Android Development Tutorial

Development with Android and Eclipse
This tutorial describes how to create Android applications. It primary uses the Eclipse IDE for development. It is based on Android 4.2 (Jelly Bean).


Table of Contents

1. What is Android?
1.1. Android Operation System
1.2. Android platform components
1.3. Task
1.4. Google Play
2. Android Development Tools
2.1. Android SDK
2.2. Android Developer Tools and Android Studio
2.3. Dalvik Virtual Machine
2.4. How to develop Android Applications
3. Security and permissions
3.1. Security concept in Android
3.2. Permission concept in Android
4. Installation
4.1. Install Android Developer Tools
4.2. Other Eclipse installation options
5. Android device emulator and Android Virtual Devices
5.1. Android emulator and Android Virtual Device
5.2. Google vs. Android AVD
5.3. Android device emulator shortcuts
6. Exercise: Create and start Android Virtual Device
6.1. Target
6.2. Create AVD
6.3. Start your AVD
7. Conventions and API level
7.1. API version
7.2. Android project and package name
8. Exercise: Use the Android project wizard
8.1. Android project wizard
8.2. Create Android project
9. Exercise: Start the generated Android application
9.1. Start AVD
9.2. Start application
10. Andoid Developer Tools
10.1. DDMS – Dalvik Debug Monitor Service
10.2. Emulator Control
10.3. File explorer
11. Android applications and tasks
11.1. Application
11.2. Tasks across application borders
12. Base user interface components in Android
12.1. Activity
12.2. Fragments
12.3. Views and layout manager
12.4. Device configuration specific layouts
13. Android application components
13.1. Android components
13.2. BroadcastReceiver
13.3. Service
13.4. ContentProvider
14. Other important Android elements
14.1. Context
14.2. Home screen and lock screen widgets
14.3. Live Wallpapers
15. The Android manifest file
15.1. AndroidManifest.xml
15.2. Fields in the AndroidManifest.xml file
16. Resources
16.1. Resource files
16.2. Resource IDs and R.java
16.3. System resources
17. Views
17.1. View class
17.2. Standard Android views
17.3. Custom views
18. Layout resource files
18.1. Activities and layouts
18.2. XML layout files
18.3. Defining IDs
18.4. Predefining IDs via a separate file
18.5. Performance considerations with layouts
19. Using Resources
19.1. Reference to resources in code
19.2. Accessing views from the layout in the activity
19.3. Reference to resources in XML files
20. Assets
20.1. Whats are assets?
20.2. Accessing assets
21. Solving Android development problems
22. Exercise: Create a temperature converter
22.1. Install the demo application
22.2. Create Project
22.3. Create attributes
22.4. Add Views
22.5. Edit view properties
22.6. Change the Activity source code
22.7. Start Project
23. Starting an installed application
24. Layout Manager and ViewGroups
24.1. Available Layout Manager
24.2. FrameLayout
24.3. LinearLayout
24.4. RelativeLayout
24.5. GridLayout
24.6. ScrollView
25. Exercise: ScrollView
26. Deployment
26.1. Overview
26.2. Deployment via Eclipse
26.3. Export your application
26.4. Via external sources
26.5. Google Play (Market)

1. What is Android?

1.1. Android Operation System

Android is an operating system based on the Linux Operating System.The project responsible for developing the Android system is called the Android Open Source Project (AOSP) and is primarily lead by Google.
The Android system supports background processing, provides a rich user interface library, supports 2-D and 3-D graphics using the OpenGL libraries, access to the file system and provides an embedded SQLite database.
Android applications consist of different components and can reuse components of other applications.

1.2. Android platform components

The Android system is a full software stack which is typically defined into the following four areas.

  • Applications – The Android Open Source project contains several default application, like the Browser, Camera, Gallery, Music, Phone and more.
  • Application framework – API which allow for high-level interaction with the Android system from Android applications.
  • Libraries and runtime – Libraries for the Application Framework for many functions (graphic rendering, data storage, web browsing, etc.) and the Dalvik runtime and the core Java libraries for running Android applications.
  • Linux kernel – Communication layer for the underlying hardware.

 
This architecture is depicted in the following graphic.
 

Android software layers

 
The developer typically works with the top two layers to create new Android applications. The Linux kernel, the libraries and the runtime is encapsulated by the Application framework.

1.3. Task

The reuse of components of other applications leads to the concept of a task in Android; an application can reuse other Android components to archive a task. For example you can trigger from your application another application which has itself registered with the Android system to handle photos. In this other application you select a photo and return to your application to use the selected photo.

1.4. Google Play

Google offers the Google Play service in which programmers can offer their Android applications to Android users. End users use the Google Play application which allows to buy and install applications from the Google Play service.
Google Play also offers an update service. If a programmer uploads a new version of his application to Google Play, this service notifies existing users that an update is available and allows them to install the update.
Google Play provides also access to services and libraries for Android application programmers. For example it provides a service to use and display Google Maps and another to synchronize application state between different Android installations. Providing these services via Google Play has the advantage that they are available for older Android releases and can be updated by Google without the need for an update of the Android release on the phone.

