Android Development Introduction

1. What is Android?

1.1. Android Operation System

Android is an operating system based on Linux with a Java programming interface. It provides tools, e.g. a compiler, debugger and a device emulator as well as its own Java Virtual machine (Dalvik Virtual Machine - DVM).
Android is officially guided by the Open Handset Alliance but in reality Google leads the project.
Android supports 2-D and 3-D graphics using the OpenGL libraries and supports data storage in a SQLite database.
Every Android applications runs in its own process and under its own user id which is generated automatically by the Android system during deployment. Therefore the application is isolated from other running applications and a misbehaving application cannot easily harm other Android applications.

1.2. Important Android components

An Android application consists out of the following parts:

• Activity - represents the presentation layer of an Android application, e.g. a screen which the user sees. An Android application can have several activities and it can be switched between them during runtime of the application.
• Views - the User interface of an Activities is built with widget classes which inherent fromandroid.view.View. The layout of the views is managed by android.view.ViewGroups. Views often have attributes which can be used to change their appearance and behavior.
• Services - perform background tasks without providing an UI. They can notify the user via the notification framework in Android.
• ContentProvider - provides data to applications, via a content provider your application can share data with other applications. Android contains a SQLite DB which can serve as data provider
• Intents - are asynchronous messages which allow the application to request functionality from other services or activities. An application can call directly a service or activity (explicit intent) or ask the Android system for registered services and applications for an intent (implicit intents). For example the application could ask via an intent for a contact application. Applications register themselves to an intent via anIntentFilter. Intents are a powerful concept as they allow the creation of loosely coupled applications.
• BroadcastReceiver - receives system messages and implicit intents, can be used to react to changed conditions in the system. An application can register as a BroadcastReceiver for certain events and can be started if such an event occurs.
• Widgets - interactive components primary used on the Android homescreen to display certain data and to allow the user to have quick access the the information

Other Android components are Live Folders and Android Live Wallpapers. Live Folders display data on the homescreen without launching the corresponding application.

1.3. Dalvik Virtual Machine

Android uses a special virtual machine, e.g. the Dalvik Virtual Machine. Dalvik uses special bytecode. Therefore you cannot run standard Java bytecode on Android. Android provides a tool dx which allows to convert Java Class files into dex (Dalvik Executable) files. Android applications are packed into an .apk (Android Package) file by the program aapt (Android Asset Packaging Tool) To simplify development Google provides the Android Development Tools (ADT) for Eclipse. The ADT performs automatically the conversion from class to dex files and creates the apk during deployment.

1.4. Security and permissions

Android defines certain permissions for certain tasks. For example if the application wants to access the Internet it must define in its configuration file that it would like to use the related permission. During the installation of an Android application the user receives a screen in which he needs to confirm the required permissions of the application.

2. Android Application Architecture

2.1. AndroidManifest.xml

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. versionNameis 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 thisActivity 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 your application is valid for. This will prevent your application being installed on devices with older SDK versions.

2.2. R.java, Resources and Assets

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 methodgetString(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 assetscan be used to store any kind of data. In Java you can access this data via the AssetsManager and the methodgetAssets() .

2.3. Reference to resources in XML files

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 .

2.4. Activities and Layouts

The user interface for Activities is defined via layouts. At runtime, layouts are instances ofandroid.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 nestleViewGroups 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 theViewGroups , 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 yourvalueto the corresponding UI element. In your Java code you can later access these UI elements via the methodfindViewById(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.

2.5. Activities and Lifecycle

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:

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

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>
   

2.6. Context

The class android.content.Context provides the connections to the Android system. It is the interface to global information about the application environment. Context also provides access to Android Services , e.g. theLocation Service. As Activities and Services extend the class Context you can directly access the context viathis .

3. Installation

The following assume that you have already Eclipse installed. For details please see Eclipse Tutorial .

3.1. Eclipse and automatic Android SDK

Use the Eclipse update manager to install all available components for the Android Development Tools (ADT) from the URL https://dl-ssl.google.com/android/eclipse/ . 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 do follow the following wizard or go to the next section to learn how to do it manually.

