Android Development Tutorial

Android Development Tutorial

Based on Android 4.2

Lars Vogel

Version 11.2

Copyright © 2009, 2010, 2011, 2012, 2013 Lars Vogel

20.01.2013

Revision History
Revision 0.1	04.07.2009	Lars Vogel	Created
Revision 0.2 - 11.2	07.07.2009 - 20.01.2013		bug fixing and enhancements

Development with Android and Eclipse
This tutorial describes how to create Android applications with Eclipse. It is based on Eclipse 4.2 (Juno), Java 1.6 and Android 4.2 (Jelly Bean).

Table of Contents
1. What is Android?
1.1. Android Operation System1.2. Google Play (Android Market)
2. Security and permissions
2.1. Security concept in Android2.2. Permission concept in Android
3. Android applications and tasks
3.1. Application3.2. Tasks across application borders
4. Android user interface components
4.1. Activity4.2. Fragments4.3. Views and layout manager4.4. Device configuration specific layouts
5. Other Android components
5.1. Intents5.2. Services5.3. ContentProvider5.4. BroadcastReceiver5.5. Home screen and lock screen widgets5.6. Live Wallpapers
6. Android Development Tools
6.1. Android SDK6.2. Android Development Tools6.3. Dalvik Virtual Machine6.4. How to develop Android Applications6.5. Resource editors
7. Android Application Architecture
7.1. AndroidManifest.xml7.2. Activities and Lifecycle7.3. Configuration Change7.4. Context
8. Resources
8.1. Using resource files8.2. Defining IDs
9. Using Resources
9.1. Reference to resources in code9.2. Reference to resources in XML files9.3. Activities and layouts
10. Assets
10.1. Whats are assets?10.2. Accessing assets
11. Installation
11.1. Options11.2. Standalone ADT installation
12. Android virtual device - Emulator
12.1. What is the Android Emulator?12.2. Google vs. Android AVD12.3. Emulator Shortcuts12.4. Parameter
13. Tutorial: Create and run Android Virtual Device
13.1. Create AVD13.2. Run AVD13.3. Stopping the emulator
14. Solving Android development problems15. Conventions and API level
15.1. API version15.2. Android project and package name
16. Tutorial: create and run Android project
16.1. Create Android project16.2. Start emulator16.3. Start application
17. LogCat view18. Using log statements and the LogCat view19. Views
19.1. Available widgets in Android19.2. View class
20. Tutorial: Create a temperature converter
20.1. Install the demo application20.2. Create Project20.3. Create attributes20.4. Add Views20.5. Edit view properties20.6. Change the Activity source code20.7. Start Project
21. Starting an installed application22. Layout Manager and ViewGroups
22.1. Available Layout Manager22.2. FrameLayout22.3. LinearLayout22.4. RelativeLayout22.5. GridLayout22.6. ScrollView
23. Tutorial: ScrollView24. DDMS perspective and important views
24.1. DDMS - Dalvik Debug Monitor Server24.2. File explorer
25. Deployment
25.1. Overview25.2. Deployment via Eclipse25.3. Export your application25.4. Via external sources25.5. Google Play (Market)
26. Thank you27. Questions and Discussion28. Links and Literature
28.1. Source Code28.2. Android Resources28.3. vogella Resources

1. What is Android?

1.1. Android Operation System

Android is an operating system based on Linux with a Java programming interface.
The Android Software Development Kit (Android SDK) provides all necessary tools to develop Android applications. This includes a compiler, debugger and a device emulator, as well as its own virtual machine to run
Android programs.
Android is currently primarily developed by Google.
Android allows background processing, provides a rich user interface library, supports 2-D and 3-D graphics using the OpenGL libraries, access to the file system and provides an embedded SQLite database.
Android applications consist of different components and can re-use components of other applications. This leads to the concept of a task in Android; an
application can re-use other Android components to archive a task. For example you can 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.

1.2. Google Play (Android Market)

Google offers the Google Play service in which programmers can offer their Android application to Android users. Google phones include the Google
Play application which allows to install applications.
Google Play also offers an update service, e.g. if a programmer uploads a new version of his application to Google Play, this service will notify existing users that an update is available and allow to install
it.
Google Play used to be called Android Market.