2. Android Development Tools

2.1. Android SDK

The Android Software Development Kit (Android SDK) contains the necessary tools to create, compile and package Android applications. Most of these tools are command line based. The primary way to develop Android applications is based on the Java programming language.
The Android SDK contains the Android debug bridge (adb) which is a tool which allows you to connect to a virtual or real Android device for the purpose of managing the device or debugging your application.

2.2. Android Developer Tools and Android Studio

Google provides graphical development environments based on the Eclipse and IntelliJ IDE to develop new applications.
The Android Developer Tools (ADT) are based on the Eclipse IDE and provide additional functionalities to develop Android applications. ADT is a set of components (plug-ins) which extend the Eclipse IDE with Android development capabilities.
Google also supports an IDE called the Android Studio for creating Android applications. This IDE is based on the IntelliJ IDE.
Both tools contain all required functionalities to create, compile, debug and deploy Android applications from the IDE. They also allow the developer to create and start Android virtual devices for testing.
Both tools provide specialized editors for Android specific files. Most of Android configuration files are based on XML. In this case these editors allow you to switch between the XML representation of the file and a structured user interface for entering the data.

2.3. Dalvik Virtual Machine

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

2.4. How to develop Android Applications

Android applications are primarily written in the Java programming language.
During development the developer creates the Android specific configuration files and writes the application logic in the Java programming language. The Java source files are converted to Java class files by the Java compiler.
The Android SDK contains a tool called dx which converts Java class files into a .dex (Dalvik Executable) file. All class files of one application are placed in one compressed .dex file. During this conversion process redundant information in the class files are optimized in the .dex file. For example if the same String is found in different class files, the .dex file contains only one reference of this String.
These .dex files are therefore much smaller in size than the corresponding class files.
 
The .dex file and the resources of an Android project, e.g. the images and XML files, are packed into an.apk (Android Package) file. The program aapt (Android Asset Packaging Tool) performs this packaging.
The resulting .apk file contains all necessary data to run the Android application and can be deployed to an Android device via the adb tool.
The ADT and Android Studio tools perform these steps transparently to the user, i.e. if the user selects that the application should be deployed, the whole Android application (.apk file) is created, deployed and started.

3. Security and permissions

3.1. Security concept in Android

The Android system installs every Android application with a unique user and group ID. Each application file is private to this generated user, e.g. other applications cannot access these files. In addition each Android application is started in its own process.
Therefore, by means of the underlying Linux operating system, every Android application is isolated from other running applications.
If data should be shared, the application must do this explicitly, e.g. via a service or a content provider.

3.2. Permission concept in Android

Android contains a permission system and predefines permissions for certain tasks. Every application can request required permissions and also define new permissions. For example an application may declare that it requires access to the Internet.
Permissions have different levels. Some permissions are automatically granted by the Android system, some are automatically rejected. In most cases the requested permissions are presented to the user before the installation of the application. The user needs to decide if these permissions are given to the application.
If the user denies a required permission, the related application cannot be installed. The check of the permission is only performed during installation, permissions cannot be denied or granted after the installation.
An Android application declares its required permissions in its AndroidManifest.xml configuration file. It can also define additional permissions which it can use to restrict access to certain components.

Note

Not all users pay attention to the required permissions during installation. But some users do and they write negative reviews on Google Play.

4. Installation

4.1. Install Android Developer Tools

4.1.1. Download packaged Android Developer Tools

Google provides a packaged and configured Android development environment based on the Eclipse IDE called Android Developer Tools. Under the following URL you find an archive file which includes all required tools for Android development: Getting the Android SDK .

4.1.2. Stand-alone ADT installation

Extract the zip file and start the Android Developer Tools (Eclipse) which are located in the eclipsefolder. You can do this, by double-clicking on the eclipse native launcher (e.g. eclipse.exe under Windows).

4.1.3. Update an existing Eclipse IDE

See ??? for a description how to update your existing Eclipse IDE to perform Android development.

4.2. Other Eclipse installation options

The simplest way to start Android development with Eclipse is to download a full packaged pre-configured Eclipse as described in Section 4.1.1, “Download packaged Android Developer Tools”. It is also possible to update an existing Eclipse installation. Please see Android installation for a detailed description

5. Android device emulator and Android Virtual Devices

5.1. Android emulator and Android Virtual Device

The Android SDK contains an Android device emulator. This emulator can be used to run an Android virtual device (AVD). AVDs allow you to test your Android applications on different Android versions and configurations without access to the real hardware.
During the creation of your AVD you define the configuration for the virtual device. This includes for example the resolution, the Android API version and the density.
You can define multiple AVDs with different configurations and start them in parallel.