3.2. Manually install Android SDK

The previous step downloads the Android SDK automatically for you. You can also download the Android SDK manuallz from the Android homepage under Android SDK download . 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.

3.3. Install a specific Android version

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

The dialog allows you to install new package and also allow 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 package. After the installation restart Eclipse.

3.4. Android Source Code

The following step is optional.
During Android development it is very useful to have the Android source code available as Android uses a lot of defaults.
Haris Peco maintains plugins which provides access to the Android Source code code. Use the Eclipse update manager to install the Android Source plugin from the following update site: "http://adt-addons.googlecode.com/svn/trunk/source/com.android.ide.eclipse.source.update".
More details can be found on the project website .

4. Emulator Shortcuts

4.1. Create an Android Emulator Device

The Android tools include an emulator. This emulator behaves like a real Android device in most cases and allows you to test your application without having a real device. You can emulate one or several devices with different configurations. Each configuration is defined via an "Android Virtual Device" (AVD).
To define an AVD open the "AVD Manager" via Windows → AVD Manager and press "New".

Enter the following.

We can also select the box "Enabled" for Snapshots. This will make the second start of the virtual device much faster.
At the end press the button "Create AVD".This will create the device and display it under the "Virtual devices". To test if your setup is correct, select your device and press "Start".
After (a long time) your device should be started.

4.2. Using the emulator

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.

4.3. Performance

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.

5. Error handling and typical problems

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

5.1. Clean Project

Several users report that get the following errors:

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

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

5.2. LogCat

The LogCat view shows you the log message of your Android device and help you analyzing problems. For example Java exceptions in your program would be shown here. To open this view, select "Window -> Show View -> Other -> Android -> LogCat" from the menu.

5.3. Emulator does not start

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

5.4. Error message for @override

The @override annotation was introduced in Java 1.6. If you receive an error message for @override change the Java compiler level to Java 1.6 via right-mouse click on the project -> Properties -> Java Compiler -> Compiler compliance level and set it to "1.6".

5.5. Missing Imports

Java requires that the classes which are not part of the standard Java Language are either fully qualified or declared via imports. In your editor use the click mouse click, select "Source-> Organize Imports" if you see error message with "XX cannot be resolved to a variable".

5.6. Eclipse Tips

To work more efficient 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.

6. Your first Android project

6.1. Create Project

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().

6.2. Two faces of things

The Android SDK allows 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 lists also the XML. 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".

6.3. Create attributes

Android allows you to create attributes for resources, e.g. for strings and / 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 to translate the application at a later point.

Table 1. String Attributes

Name	Value
celsius	to Celsius
fahrenheit	to Fahrenheit
calc	Calculate

Switch to the XML representation and validate 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>
   

6.4. Add UI Elements

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>

   

6.5. Edit UI properties

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.
We will delete the intial text for the EditText field in XML. Switch to the "main.xml" tab 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>

   

6.6. Code your application

During the generation of your new Android project you specified that an Activity called CovertActivityshould get created. The project wizard also created the correspondig 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;
 }
}
   

6.7. Start Project

To start the Android Application, select your project, right click on it, Run-As-> Android Application Be patient, the emulator starts up very slow. 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.

7. Starting an deployed application

After you ran your application on the virtual device you can start it again on the device. If you press the Home button you can also select your application.

8. Menus and Action Bar

8.1. Definition of menu entries

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 show additional actions via a popup menu. Typical 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 that 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.
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().

8.2. Action bar tabs

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

8.3. Context menus

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.

9. Tutorial: Menus and Action Bar

9.1. Project

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>
   

9.2. Add a menu XML resource

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

This will create a new file "mainmenu.xml" in the folder "res/menu" of your project. Android provides an nice editor to edit this file, unfortunately this editor is not always automatically used. To use this editor 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 avaiable.

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.

