Android is an operating system based on the Linux kernel. 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-ES (short OpenGL) standard and grants access to the file system as well as an embedded SQLite database.

An Android application typically consists of different visual and non visual components and can reuse components of other applications.

In Android the reuse of other application components is a concept known as task. An application can access other Android components to achieve a task. For example, from a component of your application you can trigger another component in the Android system, which manages photos, even if this component is not part of your application. In this component you select a photo and return to your application to use the selected photo.

Such a flow of events is depicted in the following graphic.

The Android system is a full software stack, which is typically divided into the four areas as depicted in the following graphic.

The levels can be described as:

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

Google offers the Google Play service, a marketplace in which programmers can offer their Android applications to Android users. Customers use the Google Play application which allows them 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 access to services and libraries for Android application programmers, too. For example, it provides a service to use and display Google Maps and another to synchronize the 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.

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 that allows you to connect to a virtual or real Android device, for the purpose of managing the device or debugging your application.

The Android tooling uses Gradle as build system. The Android team provides a Gradle plug-in for build Android applications which is entered in the build.gradle file in the top root of the Android project. It typically looks like the following, please note that the version might be different in your case.

// Top-level build file where you can add configuration options common to all sub-projects/modules.

buildscript {
    repositories {
        jcenter()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:1.4.0'

        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
    }
}

allprojects {
    repositories {
        jcenter()
    }
} 

You find the available versions of this plug-in under the following URL: Available Android Gradle plug-in versions.

Google provides an IDE called Android Studio as the preferred development environment for creating Android applications. This IDE is based on the IntelliJ IDE.

The Android tools provide specialized editors for Android specific files. Most of Android's 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.

This description uses Android Studio as IDE.

Android 5.0 uses the Android RunTime (ART) as runtime for all Android applications.

ART uses Ahead Of Time compilation. During the deployment process of an application on an Android device, the application code is translated into machine code. This results in approx. 30% larger compile code, but allows faster execution from the beginning of the application.

This also saves battery life, as the compilation is only done once, during the first start of the application.

The dex2oat tool takes the .dex file created by the Android tool change and compiles that into an Executable and Linkable Format (ELF file). This file contains the dex code, compiled native code and meta-data. Keeping the .dex code allows that existing tools still work.

The garbage collection in ART has been optimized to reduce times in which the application freezes.

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 Android tooling converts these application files, transparently to the user, into an Android application. When developers trigger the deployment in their IDE, the whole Android application is compiled, packaged, deployed and started.

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 the application are placed in this .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 step.

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.

Development for Android can be done on a reasonably sized computer. For a nice experience a modern computer is recommended, for example, a 2.6 GHz CPU with at least 8 GB of memory. An SSD speeds up the start of the Android emulator significantly.

The Android SDK is 32-bit, therefore on a 64-bit Linux system you need to have the package ia32-libs installed. For Ubuntu you can do this via the following command.

apt-get install ia32-libs 

Please check your distribution documentation if you are using a different flavor of Linux.

On Ubuntu 13.04 you also have to install the OpenGL support. This can be done on Ubuntu 13.04 via following command.

# install OpenGL support
sudo apt-get install libgl1-mesa-dev 

Please check your distribution documentation if you are using a different flavor of Linux.

Download Android Studio from the Android Studio website. The download comes in two flavors, SDK Tools only and Android Studio Packages. You want to download the Android Studio Package for your operation system.

Installation for Windows is simple, just lauch the .exe you downloaded. On Max OSX drag and drop Android Studio into the Applications folder.

On Linux unpack the downloaded ZIP file into an appropriate location for your applications. To launch Android Studio, navigate to the android-studio/bin/ directory in a terminal and execute studio.sh.

The first time you start Android Studio you can select if you want to import your setting from an existing installation.

Afterwards click through the setup guide.

Once you reach the last page, press the Finish button.

The Android SDK Manager allows you to install specific versions of the Android API.

The Android SDK Manager allows you to install and delete Android packages.

Select Tools → Android → SDK Manager or the SDK Manager icon in the toolbar of Android Studio to open the Android SDK manager.

In the Android SDK manager select the version of Android you would like to develop for from the tree and press the Install button. The following screenshot shows the selection for the API 18 version of Android.