5.2. Google vs. Android AVD

During the creation of an AVD you decide if you want to create an Android device or a Google device.
An AVD created for Android contains the programs from the Android Open Source Project. An AVD created for the Google API’s contains additional Google specific code.

Tip

AVDs created with the Google API allows you to test application which uses Google Play services, e.g. the new Google maps API or the new location services.

5.3. Android device emulator shortcuts

The following table lists useful shortcuts for working with an AVD.

Table 1. Android device emulator shortcuts

Shortcut Description
Alt+Enter Maximizes the emulator.
Ctrl+F11 Changes the orientation of the emulator from landscape to portrait and vice versa.
F8 Turns the network on and off.

 

6. Exercise: Create and start Android Virtual Device

6.1. Target

In this exercise you create and start an AVD. Even if you have a real Android device available you should get familiar with the creation and usage of ADVs. Virtual devices give you the possibility to test your application for selected Android versions and a specific configuration.

6.2. Create AVD

Define a new Android Virtual Device (ADV) by opening the AVD Manager via Window ? Android Virtual Device Manager and by pressing the New button.
 

Create a new AVD

 
Enter values similar to the following screenshot.
 

Settings for a new AVD

 

Tip

Ensure that the Use Host GPU option is selected. This makes the AVD use the graphical processing unit of your computer and this makes rendering much faster.

Afterwards press the OK button. This will create the AVD configuration and display it under the list of available virtual devices.

6.3. Start your AVD

Select your new entry and press the Start button. Select Launch in the following dialog.
 

Settings for a new AVD

 

Warning

Do not interrupt this startup process, as this might corrupt the AVD. The first start may take on an older machine up to 10 minutes. On a modern machine it typically takes 1-3 minutes for a new AVD to start.

After the AVD started, you can use the AVD via the mouse. The emulator also provides access to the phone buttons via a menu on the right side of the emulator.
 

Create a new AVD

 

Tip

Once started, don’t stop the AVD during your development. If you change your application and want to test a new version, you simply re-deploy your application on the AVD.

7. Conventions and API level

7.1. API version

The tutorials of this document have been developed and tested with Android 4.3, API Level 18. Please use this version for all tutorials in this tutorial. Higher versions of the API level should also work. A lower version of the Android API might also work, but if you face issues, try the recommended version.

7.2. Android project and package name

The base package for the projects is always the same as the project name, e.g. if you are asked to create a project called com.vogella.android, then the corresponding package name is com.vogella.android.
The application name, which must be entered on the Android project generation wizard, is not always predefined. In this case choose a name you like.

8. Exercise: Use the Android project wizard

8.1. Android project wizard

The Android Developer Tools provide wizards for creating Android applications. In this exercise you use one of the wizards to create an Android application.

Note

The purpose of this exercise is to demonstrate the development process, the created artifacts are explained later in this tutorial.

8.2. Create Android project

To create a new Android project select File ? New ? Other… ? Android ? Android Project from the menu. Enter the fitting data from the following table in the first wizard page.
 

Table 2. New Android project

Property Value
Application Name Test App
Project Name com.vogella.android.first
Package name com.vogella.android.first
API (Minimum, Target, Compile with) Latest

 
 

Project wizard to create an Android project - Part 1

 
Press the Next button and ensure that you have selected to create a launcher icon and an activity.
 

Project wizard to create an Android project - Part 2

 
On the wizard page for the launcher icon, create an application icon of you choosing. The following screenshot shows an example for a possible result.
 

Project wizard to create an Android project - Part 3

 
Press the Next button and select on the next page the BlankActivity template. Press the Next button.
 

Project wizard to create an Android project - Part 4

 
Enter the following data in the dialog for the BlankActivity template. The selection is depicted in the screenshot after the table.

Table 3. Values for the template

Parameter Value
Activity MainActivity
Layout activity_main
Navigation Type none

 
 

Project wizard to create an Android project - Part 5

 
Press the Finish button. The wizard may prompt you to install the support library. If you are prompted, select to install it.
 

Generated Android Project Structure

 

9. Exercise: Start the generated Android application

9.1. Start AVD

If you have not yet done so, create and start an Android virtual device (AVD) fitting for your selected API version.
 

Started emulator

 
Once you AVD is ready, unlock your emulator.
 

Started emulator

 

9.2. Start application

To build, install and run your application the Android application on the Android emulator, select your project, right click on it, and select Run-As ? Android Application.
 

Project wizard to create an Android project - Part 5

 
You may be prompted if the Android Developer Tools should monitor messages. Select Yes in this case and press the OK button.
 

Automatic monitoring of the LogCat View

 
This starts your application on the AVD. The started application is a simple Hello, world. application.
 