10. Preferences

Android supports the usage of Preferences to allow you to save data for your application. Preferences are stored as key values. The definition of Preferences can also be done via an XML resource.
Android provides the class "PreferenceActivity" which extends the class Activity. PreferenceActivity supports the simple handling of preferences. This activity can load a preference definition resources via the method addPreferencesFromResource().
To communicate between different components Android uses Intents. Typically the PreferenceActivity is started from another activity via an Intent.
In your application you can access the preference manager via the following:

SharedPreferences preferences = PreferenceManager.getDefaultSharedPreferences(this);
  

Values can get access via the key of the preference setting.

String username = preferences.getString("username", "n/a");
  

To create or change preferences you have to call the edit() methods. Once you have changed the value you have to call commit() to apply your changes.

Editor edit = preferences.edit();
edit.putString("username", "new_value_for_user");
edit.commit();

  

11. Tutorial: Preferences

11.1. Using preferences

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 maintain other properties to EditTextField, e.g. the inputMethod. Add for example the following attribute to the XML definition of your password to make the input quoted with * .

android:inputType="textPassword"
    

Create the class "MyPreferencesActivity" with extends PreferenceActivity. This activity will load the "preference.xml" and will allow to maintain 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 method onOptionsItemSelected(). 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;
}
   

11.2. Run

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.

12. Dialogs via the AlertDialog

We have already used a "Toast" which is a small message window which does not take the focus. In this chapter we will use the class "AlertDialog". AlertDialog is used to open a dialog from our activity. This modal dialog gets the focus until the user closes it.
An instance of this class can be created by the builder pattern, e.g. you can chain your method calls.
You should always open a dialog from the class onCreateDialog(int) as the Android system manages the dialog in this case for you. This method is automatically called by Android if you call showDialog(int).
Create a new Android project "de.vogella.android.alertdialog" with the activity "ShowMyDialog". Maintain the following layout for "main.xml".

<?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">

 <Button android:id="@+id/Button01" android:layout_width="wrap_content"
  android:layout_height="wrap_content" android:text="Show Simple Dialog"
  android:onClick="openMyDialog"></Button>
</LinearLayout>

  

Change the code of your activity to the following.

package de.vogella.android.alertdialog;

import android.app.Activity;
import android.app.AlertDialog;
import android.app.AlertDialog.Builder;
import android.app.Dialog;
import android.content.DialogInterface;
import android.os.Bundle;
import android.view.View;
import android.widget.Toast;

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

 }

 public void openMyDialog(View view) {
  showDialog(10);
 }

 @Override
 protected Dialog onCreateDialog(int id) {
        switch (id) {
        case 10:
            // Create out AlterDialog
            Builder builder = new AlertDialog.Builder(this);
            builder.setMessage("This will end the activity");
            builder.setCancelable(true);
            builder.setPositiveButton("I agree", new DialogInterface.OnClickListener() {
                public void onClick(DialogInterface dialog, int which) {
                    ShowMyDialog.this.finish();
                }
            });
            builder.setNegativeButton("No, no", new DialogInterface.OnClickListener() {
                public void onClick(DialogInterface dialog, int which) {
                    Toast.makeText(getApplicationContext(),"Activity will continue",Toast.LENGTH_LONG).show();
                }
            });
            AlertDialog dialog = builder.create();
            dialog.show();
        }
        return super.onCreateDialog(id);
    }

}
  

If you run your application and click your button you should see your dialog.

More on dialogs can be found on Android Dialogs standard documentation.