Press the OK button to start he installation.After the installation is completed the select option is available.

The SDK Platforms tab is used to install API versions, which the SDK Tools is used to install the development tools.

The support library allows you to use functionality provided by higher Android releases in lower Android versions.

In the Android SDK Manager select Extras and install the Android Support Repository. The Android Support Library is for the usage of the Eclipse ADT tooling.

Android currently has several versions of the library, the v4, v7 and v13 version which are valid as of the respective API level of Android. For example, the support library v7 works as of Android devices with version API 7. Higher versions of the support library require also the lower versions to work. For example, support library v7 requires the v4 library.

The Android tooling contains an Android device emulator. This emulator can be used to run an Android Virtual Device (AVD), which emulates a real Android phone.

AVDs allow you to test your Android applications on different Android versions and configurations without access to the real hardware. Even if you have a real Android device available, you should get familiar with the creation and usage of AVDs. Virtual devices give you the possibility to test your application for selected Android versions and a specific configurations.

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 of your display.

You can define multiple AVDs with different configurations and start them in parallel. This allows you to test different device configurations at once.

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

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.

Android applications must be signed before they can get installed on an Android device. During development Eclipse signs your application automatically with a self-signed certificate called the debug key.

This debug certificate has an expiration date of 365 days from its creation date. When the certificate expires, you will get a build error that the certificate has been expired.

To fix this problem, delete the debug.keystore file. The default storage location is in ~/.android/ on OS X and Linux, in C:\Documents andSettings\[username]\.android\ on Windows XP, and in C:\Users\[username]]\.android\ on Windows Vista and Windows 7.

The next time you build, the build tools will regenerate a new keystore and debug key.

The following table lists useful shortcuts for working with an 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.

AVDs created for the Google API allow you to test applications which use Google Play services, e.g., the new Google maps API or the new location services.

During the creation of an emulator you can choose if you either want Snapshot or Use Host GPU enabled.

If you select the Snapshot option, the second time you start the device it is started very fast, because the AVD stores its state if you close it. If you select Use Host GPU the AVD uses the graphics card of your host computer directly which makes the rendering on the emulated device much faster.

It is possible to run an AVD with an image based on the ARM CPU architecture or based on the Intel CPI architecture.

An Android virtual device which uses the Intel system image is much faster in execution on Intel / AMD hardware compared to the ARM based system image. This is because the emulator does not need to translate the ARM CPU instructions to the Intel / AMD CPU on your computer.

The Intel image for an API can be installed via the Android SDK Manager. In Android Studio this happens automatically if you create an device. If is possible to configure this via the package details.

At the time of this writing your also need to download and install extra drivers for MS windows.

After the download you find the driver in your Android installation directory in the extras/intel folder. You need to install the drivers by running starting the .exe file. This additional installation step is required on Window to accelerate the Intel emulator. Only downloading the driver via the Android does not make a difference.

After the download you can create a new AVD based on the Intel emulator. The emulator does not start faster but is way faster during the execution of your Android application.

There are alternatives to the default Android emulator available. For example, the Genymotion emulator is relatively fast in startup and execution of Android projects.

Turn on USB Debugging on your device in the settings. Select Settings → Development Options, then enable the USB-Debugging option.

You may also need to install the driver for your mobile phone. Linux and Mac OS usually work out of the box while Windows typically requires the installation of a driver.

For details on the driver installation on Windows please see Google guide for device deployment.

If you have several devices connected to your computer, you can select which one should be used. If only one device is connected, the application is automatically deployed on this device.

In this exercise you create an Android project and start it on an Android virtual device via Android Studio.

Press the Start a new Android Studio project link to get started. Alternatively you can select the File → New Project... entry from the menu.

Use the following data of input for your project. Project location and package name are derived from your input. If you want another package name, press the small Edit hyperlink.

Android Studio automatically downloads the required Android SDK, if that has not already been done. So depending on your installation, the next dialog might not be displayed.

Afterwards select the Empty Activity template.

On the last page, accept the defaults.

The above wizard generates based on your input an Android project. Review the generated project structure and files.

