Android is an operating system based on Linux with a Java programming interface. It provides tools, e.g. a compiler, debugger and a device emulator as well as its own Java Virtual machine (Dalvik). Android is created by the Open Handset Alliance which is lead by Google.

Android uses a special Java virtual machine (Dalvik) which is based on the Apache Harmony Java implementation. Dalvik uses special bytecode there you cannot run standard Java program on Android but you have to use the Android compiler to create this special byte-code.

Android supports 2-D and 3-D graphics using the OpenGL libraries and supports data storage in a SQLLite database.

For development Google provides the Android Development Tools (ADT) for Eclipse to develop Android applications.

An Android application consists out of the following parts:

An Android application is described the file "AndroidManifest.xml". This files contains all activities application and the required permissions for the application. For example if the application requires network access it must be specified here. "AndroidManifest.xml" can be thought as the deployment descriptor for an Android application.

Use the update manager of Eclipse to install all available plugins for the Android Development Tools (ADT) from the URL https://dl-ssl.google.com/android/eclipse/ . See Using the Eclipse update manager for details on how to use the update manager and how to install new plugins.

Download the Android SDK from the Android homepage under Android SDK download .

The download contains a zip file which you can extract to any place in your file system, e.g. I placed it under "c:\android-sdk-windows" .

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

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

Select available packages and select the latest version of the SDK.

Press "Install selected" and confirm the license for all package.

After the installation restart Eclipse.

You need to define a device which can be used for emulation. Press the device manager button, press "New" and maintain the following.

Press "Create AVD".

To test if you setup is correct, eelect your device and press "Start".

After (a long time) your device should be started.

. Select File -> New -> Other -> Android -> Android Project and create the Android project "de.vogella.android.first" Maintain the following.

Press "Finish".

This should create the following directory structure.

"R.java" is a generated class which contains the text and the UI elements. Please do not try to modify this class manually.

Select "layout/main.xml" and open the editor via double-click. The result should look like the following.

From the "Views" bar, drag in an "EditText" and three "Buttons". The result should look like the following.

Select "values/string.xml" and press "Add".

Select "Color" and maintain "white" as the name and "#FFFFFF" as the value.

Go back to "main.xml", select the complete widget and use the Properties view to set the background to this attribute.

Change your code in "Hello.java" to the following.

				package de.vogella.android.first;import android.app.Activity;import android.os.Bundle;import android.view.View;import android.widget.EditText;public class Hello extends Activity {	private EditText text;	/** Called when the activity is first created. */	@Override	public void onCreate(Bundle savedInstanceState) {		super.onCreate(savedInstanceState);		setContentView(R.layout.main); // bind the layout to the activity		text = (EditText) findViewById(R.id.EditText01);		text.setText("No button pressed");	}	// Will be connected with the buttons via XML	public void myClickHandler(View view) {		switch (view.getId()) {		case R.id.Button01:			text.setText("Button 1 was clicked");			break;		case R.id.Button02:			text.setText("Button 2 was clicked");			break;		case R.id.Button03:			text.setText("Button 3 was clicked");			break;		}	}}			

Open again "main.xml" and select your first button. Via the property view assign the method "myClickHandler" to the "on Click" property of the first button.

Assign "myClickHandler" also to the other buttons and switch to the "main.xml" tab.

The resulting "main.xml" file should look like the following.

				<?xml version="1.0" encoding="utf-8"?><LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"    android:orientation="vertical"    android:layout_width="fill_parent"    android:layout_height="fill_parent"    android:background="@color/white"><TextView      android:layout_width="fill_parent"     android:layout_height="wrap_content"     android:text="@string/hello"    /><EditText android:text="@+id/EditText01" android:id="@+id/EditText01" android:layout_width="wrap_content" android:layout_height="wrap_content"></EditText><Button android:text="@+id/Button01" android:id="@+id/Button01" android:layout_width="wrap_content" android:layout_height="wrap_content" android:onClick="myClickHandler"></Button><Button android:text="@+id/Button02" android:id="@+id/Button02" android:layout_width="wrap_content" android:layout_height="wrap_content" android:onClick="myClickHandler"></Button><Button android:text="@+id/Button03" android:id="@+id/Button03" android:layout_width="wrap_content" android:layout_height="wrap_content" android:onClick="myClickHandler"></Button></LinearLayout>			

To start the Android Application, select your project, right click on it, Run-As-> Android Application

You should get the following result.

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

We want now to create an application which displays a menu. If the user select the menu he should be able to maintain preferences.

Create a project "de.vogella.android.preferences" with the activity "HelloPreferences".

Select your project, right click on it and select New -> Other -> Android -> "Android XML File".

Press Add and select "Item". Maintain the following values.