13. Layouts

Android supports different default layout manager. LinearLayout puts all its child elements into a single column or row depending on the orientation attribute. Other types are FrameLayout, RelativeLayout and TableLayout.
All layouts allow to defined attributes. Childs can also define attributes which may be evaluated by their parent layout. For example the

14. TableLayout

14.1. Overview

In earlier chapter we have used the LinearLayout which allows you to stack widgets vertical or horizontal. LinearLayout can be nested to achieve nice effects. This chapter will demonstrate the usage of "TableLayout".
This layout allows you to organize a view into a table format. You specify via the view group "TableRow" rows for your table. Afterwards you put widgets into the individual rows.
On the "TableLayout" you can define which column should take additional space via the "android:stretchColumns" attribute. If several columns should take the available space you can specify them as a comma-separated list. Similar you can use the attribute "android:shrinkColumn", which will try to word-wrap the content of the specified widgets and the attribute "android:collapseColums" to define initially hidden columns. Via Java you can display / hide these columns via the method setColumnVisible().
Columns will be automatically created based on the maximum number of widgets in one row. Per default each widgets creates a new column in the row. You can specific via "android:layout_column" the column a widget should go and via "android:layout_span" how many columns a widget should take.
You can also put non TableRows in a table. This way you can for example add dividers between your columns.

14.2. Example

Create the project "de.vogella.android.layout.table" with the activity "DemoTableLayout". Change "main.xml" to 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">
 <TableLayout android:layout_width="match_parent"
  android:id="@+id/tableLayout1" android:layout_height="wrap_content"
  android:stretchColumns="1">
  <TableRow android:layout_width="match_parent"
   android:layout_height="wrap_content" android:id="@+id/tableRow1">
   <EditText android:text="Field1" android:id="@+id/editText1"
    android:layout_width="wrap_content" android:layout_height="wrap_content"></EditText>
   <EditText android:text="Field2" android:id="@+id/editText2"
    android:layout_width="wrap_content" android:layout_height="wrap_content"
    android:layout_column="2"></EditText>
  </TableRow>
  <View android:layout_width="wrap_content" android:id="@+id/view1"
   android:layout_height="4px" android:background="#FF0000"></View>
  <TableRow android:id="@+id/tableRow2" android:layout_width="wrap_content"
   android:layout_height="wrap_content">
   <EditText android:layout_height="wrap_content" android:text="Field3"
    android:layout_width="wrap_content" android:id="@+id/editText3"></EditText>
   <EditText android:layout_height="wrap_content"
    android:layout_width="wrap_content" android:text="Field4"
    android:id="@+id/editText4"></EditText>
  </TableRow>

 </TableLayout>
 <Button android:text="Hide second column" android:id="@+id/collapse"
  android:onClick="toggleHiddenRows" android:layout_width="wrap_content"
  android:layout_height="wrap_content"></Button>
</LinearLayout>

   

Change the activity "DemoTableLayout" to the following to use the button to hide the second column in the table.

package de.vogella.android.layout.table;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TableLayout;

public class DemoTableLayout extends Activity {
 private TableLayout layout;
 private Button button;

 
/** Called when the activity is first created. */
@Override
 public void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.main);
  layout = (TableLayout) findViewById(R.id.tableLayout1);
  button = (Button) findViewById(R.id.collapse);

 }

 public void toggleHiddenRows(View view) {
  // Second row has index 1
  layout.setColumnCollapsed(1, !layout.isColumnCollapsed(1));
  if (layout.isColumnCollapsed(1)) {
   button.setText("Show second column");
  } else {
   button.setText("Hide second column");
  }
 }
}
   

15. ContentProvider

15.1. Overview

ContentProvider are used to provide data from an application to another. ContentProvider do not store the data but provide the interface for other applications to access the data.
The following example will use an existing context provider from "Contacts".

15.2. Create contacts on your emulator

For this example we need a few maintained contacts. Select the home menu and then the menu entry "People" to create contacts.

The app will ask you if you want to login. Either login or select "Not now". Press ""Create a new contact". You can create local contacts.

Finish adding your first contact. Afterwards the app allow you to add more contacts via the + button.As a result you should have a few new contacts in your application.