Define a new Android Virtual Device (AVD) by opening the AVD Manager via Tools → Android → AVD Manager. Afterwards press the Create Virtual Device... button.

Select values similar to the following screenshots.

On the next screen select the latest API level for your AVD. You may need to select the option for additional images as highlighted in the following screenshot.

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

Select your new entry and press the Play button.

Select Run → Run 'app' to start your application. This opens a dialog in which you can select your device to deploy your application to.

After a while your application should start on the virtual device.

An Android application is a single installable unit which can be started and used independently of other Android applications.

An Android application consists of Android components, Java source and resource files.

Android application components can connect to components of other Android applications based on a task description represented by an Intent object. This way they can create cross-application tasks. The integration of these components can be done in a way that the Android application can still work flawless, even if the additional components are not installed or if different components perform the same task.

The following Android components can be defined:

An Android application can have one Application class which is instantiated as soon as the application starts and it is the last component which is stopped during application shutdown.

If not explicitly defined, Android creates a default application object for your application.

An activity is the visual representation of an Android application. An Android application can have several activities.

Activities use Section 15, “Android views - UI Widgets”, Section 16.1, “Using a layout manager” and fragments to create the user interface and to interact with the user. Both elements are described in the next sections.

A broadcast receiver (receiver) can be registered to listen to 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.

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.

A content provider (provider) defines 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.

Instances of the class android.content.Context provide the connection to the Android system which executes theapplication application. It also gives access to the resources of the project and the global information about the application environment.

For example, you can check the size of the current device display via the Context.

The Context class also provides access to Android services, e.g., the alarm manager to trigger time based events.

Activities and services extend the Context class. Therefore, they can be directly used to access the Context.

Activities are the base for the user interface in Android. They have already been introduced in Section 8.4, “Activity”.

Fragments are components which run in the context of an activity. A fragment encapsulates application code so that it is easier to reuse them and to support devices of different size.

The following picture shows an activity called MainActivity. On a smaller screen it shows only one fragment and allows the user to navigate to another fragment. On a wide screen it shows those two fragments immediately.

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 the android.view.View class which is the base class for views.

Layout managers can be nested to create complex layouts.

Widgets are interactive components which are primarily used on the Android home screen. They typically display some kind of data and allow the user to perform actions with 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.

To avoid confusion with views (which are also called widgets), this text uses the term home screen widgets, if it speaks about widgets.

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

The components and settings of an Android application are described in the AndroidManifest.xml file. This file is known as the manifest file or the manifest.

The manifest also specifies additional metadata for the application, e.g., icons and the version number of the application.

This file is read by the Android system during installation of the application. The Android system evaluates this configuration file and determines the capabilities of the application.

All activities, services and content provider 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.

The following listing shows an example for a simple Android manifest file.

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.android.rssreader"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk
        android:minSdkVersion="16"
        android:targetSdkVersion="19" />

    <uses-permission android:name="android.permission.INTERNET" />

    <application
        android:name="RssApplication"
        android:allowBackup="false"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name="RssfeedActivity"
            android:label="@string/title_activity_main" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity
            android:name=".DetailActivity"
            android:label="Details" >
        </activity>
        <activity android:name="MyPreferenceActivity" >
        </activity>

        <service android:name="RssDownloadService" >
        </service>
    </application>

</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 name. Therefore it is a good habit to use your reverse domain name here. 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 determines whether it should perform an update of the applications for the existing installation based on that versionCode. You typically start with "1" and increase this value by one if you roll-out a new version of your application.

The <application> section allows to define metadata for your application and optionally define an explicit application class. It is also a container for declaring your Android components.

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 be registered as a possible entry point into the application and made available in the launcher of the Android system. The action defines that it (android:name="android.intent.action.MAIN" ) can be started and the category android:name="android.intent.category.LAUNCHER" parameter 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 a resource file makes it easy to provide different resources (e.g., strings, colors, icons) for different devices and makes it easy to translate applications.

Similar to the <activity> tag, you can use the service, receiver and provider to declare other Android components.

The uses-sdk section in the manifest allows you to specify the minSdkVersion and targetSdkVersion version of your application.

Your application can declare permissions with the <permission> tag and declare that it required a permision with the <uses-permission> tag.

