout_width="wrap_content"
			android:id="@+id/radio1" 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>

			

During the generation of your new Android project you specified that an Activity called ConvertActivity should get created. The project wizard also created the corresponding Java classs.

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. 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.

Android provides two possible ways to display global actions which the user can select. The first one is the usage of the Action Bar in the application. The Action Bar is a window feature at the top of the activity that may display the activity title, navigation modes, and other interactive items.

The second option is that the app can open a menu which shows additional actions via a popup menu. Typically you define your menu entries in a way that they are added to the action bar if sufficient space is available in the action bar and if not, remaining menu items are displayed in the popup menu.

The option menu and the action bar of your activity is filled by the method onCreateOptionsMenu() of your activity.

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;
			

In this method you can create the menu programmatically or you can use a pre-defined XML resources which you inflate via the class "MenuInflator". Each activity has already an instance of the class available and this instance can get accessed via the method getMenuInflator().

onCreateContextMenu() is only called once. If you want to influence the menu later you have to use the method onPrepareOptionsMenu().

It is also possible to add tabs to an action bar.

You can also assign a context menu to an UI widget (view). A context menu is activated if the user "long presses" the view.

A context menu for a view is registered via the method registerForContextMenu(view). The method onCreateContextMenu() 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 and evaluate a option menu which is displayed in the action bar if sufficient space is available. This example will be extended in the chapter about preferences.

Create a project "de.vogella.android.socialapp" with the activity "OverviewActivity". Change the UI in the file "/res/layout/main.xml" to 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" >

    <Button
        android:id="@+id/Button01"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Show Preferences" >
    </Button>

    <Button
        android:id="@+id/Button02"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Change Preferences" >
    </Button>

</LinearLayout>
			

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. Android provides a nice editor to edit this file, unfortunately this editor is 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.

Switch if necessary to the "Layout" tab of the editor. Press Add and select "Item". Maintain the following value. This defines the entries in your menu. We will also define that the menu entry is displayed in the action bar if there is sufficient space available.

Change your Activity class "OverviewActivity" to the following. The OnCreateOptionsMenu method is used to create the menu. The behavior in "onOptionsItemSelected" is currently hard-coded to show a Toast and will soon call the preference settings. In case you want to disable or hide menu items you can use the method "onPrepareOptionsMenu" which is called every time the menu is called.

				
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) {
		Toast.makeText(this, "Just a test", Toast.LENGTH_SHORT).show();
		return true;
	}
}
			

Run your application. As there is enough space in the action bar your item will be displayed there. If there would be more items you could press "Menu" on the emulator to see them. If you select the menu item you should see a small info message.

The two "Preference" buttons are not yet active. We will use them in the next chapter.

We will continue using the example project "de.vogella.android.social".

Create an Android XML resource "preferences.xml" of type "PreferenceScreen".

Open the file via right-mouse click and Open-with → Android XML Resource Editor. Press Add, add a "PreferenceCategory" and add two preferences "EditTextPreferences" to this category : "User" and "Password".

You can also enter values for other properties of EditTextField, e.g. the inputMethod.

Add the following attribute to the XML definition of your password field to make the input quoted with *.

				
android:inputType="textPassword"
			

Create the class MyPreferencesActivity which extends PreferenceActivity. This Activity will load the "preference.xml" file and will allow the user to change the values.

				
package de.vogella.android.socialapp;

import android.os.Bundle;
import android.preference.PreferenceActivity;

public class MyPreferencesActivity extends PreferenceActivity {
	@Override
	public void onCreate(Bundle savedInstanceState) {
	    super.onCreate(savedInstanceState);
	    addPreferencesFromResource(R.xml.preferences);
	}
}

			

To make this class available as an activity for Android you need to register it in your "AndroidManifest.xml" file. Select "AndroidManifest.xml" and the tab "Application". Scroll to the botton of the view and add your new activity via the "Add" button.

To make use of our new preference activity and the preference values we adjust the "OverviewActivity". The first button will show the current values of the preferences via a Toast and the second button will revert the maintained user name to demonstrate how you could change the preferences via code.

				
package de.vogella.android.socialapp;

import android.app.Activity;
import android.content.SharedPreferences;
import android.content.SharedPreferences.Editor;
import android.os.Bundle;
import android.preference.PreferenceManager;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.MenuItem;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.Toast;

public class OverviewActivity extends Activity {
	SharedPreferences preferences;
	@Override
	public void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.main);
		Button button = (Button) findViewById(R.id.Button01);
		// Initialize preferences
		preferences = PreferenceManager.getDefaultSharedPreferences(this);

		button.setOnClickListener(new OnClickListener() {
			public void onClick(View v) {
				String username = preferences.getString("username", "n/a");
				String password = preferences.getString("password", "n/a");
				showPrefs(username, password);
			}
		});

		Button buttonChangePreferences = (Button) findViewById(R.id.Button02);
		buttonChangePreferences.setOnClickListener(new OnClickListener() {
			public void onClick(View v) {
				
				updatePreferenceValue();
			}
		});
	}
	
	private void showPrefs(String username, String password){
		Toast.makeText(
				OverviewActivity.this,
				"Input: " + username + " and password: "
						+ password, Toast.LENGTH_LONG).show();

	}
	
	private void updatePreferenceValue(){
		Editor edit = preferences.edit();
		String username = preferences.getString("username", "n/a");
		// We will just revert the current user name and save again
		StringBuffer buffer = new StringBuffer();
		for (int i = username.length() - 1; i >= 0; i--) {
			buffer.append(username.charAt(i));
		}
		edit.putString("username", buffer.toString());
		edit.commit();
		// A toast is a view containing a quick little message for the
		// user. We give a little feedback
		Toast.makeText(OverviewActivity.this,
				"Reverted string sequence of user name.",
				Toast.LENGTH_LONG).show();
	}

			

To open the new preference Activity we will use the onOptionsItemSelected() method. Even though we currently have only one option in our menu we use a switch to be ready for several new menu entries. To see the current values of the preferences we define a button and use the class PreferenceManager to get the sharedPreferences.

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

// This method is called once the menu is selected
@Override
public boolean onOptionsItemSelected(MenuItem item) {
	switch (item.getItemId()) {
	// We have only one menu option
	case R.id.preferences:
		// Launch Preference activity
		Intent i = new Intent(OverviewActivity.this, MyPreferencesActivity.class);
		startActivity(i);
		// Some feedback to the user
		Toast.makeText(OverviewActivity.this, "Enter your user credentials.",
			Toast.LENGTH_LONG).show();
		break;

	}
	return true;
}
			

Run your application. Press the "menu" hardware button and then select your menu item "Preferences". You should be able to enter your user settings then press the back hardware button to return to your main activity. The saved values should be displayed in a small message windows (Toast) if you press your first button. If you press the second button the username should be reversed.

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.

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.mxl" 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.os.Bundle;
import android.widget.TextView;

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

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