2. Security and permissions

2.1. Security concept in Android

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

2.2. Permission concept in Android

Android also contains a permission system. Android predefines permissions for certain tasks but every application can define additional permissions.
An Android application declare its required permissions in its

AndroidManifest.xml

configuration file. For example an application may declare that it requires
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 will be presented to the user before installation of the application. The user needs to decide if these permissions are given to the application.
If the user denies a permission required by the application, this application cannot be installed. The check of the permission is only performed during installation, permissions cannot be denied or granted after
the installation.
Not all users pay attention to the required permissions during installation. But some users do and they write negative reviews on Google Play.

3. Android applications and tasks

3.1. Application

An Android application consists out of different Android components and additional resources. The Android system knows activities, services, broadcast
receiver and content provider as components.

3.2. Tasks across application borders

Android application components can connect to components of other Android applications to createtasks. 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.

4. Android user interface components

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

4.1. Activity

An activity represents the visual representation of an Android application. activities use views,
i.e. user interface widgets as for example buttons and fragments to create the user interface and to interact with the user.
An Android application can have several activities.

4.2. 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.

4.3. Views and layout manager

Views are user interface widgets, e.g. buttons or text fields. The base class for all views is
the

android.view.View

class. Views have attributes which can be used to configure their appearance and behavior.
A layout manager is responsible for arranging other views. The base class for these layout managers
is the

android.view.ViewGroup

class which extends the

View

class.
Layout managers can be nestled to create complex layouts. You should avoid nestling them too deeply as this has a negative impact on the performance.

4.4. Device configuration specific layouts

The user interface for Activities is typcally defined via XML files (layout files). It is possible to define defined layout file 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 pictures shows an activity called MainActivity. On a smaller screen it shows one fragmentand
allows that the user navigates to another fragment. On a wide screen it shows two fragments.

5. Other Android components

Android has several more components which can be used in your Android application.

5.1. Intents

Intents are asynchronous messages which allow the application to request functionality from other Android components, e.g. from services or activities.
An application can call a component directly (explicit Intent) or ask the Android system to evaluate registered components based on the intent data
(implicit intents). For example the application could implement sharing of data via an intent and all components which allow sharing of data
would be available for the user to select. Applications register themselves to an intent via an intent filter.
Intents allow an Android application to start and to interact with components from other Android applications.

5.2. Services

Services perform tasks without providing a user interface. They can communicate with other Android components and notify the user via the notification framework
in Android.

5.3. ContentProvider

A content provider provides a structured interface to application data. Via a content provider your
application can 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 content provider.

5.4. BroadcastReceiver

Broadcast receivers can be registered to receive system messages and intents. A broadcast
receivergets notified by the Android system, if the specified event occurs.
For example you can register a broadcast receivers for the event that the Android system completed the boot processor or for the event that the state of
the phone changes, e.g. someone is calling.

5.5. 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 awidget could display a short summary of new emails and if the user selects an email, it could start the email application with the selected email.

5.6. Live Wallpapers

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

6. Android Development Tools

6.1. Android SDK

The Android Software Development Kit (SDK) contains the necessary tools to create, compile and package Android application. Most of these tools are command
line based.
The Android SDK also provides an Android device emulator, so that Android applications can be tested without a real Android phone. You can create Android virtual
devices (AVD) via the Android SDK, which run in this emulator.
The Android SDK contains the Android debug bridge (adb) tool which allows to connect to an virtual or real Android device.

6.2. Android Development Tools

Google provides the Android Development Tools (ADT) to develop Android applications with Eclipse. ADT is a set of components (plug-ins) which extend the
Eclipse IDE with Android development capabilities.
ADT contains all required functionalities to create, compile, debug and deploy Android applications from the Eclipse IDE. ADT also allows to create and start AVDs.
The Android Development Tools (ADT) provides specialized editors for resources files, e.g. layout files. These editors allow to switch between the XML representation of the file and a richer user interface via
tabs on the bottom of the editor.

6.3. Dalvik Virtual Machine

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

6.4. How to develop Android Applications

Android applications are primarily written 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 once reference of this String.
These dex files are therefore much smaller in size than the corresponding class files.

The

.dex

file and the resources of an Android project, e.g. the images and XML files, are packed into an

.apk

(Android
Package) file. The program aapt (Android Asset Packaging Tool) performs this packaging.
The resulting

.apk

file contains all necessary data to run the Android application and can be deployed to an Android device via the adb tool.
The Android Development Tools (ADT) performs these steps transparently to the user.
If you use the ADT tooling you press a button the whole Android application (

.apk

file) will be created and deployed.