15.3. Using the Contact Content Provider

Create a new Android project "de.vogella.android.contentprovider" with the activity "ContactsView".
Rename the id of the existing TextView from the example wizard to "contactview". Delete the default text. Also change the layout_height to "fill_parent".
The resulting main.xml should look 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">
 <TextView android:layout_width="fill_parent"
  android:layout_height="fill_parent" android:id="@+id/contactview" />
</LinearLayout>

   

Access to the contact content provider require a certain permission as not all applications should have access to the contact information. Open the AndroidManifest.xml, and select the Permissions tab. On that tab click the "Add" button, and select "Uses Permission". From the drop-down list select the entry "android.permission.READ_CONTACTS".
Change the coding of the activity.

package de.vogella.android.contentprovider;

import android.app.Activity;
import android.database.Cursor;
import android.net.Uri;
import android.os.Bundle;
import android.provider.ContactsContract;
import android.widget.TextView;

public class ContactsView extends Activity {
 
/** Called when the activity is first created. */
@Override
 public void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.main);
  TextView contactView = (TextView) findViewById(R.id.contactview);

  Cursor cursor = getContacts();

  while (cursor.moveToNext()) {

   String displayName = cursor.getString(cursor
     .getColumnIndex(ContactsContract.Data.DISPLAY_NAME));
   contactView.append("Name: ");
   contactView.append(displayName);
   contactView.append("\n");
  }
 }

 private Cursor getContacts() {
  // Run query
  Uri uri = ContactsContract.Contacts.CONTENT_URI;
  String[] projection = new String[] { ContactsContract.Contacts._ID,
    ContactsContract.Contacts.DISPLAY_NAME };
  String selection = ContactsContract.Contacts.IN_VISIBLE_GROUP + " = '"
    + ("1") + "'";
  String[] selectionArgs = null;
  String sortOrder = ContactsContract.Contacts.DISPLAY_NAME
    + " COLLATE LOCALIZED ASC";

  return managedQuery(uri, projection, selection, selectionArgs,
    sortOrder);
 }

}
   

Typically you would display such data in a ListView.

16. ScrollView

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.
Create an android project "de.vogella.android.scrollview" with the activity "ScrollView". Create the following layout and class.

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

<LinearLayout android:id="@+id/LinearLayout01" android:orientation="vertical" android:layout_width="fill_parent" android:layout_height="wrap_content">
 <TextView android:id="@+id/TextView01" android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="This is a header" android:textAppearance="?android:attr/textAppearanceLarge" android:paddingLeft="8dip" android:paddingRight="8dip" android:paddingTop="8dip"></TextView>
 <TextView android:text="@+id/TextView02" android:id="@+id/TextView02" android:layout_width="wrap_content" android:layout_height="fill_parent" android:layout_weight="1.0"></TextView>
 
 <LinearLayout android:id="@+id/LinearLayout02" android:layout_width="wrap_content" android:layout_height="wrap_content">
  <Button android:id="@+id/Button01" android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="Submit" android:layout_weight="1.0"></Button>
  <Button android:id="@+id/Button02" android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="Cancel" android:layout_weight="1.0"></Button>
 </LinearLayout>
</LinearLayout>
</ScrollView>

  

package de.vogella.android.scrollview;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.TextView;

public class ScrollView extends Activity {
    
/** Called when the activity is first created. */
@Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        TextView view = (TextView) findViewById(R.id.TextView02);
        String s="";
        for (int i=0; i < 100; i++) {
         s += "vogella.de ";
        }
        view.setText(s);
    }
}
  

The attribute "android:fillViewport="true"" ensures that the the scrollview is set to the full screen even if the elements are smaller then one screen and the "layout_weight" tell the android system that these elements should be extended.

17. DDMS perspective and important views

17.1. DDMS - Dalvik Debug Monitor Server

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 to place calls and send SMS to the device. It also allow to set the current geo position and to perform a performance trace of your application.

17.2. LogCat View

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

17.3. File explorer

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

18. Shell

18.1. Android Debugging Bridge - Shell

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 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

   

18.2. Uninstall an application via adb

You can uninstall an android application via the shell. Switch the 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>
   

18.3. Emulator Console via telnet

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 conntect 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

19. Deploy your application on a real device

Turn on "USB Debugging" on your device in the settings. Select in the settings Applications > Development, then enable USB debugging. You also need to install the driver for your mobile phone. 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.