Project wizard to create an Android project - Part 5

 

10. Andoid Developer Tools

10.1. DDMS – Dalvik Debug Monitor Service

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 groups several Eclipse views which can also be used independently.
On the left side it shows you the connected Android devices and the running processes on the device. The right side is a stack of views with different purposes. You can select processes and trigger actions from the toolbar, e.g. start a trace or stop the process.
 

DDMS perspective

 
The following description highlights a few views in this perspective. Others are described once they are needed.

10.2. Emulator Control

The Emulator Control view allows you to simulate phone calls and SMS on the AVD. It also allows the application to set the current geo position.
 

Emulator control view in the DDMS

 

10.3. File explorer

The file explorer allows you to browse the file system on your Android virtual device.
 

Showing the File Explorer View

 

11. Android applications and tasks

11.1. Application

An Android application is a single installable unit which can be started and used independently of other Android applications. It consists out of Android components and resource files. The Android system knows activities, services, broadcast receiver and content provider as components.
An Android application can have one application class which is instantiated as soon as the application starts and the last components which is stopped if the application gets stopped.

11.2. Tasks across application borders

Android application components can connect to components of other Android applications. This way they can create cross-application tasks.
For example an application which allows you to make a photo can start an email application and instruct this application to create a new email and attach a photo to this email. This reuse of existing components is an important design concept of Android applications and allows the developer to leverage the applications installed on the device.

12. Base user interface components in Android

The following description gives a overview of the most important user interface related component and parts of an Android application.

12.1. Activity

An activity represents the visual representation of an Android application. An Android application can have several activities.
Activities use views and fragments to create the user interface and to interact with the user. Both elements are described in the next sections.

12.2. Fragments

Fragments are components which run in the context of an activity. A fragment encapsulates application code so that it is easier to reuse it and to support different sized devices.
Fragments are optional components which allow you to reuse user interface and non user interface components for different devices configurations.

12.3. Views and layout manager

Views are user interface widgets, e.g. buttons or text fields. Views have attributes which can be used to configure their appearance and behavior.
A ViewGroup is responsible for arranging other views. It is also known as layout manager. The base class for these layout managers is the android.view.ViewGroup class which extends theandroid.view.View class which is the base class for views.
Layout managers can be nestled to create complex layouts.

12.4. Device configuration specific layouts

The user interface for activities is typically defined via XML files (layout files). It is possible to define layout files for different device configuration, e.g. based on the available width of the actual device running the application.
Fragments are designed to support such a setup.
The following picture shows an activity called MainActivity. On a smaller screen it shows only one fragment and allows that the user navigates to another fragment. On a wide screen it shows two fragments.
 

Activities and Fragments combined

 
 

 

13. Android application components

13.1. Android components

Android applications can consists out of four components:

  • Activities
  • Services
  • Broadcast receivers
  • Content provider

Activities have already been introduced in Section 12.1, “Activity”, this section gives an overview of the other components.

13.2. BroadcastReceiver

A broadcast receiver (receiver) can be registered to receive system messages and intents. A receiver gets notified by the Android system, if the specified event occurs.
For example you can register a receiver for the event that the Android system finished the boot process. Or you can register for the event that the state of the phone changes, e.g. someone is calling.

13.3. Service

A service performs tasks without providing an user interface. They can communicate with other Android components for example via broadcast receivers and notify the user via the notification framework in Android.

13.4. ContentProvider

A content provider (provider) provides a structured interface to application data. A provider can be used for accessing data within one application but can also be used to share data with other applications.
Android contains an SQLite database which is frequently used in conjunction with a content provider. The SQLite database would store the data, which would be accessed via the provider.

14. Other important Android elements

14.1. Context

The class android.content.Context provides the connection to the Android system and the resources of the project. It is the interface to global information about the application environment.
The Context also provides access to Android services, e.g. the Location Service.
Activities and services extend the Context class.

14.2. Home screen and lock screen widgets

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

14.3. Live Wallpapers

Live wallpapers allow you to create animated backgrounds for the Android home screen.

15. The Android manifest file

15.1. AndroidManifest.xml

The components and settings of an Android application are described in the AndroidManifest.xml file. This file is known as the Android manifest file.
All activities, services and content providers components of the application must be statically declared in this file. Broadcast receiver can be defined statically in the manifest file or dynamically at runtime in the application.
The Android manifest file must also contain the required permissions for the application. For example if the application requires network access it must be specified here.

15.2. Fields in the AndroidManifest.xml file

The following listing shows an example for the AndroidManifest.xml file.
 