The uses-configuration section in the manifest allows you to specify required input methods for your device. For example, the following snippet would require that the device has a hardware keyboard.

<uses-configuration android:reqHardKeyboard="true"/> 

The uses-feature section allows you to specify the required hardware configuration for your device. For example, the following snippet would require that the device has a camera.

<uses-feature android:name="android.hardware.camera" /> 

Via the installLocation attribute of your application you can specify if your application can be installed on the external storage of the device. Use auto or preferExternal to permit this.

In reality this option is rarely used, as an application installed on the external storage is stopped once the device is connected to a computer and mounted as USB storage.

You find more information about the attributes and sections of the manifest in the Android Manifest documentation.

Android allows you to create static resources like images and XML configuration files. This allows you to keep these resources separate from the source code of your Android application.

Resource files must be placed in the /res directory of your application in a predefined sub-folder. The specific sub-folder depends on type of resource which is stored.

The following table gives an overview of the supported resources and their standard folder prefixes.

The following listing is an example for file called values.xml in the /res/values which defines a few String constants, a String array, a color and a dimension.

<?xml version="1.0" encoding="utf-8"?>
<resources>

    <string name="app_name">Test</string>
    <string name="action_settings">Settings</string>
    <string name="hello_world">Hello world!</string>

    <string-array name="operationsystems">
        <item>Ubuntu</item>
        <item>Android</item>
        <item>Microsoft Windows</item>
    </string-array>

    <color name="red">#ffff0000</color>

    <dimen name="mymargin">10dp</dimen>

</resources> 

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.

Every resource 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 a R.java file. Manual changes in the R.java file are not necessary and will be overwritten 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 defined on the Context class.

The Android SDK uses the camelCase notation for most of its IDs, e.g., buttonRefresh. It is good practice to follow this approach.

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

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

Defining layouts via XML layout files is the preferred way. This separates the programming logic from the layout definition. It also allows the definition of different layouts for different devices.

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);
    }

} 

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 android:id attribute of the corresponding element in the layout file. The following shows an example in which a button gets the button1 ID assigned via the android:id="@+id/button1" parameter.

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

By conversion this statement creates a new ID if necessary in the R.java file and assigns the defined ID to the corresponding view.

As described in the last section Android allows you to define IDs of user interface components dynamically in the layout files. To have one central place to define ID, you can also define them in a configuration file.

To control your IDs, you can also create a file, typically called ids.xml, in your /res/values folder and define your IDs in this file. The following listing shows an example for such a file.

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

This allows you to use the ID 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> 

Calculating the layout and drawing the views is a resource intensive operation. You should use the simplest layout possible to achieve 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.

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.

AbsoluteLayout is deprecated and TableLayout can be implemented more effectively via GridLayout.

All layouts allow the developer to define attributes. Children can also define attributes which may be evaluated by their parent layout.

Children can specify their desired width and height via the following attributes.

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 application to maximize the widget in its parent. The wrap_content value tells the layout to allocate the minimum amount so that the widget is rendered correctly. The effect of these elements is demonstrated in the following graphics.

FrameLayout is a layout manager which draws all child elements on top of each other. This 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.

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 while horizontal is the default value.

If horizontal is used, the child elements are layouted as indicated by the following picture.

Vertical 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 corresponding 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 always have a certain ratio.

RelativeLayout allows positioning the widget relative to each other. This can be used for complex layouts. RelativeLayout is a complex layout manager and should only be used if such a complex layout is required, as it performs a resource intensive calculation to layout its children.

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> 

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 to define for each View, in which row and column it should be placed as well as 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.

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/input2"
        android:layout_column="1"
        android:layout_columnSpan="2"
        android:layout_row="2"
        android:inputType="textPassword"
        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.

The ScrollView or the HorizontalScrollView class is not a layout manager, but useful to make views available, even if they do not fit onto the screen. A scroll view can contain one view, e.g., a layout manager containing more views. s If the child view is to large, scroll view allows scrolling the content.

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 than one screen.

In this exercise you continue to use the application you created in Section 7, “Exercise: Getting started with Android Studio”. Open the activity_main.xml layout file in the res/layout folder.

