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.

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.

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.

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.

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.

Android application components can connect to components of other Android applications to create tasks. For example an application which allows you to make a photo can start an email application and instruct this application to create a new email and attach a photo to this email.

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.

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.

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.

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 fragment and allows that the user navigates to another fragment. On a wide screen it shows two fragments.

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.

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.

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.

Broadcast receivers can be registered to receive system messages and intents. A broadcast receiver gets 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.

Widgets are interactive components which are primarily used on the Android homescreen. They typically display some kind of data and allow the user to perform actions via them. For example a widget 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.

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

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.

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.

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.

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.

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.

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.

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 for activities via predefined methods. The most important methods are:

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 activity might 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> 

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.

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.

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.

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> 

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); 

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.

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.

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 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();
        }
      }
    } 

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

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.

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.

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.

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.

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.

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.

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

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.

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 is de.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.

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.

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.

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.

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.

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.

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.

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.

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

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

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.

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> 

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> 

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> 

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;
  }
} 

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.

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.

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.

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.

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.

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> 

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.

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.

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.

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

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.

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.

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

If you want to install your application without the Eclipse IDE you can right-click on it and select Android Tools → Export Signed Application Package.

This wizard allows to use an existing key or to create a new one.

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

Make sure to backup your key.

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.

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.

Source Code of Examples

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

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