6.5. Resource editors

The ADT allows the developer to define certain artifacts, e.g. Strings and layout files, in two ways: via a rich editor, and directly via XML. This is done via multi-page editors in Eclipse. In these editors you
can switch between both representations by clicking on the tab on the lower part of the screen.
For example if you open the

res/layout/main.xml

file in the Package Explorer View of Eclipse, you
can switch between the two representations as depicted in the following screenshot.

7. Android Application Architecture

7.1. AndroidManifest.xml

The components and settings of an Android application are described in the

AndroidManifest.xml

file. For example all activities and services of
the application must be declared in this file.
It must also contain the required permissions for the application. For example if the application requires network access it must be specified here.

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

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

</manifest>

The

package

attribute defines the base package for the Java objects referred to in this file. If a Java object lies within a different package,
it must be declared with the full qualified package name.
Google Play requires that every Android application uses its own unique package. Therefore it is a good habit to use your reverse domain name as package name. This will avoid collisions with other Android applications.

android:versionName

and

android:versionCode

specify the version of your
application.

versionName

is what the user sees and can be any String.

versionCode

must be an integer. The Android Market determine based on the

versionCode

, if it
should perform an update of the applications for the existing installations. You typically start with "1" and increase this value by one, if you roll-out a new version of your application.
The

<activity>

tag defines an activity, in this example pointing to the

Convert

class
in the

de.vogella.android.temperature

package. An intent filter is registered for this class which defines that this activity is started once the application
starts (action

android:name="android.intent.action.MAIN"

). The category definition

category android:name="android.intent.category.LAUNCHER"

defines
that this application is added to the application directory on the Android device.
The

@string/app_name

value refers to resource files which contain the actual value of the application name. The usage of resource file makes it easy to provide different
resources, e.g. strings, colors, icons, for different devices and makes it easy to translate applications.
The

uses-sdk

part of the

AndroidManifest.xml

file defines the minimal SDK version for
which your application is valid. This will prevent your application being installed on unsupported devices.

7.2. Activities and Lifecycle

The Android system controls the lifecycle of your application. At any time the Android system may stop or destroy your application, e.g. because of an incoming call. The Android system defines a lifecycle foractivities via
predefined methods. The most important methods are:

onSaveInstanceState()

- called after the activity is stopped. Used to save data so that the activitycan
restore its states if re-started

onPause()

- always called if the activity ends, can be used to release resource or save data

onResume()

- called if the activity is re-started, can be used to initialize fields

7.3. Configuration Change

An activity will also be restarted, if a so called "configuration change" happens. A configuration change happens if an event is triggered which may be
relevant for the application. For example if the user changes the orientation of the device (vertically or horizontally). Android assumes that an activitymight want to use different resources for these
orientations and restarts the activity.
In the emulator you can simulate the change of the orientation via Ctrl+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>

7.4. 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.

8. Resources

8.1. Using resource files

Android supports that resources, like images and certain XML configuration files, can be keep separate from the source code.
These resources must be defined in the

res

directory in a special folder dependent on their purpose. You can also append additional qualifiers to the folder
name to indicate that the related resources should be used for special configurations, e.g. you can specify that a resource is only valid for a certain screen size.
The following table give an overview of the supported resources and their standard folder prefix.

Table 1. Resources

Resource	Folder	Description
Simple Values	/res/values	Used to define strings, colors, dimensions, styles and static arrays of strings or integers. 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 foractivities and 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.
Menus	/res/menu	Define the properties of entries for a menu.

The

gen

directory in an Android project contains generated values.

R.java

is a generated class which
contains references to certain resources of the project.
If you create a new resource, the corresponding reference is automatically created in

R.java

via the Eclipse ADT tools. These references are static integer
values and define IDs for the resources.
The Android system provides methods to access the corresponding resource via these IDs.
For example to access a String with the

R.string.yourString

ID, you would use the

getString(R.string.yourString))

method.

R.java

is automatically created by the Eclipse development environment, manual changes are not necessary and will be overridden by the tooling.

8.2. Defining IDs

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>

9. Using Resources

9.1. 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);

9.2. 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

.