Investigate the XML layout in the visual editor as well in as the XML structure.

Remove all views, except the top level entry which is the layout manager. In the visual design mode you can remove a view by right-clicking it and by selecting the Delete entry for the context menu.

The result layout file should look similar to the following 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" >


</RelativeLayout> 

Add an Plain Text (EditText) and a Button to your layout. The easiest way is to find these elements in the Palette and drag and drop them into your layout.

Use the text (XML) editor to change the ID of the new EditText field to main_input. In the XML file this looks like @+id/main_input.

Change the button text to Start via the android:text property in your layout file. Assign the name onClick to the android:onClick property of your Button. This defines that a public void onClick (View view) method is be called in the activity once the button is pressed. This method is implemented in the next step.

After these changes your layout file should be similar to the following code.

<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" >

    <EditText
        android:id="@+id/main_input"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentTop="true"
        android:layout_alignParentStart="true"
        android:layout_alignParentEnd="true" />

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/main_input"
        android:layout_below="@+id/main_input"
        android:layout_marginTop="31dp"
        android:onClick="onClick"
        android:text="Start" />

</RelativeLayout> 

You can start your application in the emulator or use the layout review in your IDE. The resulting layout should look like the following screenshot.

If you press the button, the application crashes because you need still need to implement the onClick method you defined in your layout file for your button in your activity. This is done in the next section.

Implement the following method in your MainActivity class.

public void onClick (View view) {
  Toast.makeText(this, "Button 1 pressed",
  Toast.LENGTH_LONG).show();
} 

Start your application. Press your button and validate that a popup message (Toast) is shown.

Go back to the source code and use the findViewById(id) method with the correct id and cast the returned object into EditText, e.g. EditText text = (EditText) findViewById(id). You can get the right id via the R class. It should be stored under ID and called main_input.

Use the text.getText().toString() method to read the string in the editor field and add the text to your Toast message.

package com.vogella.testapp;

import android.app.Activity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.EditText;
import android.widget.Toast;

public class MainActivity extends Activity {

  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    if (BuildConfig.DEBUG) {
      Log.d(Constants.LOG, "onCreated called");
    }
    setContentView(R.layout.activity_main);
  }

  // you may have here an onCreateOptionsMenu method
  // this method is not required for this exercise
  // therefore you can delete it

  public void onClick(View view) {
    EditText input = (EditText) findViewById(R.id.main_input);
    String string = input.getText().toString();
    Toast.makeText(this, string, Toast.LENGTH_LONG).show();
  }
} 

Restart your application and ensure that the Toast displays the text which the EditText field contains.

In this exercise you add radio buttons the layout of the application you created in Section 7, “Exercise: Getting started with Android Studio” and extended in Section 17, “Exercise: Use layouts and view interaction”. Depending on the user selection the radio button arrangement changes from horizontal to vertical.

Open your layout file and add a radio group with two radio buttons to your layout.

Assign them based on the following table.

The resulting layout file should be similar to the following listing. Note: Only the RadioGroup part is new.

<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" >


    <EditText
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/main_input"
        android:layout_alignParentTop="true"
        android:layout_alignParentStart="true"
        android:layout_alignParentEnd="true" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Start"
        android:id="@+id/button"
        android:layout_below="@id/main_input"
        android:layout_alignParentStart="true"
        android:onClick="onClick"/>


    <RadioGroup
        android:id="@+id/orientation"
        android:layout_below="@id/button"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="16dp">

        <RadioButton
            android:id="@+id/horizontal"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Horizontal" >
        </RadioButton>

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


</RelativeLayout> 

The resulting layout should look like the following screenshot.

Change the onCreate() method in your activity. Use the findViewById() method to find the RadioGroup in your layout.

Implement a listener on the radio group which changes the orientation of the radio buttons based on the current selection of the buttons. Which button is selected, can be identified by the ID parameter.

RadioGroup group1 = (RadioGroup) findViewById(R.id.orientation);
group1.setOnCheckedChangeListener(new RadioGroup.OnCheckedChangeListener() {
  @Override
  public void onCheckedChanged(RadioGroup group, int checkedId) {
    switch (checkedId) {
      case R.id.horizontal:
        group.setOrientation(LinearLayout.HORIZONTAL);
        break;
      case R.id.vertical:
        group.setOrientation(LinearLayout.VERTICAL);
        break;
    }
  }
}); 

Run your application and select the different radio button. Ensure that the orientation of the buttons is changed based on your selection.