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

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

</manifest>

 
The package attribute defines the base package for the Java objects referred to in this file. If a Java object lies within a different package, it must be declared with the full qualified package name.
Google Play requires that every Android application uses its own unique package. Therefore it is a good habit to use your reverse domain name as package name. This will avoid collisions with other Android applications.
android:versionName and android:versionCode specify the version of your application. versionName is what the user sees and can be any String.
versionCode must be an integer. The Android Market determine based on the versionCode, if it should perform an update of the applications for the existing installations. You typically start with “1” and increase this value by one, if you roll-out a new version of your application.
The <activity> tag defines an activity component. The name attribute points to class, which (if not fully qualified), is relative to the package defined in the package attribute.
The intent filter part in the Android manifest file, tells the Android runtime that this activity should to registered as possible entry point into the application and made available in the launcher of the Android system. The action define that is can be started android:name="android.intent.action.MAIN" ) and thecategory android:name="android.intent.category.LAUNCHER" category tells the Android system to add the activity to the launcher.
The @string/app_name value refers to resource files which contain the actual value of the application name. The usage of resource file makes it easy to provide different resources, e.g. strings, colors, icons, for different devices and makes it easy to translate applications.
The uses-sdk part of the AndroidManifest.xml file defines the minimal SDK version for which your application is valid. This will prevent your application being installed on unsupported devices.

16. Resources

16.1. Resource files

Android supports that resources, like images and certain XML configuration files, can be keep separate from the source code.
Resource files must be placed in the /res directory in a predefined sub-folder dependent on their type. You can also append additional qualifiers to the folder name to indicate that the related resources should be used for special configurations. For example you can specify that layout file is only valid for a certain screen size.
The following table give an overview of the supported resources and their standard folder prefix.

Table 4. Resources

Resource Folder Description
Drawables /res/drawables Images (e.g. png or jpeg files) or XML files which describe a drawable.
Simple Values /res/values Used to define strings, colors, dimensions, styles and static arrays of strings or integers via XML files. By convention each type is stored in a separate file, e.g. strings are defined in the res/values/strings.xml file.
Layouts /res/layout XML file with layout description files used to define the user interface for activitiesand Fragments.
Styles and Themes /res/values Files which define the appearance of your Android application.
Animations /res/animator Define animations in XML for the property animation API which allows to animate arbitrary properties of objects over time.
Raw data /res/raw Arbitrary files to save in their raw form. You access them via an InputStream.
Menus /res/menu Define the properties of entries for a menu.

 

16.2. Resource IDs and R.java

Every resource file gets an ID assigned by the Android build system. The gen directory in an Android project contains the R.java references file which contains these generated values. These references are static integer values.
If you add a new resource file, the corresponding reference is automatically created in R.java file. Manual changes in the R.java file are not necessary and are overridden by the tooling.
The Android system provides methods to access the corresponding resource files via these IDs.
For example to access a String with the R.string.yourString ID in your source code, you would use the getString(R.string.yourString) method.

16.3. System resources

Android provides also resources, these are called system resources. System resources are distinguished from local resources by the android namespace prefix. For example android.R.string.canceldefines the platform string for a cancel operation.

17. Views

17.1. View class

All views in Android extends the android.view.View class. This class is relatively larger (greater than 18 000 lines of code) and provides a lot of base functionality for subclasses. Customer can implement their own views by extending android.view.View.

17.2. Standard Android views

Android provides standard views (widgets), e.g. the Button, TextView, EditText classes and well as more complex widgets, for example ListView or GridView to show structured data.
The main packages for views are android.view for all the base classes and android.widget for the default widgets of the Android platform.
The usage of standard views is demonstrated in the following exercises.

17.3. Custom views

Developers are free to develop their own views by extending the android.view.View class.

18. Layout resource files

18.1. Activities and layouts

Android activities define their user interface with views (widgets) and fragments. This user interface can be defined via XML layouts resource files in the /res/layout folder or via Java code. You can also mix both approaches.
Defining layouts via XML layout files 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.

18.2. XML layout files

A layout resource file is referred to as layout. A layout specifies the ViewGroups, Views, their relationship and their attributes via an XML representation.
The following code is an example for a simple layout file.
 

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".MainActivity" >

    <TextView
        android:id="@+id/mytext"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/hello_world" />

</RelativeLayout>

 
A layout is assigned to an activity via the setContentView() method calls, as demonstrated in the following example code.
 

package com.vogella.android.first;

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

public class MainActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }

}

 

18.3. Defining IDs

If a view needs to be accessed via Java code (see Section 19.2, “Accessing views from the layout in the activity”), you have to give the View a unique ID via the android:id attribute. To assign a new ID to a View use . The following shows an example in which a @+id/yourvalue Button gets the button1ID assigned.
 

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

 
By conversion this will create and assign a new yourvalue ID to the corresponding view.

18.4. Predefining IDs via a separate file

Android allows that you define ID of user interface components dynamically in the layout files, via the@+id/your_id notation.
To control your IDs you can also create a file called ids.xml in your /res/values folder and define all IDs in this file.
 

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <item name="button1" type="id"/>
</resources>

 
This allow you to use the ID directly in your layout file.
 

