Android Development

1. Android Development

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 created by the Open Handset Alliance which is lead by Google.
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.
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 userid 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 build with widgets classes which inherent from "android.view.View". The layout of the views is managed by "android.view.ViewGroups".
• Services - perform background tasks without providing an UI. They can notify the user via the notification framework in Android.
• Content Provider - 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. Application register themself to an intent via an IntentFilter. Intents are a powerful concept as they allow to create loosely coupled applications.
• Broadcast Receiver - receives system messages and implicit intents, can be used to react to changed conditions in the system. An application can register as a broadcast receiver for certain events and can be started if such an event occurs.

Other Android parts are Android Widgets or Live Folders and Live Wallpapers . Live Folders display any source of data on the homescreen without launching the corresponding application.

1.3. Security and permissions

Android defines certain permissions for certain tasks. For example if the application want 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 get a screen in which he needs to confirm the required permissions of the application.

1.4. AndroidManifest.xml

An Android application is described the file "AndroidManifest.xml". This file must declare all activities, services, broadcast receivers and content provider 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 specfic 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.
"activity" defines an activity in this example pointing to the class "de.vogella.android.temperature.Convert". For this class an intent filter is registered 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 @ values 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.

1.5. 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 files which defines strings.
If you create a new resources, 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)); Please do not try to modify "R.java" manually.
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().

1.6. Activities and Layouts

The user interface for Activities is defined via layouts. Layouts are at runtime instances of "android.view.ViewGroups". The layout defines the UI elements, their properties and their arragement. UI elements are based on the class "android.view.View". ViewGroup is a subclass of View A and a layout can contain UI components (Views) or other layouts (ViewGroups). You should not nestle ViewGroups to deeply as this has a negativ 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 view groups, 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 to define different layouts for different devices. You can also mix both approaches.

1.7. Activities and Lifecyle

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 an 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 ressource or save data
• onResume() - called if the Activity is re-started, can be used to initiaze fields

The activity will also be restarted if a so called "configuration change" happens. A configuration change for examples 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>
   

1.8. Context

The class android.content.Context provides the connections to the Android system. It is the interface to global information about the application environment. Context also provides the method which allows to receive Android services, e.g. the Location Service . As Activities and Services extend the class "Context" you can directly access the context via "this".

2. Installation

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

2.1. Android SDK

Download the Android SDK 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.

2.2. Eclipse

Use the Eclipse update manager to install all available plugins for the Android Development Tools (ADT) from the URL https://dl-ssl.google.com/android/eclipse/ .

2.3. Configuration

In Eclipse open the Preferences dialog via Windows -> Preferences. Select Android and enter the installation path of the Android SDK.

Select Window -> Android SDK and AVD Manager from the menu.

Select "Available packages" and open the "Third Party Add-ons". Select the Google API 10 (Android 2.3.4) version of the SDK. We are using the Google version as this includes also the Google Maps which we might want to use later at a certain point.

Press "Install selected" and confirm the license for all package. After the installation restart Eclipse.

2.4. 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 .

3. Create an Android Emulator Device

The Android tools include an emulator. This emulator behaves like a real Android device in most cases and allow 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 press the device manager button, press "New" and enter the following.

We will 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. Error handling

Things are not always working as they should be. 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 this error select from the menu Project -> Clean.
If you having problems with your own code you can use the LogCat viewer as described in LogCat Viewer .

5. Your first Android project

5.1. Create Project

Tip:  

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().

5.2. 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".

5.3. 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

NameValue

myClickHandlermyClickHandler
celsiusto Celsius
fahrenheitto Fahrenheit
calcCalculate


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>
   

5.4. 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 postion 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 and the corresponding XML is listed below. Make sure that your code is the same as listed below.

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>

   

5.5. Edit UI properties

If you select a UI element you can change its properties via the properties view. Most of the properties can also be changed via the right mouse menu. Select the EditText field, right mouse click on it, select Properties-> Text and delete the content. This means no text will be initially shown in the text field.

Assign the "celsius" string attribute to your "text" property of the first radio button and "fahrenheit" to the second.

From now on I assume you are able to use the properties menu on the UI elements. 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.
Right-click on the view in Graphical Layout mode, then select “Properties”/”Background...” from the popup menu. 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>

   

5.6. Code your application

Change your code in "Convert.java" to the following. Note that the "myClickHandler" will be called based on the "On Click" 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 Convert 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;
 }
}

   

5.7. 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.