In this exercise you learn how to create and consume Android resources and repeat the creation of an interactive application.

This application is available on Google Play under the following URL: Android Temperature converter

Alternatively you can also scan the following barcode with your Android phone to install it via the Google Play application.

Create a new Android project with the following data.

Select the res/values/strings.xml file to open the editor for this file. Add the Color and String definitions to the file as described by the following table.

The entered values should be similar to the following listing.

<?xml version="1.0" encoding="utf-8"?>
<resources>

    <string name="app_name">Temperature Converter</string>
    <string name="action_settings">Settings</string>
    <string name="hello_world">Hello world!</string>
    <color name="myColor">#F5F5F5</color>
    <string name="celsius">to Celsius</string>
    <string name="fahrenheit">to Fahrenheit</string>
    <string name="calc">Calculate</string>

</resources> 

Select the res/layout/activity_main.xml file and open the associated Android editor via a double-click on the file.

Remove any existing view from your layout, either directly from the XML source or via the graphical editor.

Afterwards add an LinearLayout, another LinearLayout with one ViewText, EditText as children. Afterwards a RadioGroup with two radio buttons and a Button to your layout. Do this either directly in the XML file or via the graphical editor. A simple way of organizing the components is to drag and drop them onto the Component Tree view.

The result should look like the following screenshots. The first one shows the component view the second one the preview.

Switch to the XML tab of your layout file and verify that the file looks similar to the following listing. The Android tools team changes the generated code from time to time, so your XML might look slightly different.

<LinearLayout 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:orientation="vertical"
    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"
    android:background="@color/myColor">


    <EditText
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/editText1" />

    <RadioGroup
        android:id="@+id/radioGroup1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignStart="@+id/editText1"
        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_alignStart="@+id/radioGroup1"
        android:layout_below="@+id/radioGroup1"
        android:layout_marginTop="22dp"
        android:text="Button" />

</LinearLayout> 

Switch to the XML representation of the file and assign the @string/celsius value to the android:text property of the first radio button. Assign the fahrenheit string attribute to the text property of the second radio button.

Ensure that the Checked property is set to true for the first RadioButton.

Assign @string/calc to the text property of your button and assign the value onClick to the OnClick property.

Set the inputType property to numberSigned and numberDecimal on the EditText. As an example you can use the last line in the following XML snippet. Also change its ID to "inputValue".

<EditText
  android:id="@+id/inputValue"
  android:layout_width="match_parent"
  android:layout_height="wrap_content"
  android:layout_alignParentEnd="true"
  android:layout_below="@+id/textView"
  android:ems="10"
  android:inputType="numberSigned|numberDecimal" /> 

All your user interface components are contained in a layout. Assign the background color to this Layout. Select Color and then select myColor in the dialog. As an example you can use the last line in the following XML snippet.

<LinearLayout 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:orientation="vertical"
    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"
    android:background="@color/myColor"> 

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

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

<LinearLayout 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:orientation="vertical"
    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"
    android:background="@color/myColor">


    <EditText
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/inputValue"
        android:inputType="numberSigned|numberDecimal"/>

    <RadioGroup
        android:id="@+id/radioGroup1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignStart="@+id/editText1"
        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_alignStart="@+id/radioGroup1"
        android:layout_below="@+id/radioGroup1"
        android:layout_marginTop="22dp"
        android:text="@string/calc"
        android:onClick="onClick"/>

</LinearLayout> 

Create the following utility class to convert from celsius to fahrenheit and vice versa.

package com.vogella.android.temperatureconverter;

public class ConverterUtil {
  // converts to celsius
  public static float convertFahrenheitToCelsius(float fahrenheit) {
    return ((fahrenheit - 32) * 5 / 9);
  }

  // converts to fahrenheit
  public static float convertCelsiusToFahrenheit(float celsius) {
    return ((celsius * 9) / 5) + 32;
  }
} 