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity" >

    <Button
        android:id="@id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerHorizontal="true"
        android:layout_centerVertical="true"
        android:layout_marginRight="27dp"
        android:text="Button" />

</RelativeLayout>

 

18.5. Performance considerations with layouts

Calculating the layout and drawing the views is an resource intensive operation. You should use the most simple layout possible to archive good performance. For example you should avoid nesting layout managers too deeply or avoid using complex layout managers in case a simple layout manager is sufficient.

19. Using Resources

19.1. Reference to resources in code

 
The Resources class allows to access individual resources. An instance of Resources can get access via the getResources() method of the Context class.
 
The Resources class is also used by other Android classes, for example the following code shows how to create a Bitmap file from a reference ID.
 
 

BitmapFactory.decodeResource(getResources(), R.drawable.ic_action_search);

 
 

19.2. Accessing views from the layout in the activity

In your code you typically need to access the views in your activity or fragment to access and modify their properties.
In an activity you can use the findViewById(id) method call to search for a view in the current layout. The id is the ID attribute of the view in the layout. The usage of this method is demonstrated by the following code.
 

package com.vogella.android.first;

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

public class MainActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        TextView textView = (TextView) findViewById(R.id.mytext);
        // TODO do something with the TextView
    }

}

 

19.3. Reference to resources in XML files

In your XML files, for example your layout files, you can refer to other resources via the @ sign.
For example, if you want to refer to a color which is defined in an XML resource, you can refer to it via@color/your_id. Or if you defined a “hello” string in an XML resource, you could access it via@string/hello.
To use an Android system resource, include the android namespace into the references, e.g.android.R.string.cancel.

20. Assets

20.1. Whats are assets?

While the res directory contains structured values which are known to the Android platform, the assetsdirectory can be used to store any kind of data.
While you could also store uses data in the /res/raw folder. If you need access to original file names and file hierarchy, you can save these resources in the assets directory.

20.2. Accessing assets

You access this data via the AssetsManager which you can access the getAssets() method.
The AssetsManager class allows to read a file in the assets folder as InputStream with theopen() method. The following code shows an example for this.
 

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

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

 

21. Solving Android development problems

Things are not always working as they should. You find a list of typical Android development problems and their solution under the following link: Solutions for common Android development problems.

 

22. Exercise: Create a temperature converter

22.1. Install the demo application

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

QR Code to install the Android Temperature converter

 

22.2. Create Project

Select File ? New ? Other… ? Android ? Android Application Project to create a new Android project with the following data.
 

Table 5. New Android project

Property Value
Application Name Temperature Converter
Project Name de.vogella.android.temperature
Package name de.vogella.android.temperature
API (Minimum, Target, Compile with) Latest
Template BlankActivity
Activity MainActivity
Layout activity_main

 
After the wizard ends, a project structure similar to the following picture is created.
 

Generated Android Project Structure

 

22.3. Create attributes

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

Adding Android Attributes

 
Select the Color entry in the following dialog and press the OK button. Enter myColor as the name and#F5F5F5 as the value.
 

Details for a String

 
Add more attributes, this time of the String type. String attributes allow the developer to translate the application at a later point.
 

Table 6. String Attributes

Name Value
celsius to Celsius
fahrenheit to Fahrenheit
calc Calculate

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

<resources>

    <string name="app_name">Temparature Convertor</string>
    <string name="hello_world">Hello world!</string>
    <string name="menu_settings">Settings</string>
    <string name="title_activity_main">Temparature Convertor</string>
    <color name="myColor">#3399CC</color>
    <string name="celsius" >to Celsius</string>
    <string name="fahrenheit">to Fahrenheit</string>
    <string name="calc">Calculate</string>

</resources>

 

22.4. Add Views

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

Plain Textr Fields

 
You will now create the layout for your Android application.
Right-click on the existing Hello World! text object in the layout. Select Delete from the popup menu to remove the text object.
Afterwards select the Text Fields section in the Palette and locate the Plain Text (via the tooltip).
 
 

Adding a text field

 
 
All entries in the Text Fields section define text fields. The different entries define additional attribute for them, e.g. if a text field should only contain numbers.
Drag this onto the layout to create a text input field.
Afterwards select the Form Widgets section in the Palette and drag a RadioGroup entry into 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.
Drag a Button from the Form Widgets section into the layout.
The result should look like the following screenshot.
 

Current layout of activity_main.xml

 
Switch to the XML tab of your layout file and verify that the file looks similar to the following listing. ADT changes the templates very fast, so your XML might look slighty different.
 