9.3. Activities and layouts

The user interface for activities is defined via layouts. The layout defines the included

Views

(widgets)
and their properties.
A layout can be defined via Java code or vian XML. In most cases the layout is defined as an XML file.
XML based layouts are defined via a resource file in the

/res/layout

folder. This file specifies the

ViewGroups

,

Views

,
their relationship and their attributes for this specific layout.
If a

View

needs to be accessed via Java code, you have to give the

View

a unique ID via the

android:id

attribute.
To assign a new ID to a

View

use . The following shows an example in which a

@+id/yourvalue

Button

gets
the

button1

ID assigned.

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

By conversion this will create and assign a new

yourvalue

ID to the corresponding

View

. In
your Java code you can later access a

View

via the method

findViewById(R.id.yourvalue)

.
Defining layouts via XML is usually the preferred way as this separates the programming logic from the layout definition. It also allows the definition of different layouts for different devices. You can also mix
both approaches.

10. Assets

10.1. Whats are assets?

While the

res

directory contains structured values which are known to the Android platform, the

assets

directory
can be used to store any kind of data.

10.2. 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
the

open()

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();
}
}
}

11. Installation

11.1. Options

You have different options to install the Android development tools. The simplest way is to download a full packaged pre-configured Eclipse.
For other options please see Android installation

11.2. Standalone ADT installation

11.2.1. Download

Google provides a pre-packaged and configured Eclipse based Android development environment. The following link allows to download a archive file which includes all required tools for Android development.

http://developer.android.com/sdk/index.html

11.2.2. Standalone ADT installation

Extract the zip file and start Eclipse from the

eclipse

folder via the

eclipse

native launcher, e.g.

eclipse.exe

under
Windows.

12. Android virtual device - Emulator

12.1. What is the Android Emulator?

The Android Development Tools (ADT) include an emulator to run an Android system. The emulator behaves like a real Android device (in most cases) and allows you to test your application without having a real device.
You can configure the version of the Android system you would like to run, the size of the SD card, the screen resolution and other relevant settings. You can define several of them with different configurations.
These devices are called Android Virtual Device and you can start several in parallel.

12.2. Google vs. Android AVD

During the creation of an AVD you decide if you want an Android device or a Google device.
An AVD created for Android will contain the programs from the Android Open Source Project. An AVD created for the Google API's will also contain several Google applications, most notable the Google Maps application.
If you want to use functionality which is only provided via the Google API's, e.g. Google Maps you must run this application on an AVD with Google API's.

12.3. Emulator Shortcuts

The following shortcuts are useful for working with the emulator.
Alt+Enter Maximizes the emulator. Nice for demos.
Ctrl+F11 changes the orientation of the emulator.
F8 Turns network on / off.

12.4. Parameter

The graphics of the emulator can use the native GPU of the computer. This makes the rendering in the emulator very fast. To enable this, add the

GPU Emulation

property
to the device configuration and set it to

true

.



You can also set the

Enabled

flag for Snapshots. This will save the state of the emulator and will let it start much faster. Unfortunately currently
native GPU rendering and Snapshots do not work together.
Android devices do not have to have hardware button. If you want to create such an AVD, add the

Hardware Back/Home keys

property to the device
configuration and set it to

false

.

13. Tutorial: Create and run Android Virtual Device

13.1. Create AVD

To define an Android Virtual Device (ADV) open the AVD Manager dialog via Window → Android
Virtual Device Manager and press the New button.



Enter the values similar to the following screenshot.



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

13.2. Run AVD

To test if your setup is correct, select your new entry and press the Start button



After some time your AVD starts. Do not interrupt this startup process, as this might corrupt the AVD.
After the AVD started, you can use the AVD via the mouse and via the virtual keyboard of the emulator.

13.3. Stopping the emulator

During development you don't stop the AVD, you just re-deploy your application.

14. 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.

15. Conventions and API level

15.1. API version

The tutorials of this document have been developed and tested with Android 4.2, API Level 17. 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.

15.2. 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 de.vogella.android.example.test,
then the corresponding package name isde.vogella.android.example.test.
The application name, which must be entered on the Android project generation wizard, will not be predefined. Choose a name you like.

16. Tutorial: create and run Android project

16.1. Create Android project

In this section you create a simple Android project and run it.
You create an Android application with the data from the following table. The process of creating the Android application is described and depicted below the table.