The Android project wizard created the corresponding MainActivity class for your activity code. Adjust this class so that it is similar to the following code.

package com.vogella.android.temperatureconverter;

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.inputValue);

  }

  // 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(ConverterUtil.convertFahrenheitToCelsius(inputValue)));
        celsiusButton.setChecked(false);
        fahrenheitButton.setChecked(true);
      } else {
        text.setText(String
            .valueOf(ConverterUtil.convertCelsiusToFahrenheit(inputValue)));
        fahrenheitButton.setChecked(false);
        celsiusButton.setChecked(true);
      }
      break;
    }
  }

} 

Start your Android application and type in a number, select your conversion and press the button. The result should be displayed and the other option should get selected.

The Resources class allows to access individual resources. An instance of the Resources class can be retrieved via the getResources() method of the Context class. As activities and services extend the Context class, you can directly use this method in implementations of these components.

An instance of the Resources class is also required by other Android framework classes. For example, the following code shows how to create a Bitmap file from a reference ID.

// BitmapFactory requires an instance of the Resource class
BitmapFactory.decodeResource(getResources(), R.drawable.ic_action_search); 

In your activity (and fragment) code you frequently need to access the views 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
    }

} 

It is also possible to search in a view hierarchy with the findViewById(id) method, as demonstrated in the following code snippet.

// search in the layout of the activity
LinearLayout linearLayout = (LinearLayout) findViewById(R.id.mylayout);

// afterwards search in linearLayout for another view
TextView textView = (TextView) linearLayout.findViewById(R.id.mytext);

// note, you could have directly searched for R.id.mytext, the above coding
// is just for demonstration purposes 

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 String with the "titlepage" key in an XML resource, you could access it via @string/titlepage.

To use an Android system resource, include the android namespace into the references, e.g., android.R.string.cancel.

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.

You can access files stored in this folder based on their path. The assets directory also allows you to have sub-folders.

You access this data via the AssetsManager which you can access via the getAssets() method from an instance of the Context class.

The AssetsManager class allows you 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 the "logo.png" bitmap from the assets folder
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();
      }
    }
} 

In this exercise you continue to use the application you created in Section 7, “Exercise: Getting started with Android Studio”.

Create two new images to your project called ic_tethering and ic_offline. The creation process is described in Section 25.1, “Create an image via Android studio”.

Open your layout file and add a new Button and an ImageView to it.

Assign the ic_offline file to your ImageView, via your layout file as demonstrated in the following XML snippet.

<!--
        NOTE: More attributes are required 
        for the correct layout of the ImageView. These are left 
        out for brevity
    -->

<ImageView
  android:id="@+id/myicon"
  .... more attributes
  android:src="@drawable/ic_offline" /> 

If the button is clicked, use the findViewById() to search for the ImageView. Use its setImageResource() method to assign the png file (which is represented at runtime via a Drawable object) to your ImageView. The parameter of the setImageResource() method is the R reference to your file, e.g., R.drawable.your_png_file.

Ensure that if you press your new button, the displayed image is replaced.

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

The application can define via a <uses-feature> declaration in the manifest file define which hardware of software features is requires. Via the android:required property the application can define if such a feature is required for the application to work correctly (true) or if application prefers to use the feature if present on the device, but that it is designed to function without the specified feature, if necessary.

Examples for such definitions are the presence of a certain hardware sensor or the availability of a camera.

For an overview of the available restrictions see uses-features official documentation.

Android applications must be signed before they can get installed on an Android device. During development Eclipse signs your application automatically with a debug key.

To install the Android application via another channel you need to sign the Android apk with a self-created signature key.

Please note that you need to use the same signature key in Google Play (Google Market) to update your application. If you lose the key, you will NOT be able to update your application ever again.

Make sure to backup your key.

Use the Build → Generate Signed APK... menu entry to start the export from Android Studio.

If you want to export your production application via 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.

Android also allows to install applications 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.

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.

See Creating an image asset to learn how to create an image asset with Android Studio.

See Creating a new resource file to learn how to a new resource file with Android Studio.

See Add a new menu XML resource to learn how to a new XML menu resource with Android Studio.

See Android support library how to remove the usage of the support library.