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent" >

    <EditText
        android:id="@+id/editText1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_alignParentTop="true"
        android:ems="10" >

        <requestFocus />
    </EditText>

    <RadioGroup
        android:id="@+id/radioGroup1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_below="@+id/editText1" >

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

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

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_below="@+id/radioGroup1"
        android:text="Button" />

</RelativeLayout>

 

22.5. Edit view properties

If you select a user interface component (an instance of View), you can change its properties via the Eclipse Properties view. Most of the properties can be changed via the menu which can be opened via right-click. You can also edit properties of fields directly in XML. Changing properties in the XML file is much faster, if you know what you want to change. But the right-click menu is nice, if you are searching for a certain property.
Open your layout file.
Use a right-click on the first radio button to assign the celsius String attribute to its text property. Assign the fahrenheit string attribute to the text property of the second radio button.
 

Change the text property of the radio button

 
 

Selection of the right text from the pre-defined string values

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

Attribute selection

 
Afterwards the background should change to the whitesmoke color. It might difficult to see the difference.
 

New look of the layout after the changes

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

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@color/myColor" >

    <EditText
        android:id="@+id/editText1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_alignParentTop="true"
        android:ems="10"
        android:inputType="numberSigned|numberDecimal"
         >

        <requestFocus />
    </EditText>

    <RadioGroup
        android:id="@+id/radioGroup1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_below="@+id/editText1" >

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

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

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_below="@+id/radioGroup1"
        android:onClick="onClick"
        android:text="@string/calc" />

</RelativeLayout>

 

22.6. Change the Activity source code

During the generation of your new Android project you specified that an activity called MainActivityshould be created. The project wizard created the corresponding Java class.
Change your MainActivity class to the following isting. Note that the onClick will be called based on the OnClick property of your button. I use the same name as this is easier to remember.
 

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 MainActivity extends Activity {
  private EditText text;

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

  }

  // This method is called at button click because we assigned the name to the
  // "OnClick property" of the button
  public void onClick(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;
  }
}

 

22.7. Start Project

To start the Android Application, select your project, right click on it, and select Run-As ? Android Application. If an emulator is not yet running, it will be started. Be patient, the emulator starts up very slowly.
Type in a number, select your conversion and press the button. The result should be displayed and the other option should get selected.
 

The running application in the emulator

 

23. Starting an installed application

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

How to select your application from the Android home menu

 
 

Selecting the application from the application choicer

 

24. Layout Manager and ViewGroups

24.1. Available Layout Manager

A layout manager is a subclass of ViewGroup and is responsible for the layout of itself and its childViews. Android supports different default layout managers.
As of Android 4.0 the most relevant layout managers are LinearLayout, FrameLayout,RelativeLayout and GridLayout.
All layouts allow the developer to define attributes. Children can also define attributes which may be evaluated by their parent layout.
AbsoluteLayoutLayout is deprecated and TableLayout can be implemented more effectively via GridLayout
Children can specify there desired width and height via the following attributes.

Table 7. Width and height definition

Attribute Description
android:layout_width defines the width of the widget
android:layout_height defines the height of the widget

 
Widgets can uses fixed sizes, e.g. with the dp definition, for example 100dp. While dp is a fixed size it will scale with different device configurations.
The match_parent value tells the to maximize the widget in its parent. The wrap_content value tells the layout to allocate the minimum amount so that widget is rendered correctly.

24.2. FrameLayout

FrameLayout is a layout manager which draws all child elements on top of each other. Which allows to create nice visual effects.
The following screenshot shows the Gmail application which uses FrameLayout to display several button on top of another layout.
 

FrameLayout

 

24.3. LinearLayout

LinearLayout puts all its child elements into a single column or row depending on theandroid:orientation attribute. Possible values for this attribute are horizontal and vertical,horizontal is the default value.
If horizontal is used the child elements are layouted as indicated by the following picture.
 

 
Vertial would result in a layout as depicted in the following picture.
 

 
LinearLayout can be nested to achieve more complex layouts.
LinearLayout supports assigning a weight to individual children via the android:layout_weightlayout parameter. This value specifies how much of the extra space in the layout is allocated to theView. If for example you have two widgets and the first one defines a layout_weight of 1 and the second of 2, the first will get 1/3 of the available space and the other one 2/3. You can also set the layout_width to zero to have always a certain ratio.

24.4. RelativeLayout

RelativeLayout allow to position the widget relative to each other. This allows for complex layouts.
A simple usage for RelativeLayout is if you want to center a single component. Just add one component to the RelativeLayout and set the android:layout_centerInParent attribute to true.
 

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

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

</RelativeLayout>

 

24.5. GridLayout

GridLayout was introduced with Android 4.0. This layout allows you to organize a view into a Grid. GridLayout separates its drawing area into: rows, columns, and cells.
You can specify how many columns you want for define for each View in which row and column it should be placed and how many columns and rows it should use. If not specified GridLayout uses defaults, e.g. one column, one row and the position of a View depends on the order of the declaration of the Views.
The following layout file defines a layout using GridLayout.
 