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
Template	BlankActivity
Activity	MainActivity
Layout	activity_main

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



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



On the wizard page for the launcher icon, create a nice looking icon. The following screenshot shows an example.



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



Enter the following data which was also described in the above table.



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

16.2. Start emulator

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



Unlock your emulator.

Note

You typically don't stop the emulator during development. The emulator is similar to a real Android device and re-starting the emulator is similar to rebooting your Android phone.

16.3. 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.



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

17. LogCat view

You can see the Android log statements via the LogCat view.
You can open this view via Window → Show View → Other... → Android → LogCat.



The LogCat view allows you also to define a filter for the log messages, e.g. for your category.

18. Using log statements and the LogCat view

Create the following interface to define your log constant.

package com.vogella.android.first;

public interface Constants {
String LOG = "com.vogella.android.first";
}

Add a log statement to your

onCreate

method.

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);

if (BuildConfig.DEBUG) {
Log.d(Constants.LOG, "onCreated called");
}
setContentView(R.layout.activity_main);
}

Add a filter to the LogCat view for your category.

19. Views

19.1. Available widgets in Android

Android provides lots of simple views (widgets), e.g. the

Button

,

TextView

,

EditText

classes
and well as more complex widgets, for example

ListView

or

GridView

to show structured data.

19.2. 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

.

20. Tutorial: Create a temperature converter

20.1. 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.

20.2. Create Project

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

Table 3. 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.

20.3. 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.



Select the Color entry in the following dialog and press the OK button. Enter

myColor

as
the name and

#F5F5F5

as the value.



Add more attributes, this time of the

String

type. String attributes allow the developer to translate the application at a later point.

Table 4. 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>

20.4. 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.



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).



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.



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>

20.5. 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.





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.



Afterwards the background should change to the

whitesmoke

color. It might difficult to see the difference.



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>

20.6. Change the Activity source code

During the generation of your new Android project you specified that an activity called

MainActivity

should
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;
}
}

20.7. 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.

21. Starting an installed application

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

22. Layout Manager and ViewGroups

22.1. Available Layout Manager

A layout manager is a subclass of

ViewGroup

and is responsible for the layout of itself and its child

Views

. Android
supports different default layout managers.
As of Android 4.0 the most relevant layout 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 5. 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.

22.2. 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.

22.3. LinearLayout

LinearLayout

puts all its child elements into a single column or row depending on the

android:orientation

attribute.
Possible values for this attribute are

horizontal

and

vertical

,

horizontal

is
the default value.
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_weight

layout
parameter. This value specifies how much of the extra space in the layout is allocated to the

View

. 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.

22.4. 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>

22.5. 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.

22.6. 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.

23. Tutorial: ScrollView

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: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>

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.com ";
}
view.setText(s);
}
}

24. DDMS perspective and important views

24.1. DDMS - Dalvik Debug Monitor Server

Eclipse provides a perspective for interacting with your Android (virtual) device and your Android application program. Select Window → Open
Perspective → Other... → DDMS to open this perspective. It includes several

Views

which
can also be used independently and allows for example the application to place calls and send SMS to the device. It also allows the application to set the current geo position and allows you to perform a performance trace of your application.

24.2. File explorer

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

25. Deployment

25.1. 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 describes the most common ones.

25.2. 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.

25.3. 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 selectAndroid 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.

25.4. 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.

25.5. 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.

26. Thank you

Please help me to support this article:

27. Questions and Discussion

Before posting questions, please see the vogella FAQ. If you have questions or find an error
in this article please use the www.vogella.com Google Group. I have created a short list how
to create good questions which might also help you.

28. Links and Literature

28.1. Source Code

Source Code of Examples

28.2. Android Resources

Android ListView, ListActivity and ListFragment
Android SQlite Database
Android Widgets
Android Live Wallpaper
Android Services
Android Location API and Google Maps
Android Intents
Android and Networking
Android Homepage
Android Developer Homepage
Android Issues / Bugs
Android Google Groups
Android Live Folder

28.3. vogella Resources

vogella Training Android and Eclipse Training from the vogella team
Android Tutorial Introduction to Android Programming
GWT Tutorial Program in Java and compile to JavaScript and HTML
Eclipse RCP Tutorial Create native applications in Java
JUnit Tutorial Test your application
Git Tutorial Put everything you have under distributed version control system