5.8. Using the home menu

If you press the Home button you can also select your application.

6. Menus

6.1. Menus

To use menus Android provides two ways. First is the option menu which can be opened via the menu button. The option menu of your activity is filled in the method onCreateOptionsMenu() of your activity. You can register here a menu via your code or use a XML menu resources which you inflate via a "MenuInflator". You get a MenuInflator via your activity with the method getMenuInflator().
onCreateContextMenu() is only called once. If you want to influence the menu later you have to use the method onPrepareOptionsMenu().
The second option to display a menu is to use the context menu for a 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.

6.2. Project

This chapter will demonstrate how to create and evaluate a option menu, how to define preferences and how to navigate between activities via an intent . Create a project "de.vogella.android.preferences" with the activity "HelloPreferences". 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: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 Preferences"></Button>
 <Button android:id="@+id/Button02" android:layout_width="wrap_content"
  android:layout_height="wrap_content" android:text="Change Preferences"></Button></LinearLayout>

   

6.3. Add a menu XML resource

Menus can be defined via XML files. Select your project, right click on it and select New -> Other -> Android -> "Android XML File". Select the option "Menu", enter as File "menu.xml" and press the button "Finish".

Press Add and select "Item". Maintain the following value. This defines the entries in your menu. We will have only one entry.

Change your class "HelloPreferences" 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.preferences;

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 HelloPreferences extends Activity {
 
/** Called when the activity is first created. */
@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.menu, menu);
  return true;
 }

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

   

Run your application and press "Menu" on the emulator. Your menu should be displayed. If you select the menu entry you should see a small info message.

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

7. Preferences and Intents

7.1. Overview

Preferences allow you to save data for your application. Preferences are stored as key values. Intents allow you to start Activities from other Activities.

7.2. Using preferences

We will continue using the example project "de.vogella.android.preferences" from the last chapter.
Preference values can also be stored as a XML resource. Create another Android XML File "preferences.xml" this time of type "Preference".

Press Add, add a category and add two preferences "EditTextPreferences" to this category : "User" and "Password".

To allow the user to enter the preference value you can define a Activity with extends PreferenceActivity. This activity can load a preference definition resources via the method addPreferencesFromResource(). Create the class "Preferences" which will load the "preference.xml".

package de.vogella.android.preferences;

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

public class Preferences extends PreferenceActivity {

 
/** Called when the activity is first created. */
@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". Add the activity "Preferences".

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.preferences;

import android.app.Activity;
import android.content.Intent;
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 HelloPreferences extends Activity {
 SharedPreferences preferences;

 
/** Called when the activity is first created. */
@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");
    Toast.makeText(
      HelloPreferences.this,
      "You entered user: " + username + " and password: "
        + password, Toast.LENGTH_LONG).show();

   }
  });

  Button buttonChangePreferences = (Button) findViewById(R.id.Button02);
  buttonChangePreferences.setOnClickListener(new OnClickListener() {
   public void onClick(View v) {
    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(HelloPreferences.this,
      "Reverted string sequence of user name.",
      Toast.LENGTH_LONG).show();
   }
  });
 }

}
   

We will update the method onOptionsItemSelected() to open the activity "Preferences" once you select the option menu. 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.menu, 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(HelloPreferences.this, Preferences.class);
   startActivity(i);
   // Some feedback to the user
   Toast.makeText(HelloPreferences.this,
     "Here you can enter your user credentials.",
     Toast.LENGTH_LONG).show();
   break;

  }
  return true;
 }

   

7.3. 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.

8. 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.

9. TableLayout

9.1. 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.

9.2. 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="toogleHiddenRows" 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 toogleHiddenRows(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");
  }
 }
}
   

10. ContentProvider

10.1. 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".

10.2. Create contacts on your emulator

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

Press Menu and select "New Contact".

As a result you should have a few new contacts.

10.3. 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 . Please see the ListView Tutorial for details.

11. 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.

12. DDMS perspective and important views

12.1. DDMS - Dalvik Debug Monitor Server

Eclipse provides a perspective for interacting with your device and program. Open it to see the possible options. This perspective includes the following 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.

12.2. LogCat View

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

12.3. File explorer

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

13. Shell

13.1. 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 individuel 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

   

13.2. 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.

13.3. 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

14. 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.