<?xml version="1.0" encoding="utf-8"?>
<GridLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/GridLayout1"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:columnCount="4"
    android:useDefaultMargins="true" >

    <TextView
        android:layout_column="0"
        android:layout_columnSpan="3"
        android:layout_gravity="center_horizontal"
        android:layout_marginTop="40dp"
        android:layout_row="0"
        android:text="User Credentials"
        android:textSize="32dip" />

    <TextView
        android:layout_column="0"
        android:layout_gravity="right"
        android:layout_row="1"
        android:text="User Name: " >
    </TextView>

    <EditText
        android:id="@+id/input1"
        android:layout_column="1"
        android:layout_columnSpan="2"
        android:layout_row="1"
        android:ems="10" />

    <TextView
        android:layout_column="0"
        android:layout_gravity="right"
        android:layout_row="2"
        android:text="Password: " >
    </TextView>

    <EditText
        android:id="@+id/input1"
        android:layout_column="1"
        android:layout_columnSpan="2"
        android:layout_row="2"
        android:ems="8" />

    <Button
        android:id="@+id/button1"
        android:layout_column="2"
        android:layout_row="3"
        android:text="Login" />

</GridLayout>

 
This creates a user interface similar to the following screenshot.
 

GridLayout Activity result

 

24.6. ScrollView

The ScrollView class can be used to contain one View that might be to big too fit on one screen.ScrollView will is this case display a scroll bar to scroll the context.
Of course this View can be a layout which can then contain other elements.
The following code shows an example layout file which uses a ScrollView.
 

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

    <TextView
        android:id="@+id/TextView01"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:paddingLeft="8dip"
        android:paddingRight="8dip"
        android:paddingTop="8dip"
        android:text="This is a header"
        android:textAppearance="?android:attr/textAppearanceLarge" >
    </TextView>

</ScrollView>

 
The android:fillViewport="true" attribute ensures that the scrollview is set to the full screen even if the elements are smaller then one screen.

25. Exercise: ScrollView

This exercise demonstrates the usage of the ScrollView view to provide a scrollable user interface component. Create an android project de.vogella.android.scrollview ” with the activity calledScrollViewActivity.
Change the layout file used in the activity to the following.
 

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

    <LinearLayout
        android:id="@+id/LinearLayout01"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="vertical" >

        <TextView
            android:id="@+id/TextView01"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:paddingLeft="8dip"
            android:paddingRight="8dip"
            android:paddingTop="8dip"
            android:text="This is a header"
            android:textAppearance="?android:attr/textAppearanceLarge" >
        </TextView>

        <TextView
            android:id="@+id/TextView02"
            android:layout_width="wrap_content"
            android:layout_height="match_parent"
            android:layout_weight="1.0"
            android:text="@+id/TextView02" >
        </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:layout_weight="1.0"
                android:text="Submit" >
            </Button>

            <Button
                android:id="@+id/Button02"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_weight="1.0"
                android:text="Cancel" >
            </Button>
        </LinearLayout>
    </LinearLayout>

</ScrollView>

 
Change your ScrollViewActivity class to the following code.
 

package de.vogella.android.scrollview;

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

public class ScrollViewActivity 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 < 500; i++) {
          s += "vogella.com ";
        }
        view.setText(s);
    }
}

 
Start your application and ensure that you can scroll down to the buttons.
 

Showing the running application with the ScrollView in action

 

26. Deployment

26.1. Overview

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

26.2. Deployment via Eclipse

Turn on USB Debugging on your device in the settings. Select in the settings of your device Applications? Development, then enable USB debugging.
You may also need to install the a driver for your mobile phone. Linux and Mac OS usually work out of the box while an Windows OS typically requires the installation of a driver.
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.
If you have only one device connected and no emulator running, the Android develoment tools will automatically deploy to this device. If you have several connected you can selected which one shoudl be used.
 

The Android Run Configuration on the Target Tab, selecting the "Manual" flag.

 
 

Show the selection of the real device to deploy on it.

 

26.3. Export your application

Android application must be signed before they can get installed on an Android device. During development Eclipse signs your application automatically with a debug key.
If you want to install your application without the Eclipse IDE you can right-click on it and select Android Tools ? Export Signed Application Package.
This wizard allows to use an existing key or to create a new one.
Please note that you need to use the same signature key in Google Play (Google Market) to update your application. If you loose the key you will NOT be able to update your application ever again.
Make sure to backup your key.

26.4. Via external sources

Android allow to install applications also directly. Just click on a link which points to an .apk file, e.g. in an email attachment or on a webpage. Android will prompt you if you want to install this application.
This requires a setting on the Android device which allows the installation of non-market application. Typically this setting can be found under the “Security” settings.

26.5. Google Play (Market)

Google Play requires a one time fee, currently 25 Dollar. After that the developer can directly upload his application and the required icons, under Google Play Publishing .
Google performs some automatic scanning of applications, but no approval process is in place. All application, which do not contain malware, will be published. Usually a few minutes after upload, the application is available.

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