Change your class "HelloPreferences" to the following.

				package de.vogella.android.preferences;import android.app.Activity;import android.os.Bundle;import android.view.Menu;import android.view.MenuInflater;public class HelloPreferences extends Activity {    /** Called when the activity is first created. */    @Override    public void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.main);    }    @Override    public boolean onCreateOptionsMenu(Menu menu) {    	MenuInflater inflater = getMenuInflater();    	inflater.inflate(R.menu.menu, menu);    	return true;    }}			

If you run your application and press "Menu" on the emulator. Your menu should alread be displayed.

Create another "Android XML File" this time "preferences.xml".

Press Add and add two "EditTextPreferences": "User" and "Password".

Create the class "Preferences" which will load the "preference.xml".

				package de.vogella.android.preferences;import android.os.Bundle;import android.preference.PreferenceActivity;public class Preferences extends PreferenceActivity {	/** Called when the activity is first created. */	@Override	public void onCreate(Bundle savedInstanceState) {	    super.onCreate(savedInstanceState);	    addPreferencesFromResource(R.xml.preferences);	}		}			

Select "AndroidManifest.xml" and the tab "Application". Add the activity "Preferences".

To use the preferences add a button to your main.xml with the id "@+id/Button01" and change the coding of HelloPreferences to the following.

				package de.vogella.android.preferences;import android.app.Activity;import android.content.Intent;import android.content.SharedPreferences;import android.os.Bundle;import android.preference.PreferenceManager;import android.view.Menu;import android.view.MenuInflater;import android.view.MenuItem;import android.view.View;import android.view.View.OnClickListener;import android.widget.Button;import android.widget.Toast;public class HelloPreferences extends Activity {	SharedPreferences preferences;	/** Called when the activity is first created. */	@Override	public void onCreate(Bundle savedInstanceState) {		super.onCreate(savedInstanceState);		setContentView(R.layout.main);		Button button = (Button) findViewById(R.id.Button01);		// Initialize preferences		preferences = PreferenceManager.getDefaultSharedPreferences(this);				button.setOnClickListener(new OnClickListener() {			@Override			public void onClick(View v) {				String username = preferences.getString("username", "n/a");				String password = preferences.getString("password", "n/a");				Toast.makeText(HelloPreferences.this,						"You maintained user: " + username + " and password: " + password,						Toast.LENGTH_LONG).show();			}		});	}	@Override	public boolean onCreateOptionsMenu(Menu menu) {		MenuInflater inflater = getMenuInflater();		inflater.inflate(R.menu.menu, menu);		return true;	}	// This method is called once the menu is selected	@Override	public boolean onOptionsItemSelected(MenuItem item) {		switch (item.getItemId()) {		// We have only one menu option		case R.id.preferences:			// Launch Preference activity			Intent i = new Intent(HelloPreferences.this, Preferences.class);			startActivity(i);			// A toast is a view containing a quick little message for the user.			Toast.makeText(HelloPreferences.this,					"Here you can maintain your user credentials.",					Toast.LENGTH_LONG).show();			break;		}		return true;	}}			

Run your application. Select your menu "Preferences". You should be able to maintain your user settings and if you press the button in your main activities the maintained valued should be displayed in a small message windows (Toast).

ContentProvider are used to provide data from an application to another. ContentProvider do not store the data but provide the interface for other applications to access the data.

The following example will use an existing context provider from "Contacts".

Start the contacts application and create a few contacts.

Use the Android emulator and create a view contacts.

Press Menu and select "New Contact".

As a result you should have a few new contacts.

Create a new Android project "de.vogella.android.contentprovider" with the activity "ContactsView".

Rename the id of the the existing TextView from the example wizard to "contactview". Delete the default text. Also change the layout_height to "fill_parent".

The resulting main.xml should look like the following.

				<?xml version="1.0" encoding="utf-8"?><LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"	android:orientation="vertical" android:layout_width="fill_parent"	android:layout_height="fill_parent">	<TextView android:layout_width="fill_parent"		android:layout_height="fill_parent" android:id="@+id/contactview" /></LinearLayout>			

In AndroidManifest.xml add the User Permission that the application can use "android.permission.READ_CONTACTS".

Change the coding of the activity.

				package de.vogella.android.contentprovider;import android.app.Activity;import android.database.Cursor;import android.net.Uri;import android.os.Bundle;import android.provider.ContactsContract;import android.widget.TextView;public class ContactsView extends Activity {	/** Called when the activity is first created. */	@Override	public void onCreate(Bundle savedInstanceState) {		super.onCreate(savedInstanceState);		setContentView(R.layout.main);		TextView contactView = (TextView) findViewById(R.id.contactview);		Cursor cursor = getContacts();		while (cursor.moveToNext()) {			String displayName = cursor.getString(cursor					.getColumnIndex(ContactsContract.Data.DISPLAY_NAME));			contactView.append("Name: ");			contactView.append(displayName);			contactView.append("\n");		}	}	private Cursor getContacts() {		// Run query		Uri uri = ContactsContract.Contacts.CONTENT_URI;		String[] projection = new String[] { ContactsContract.Contacts._ID,				ContactsContract.Contacts.DISPLAY_NAME };		String selection = ContactsContract.Contacts.IN_VISIBLE_GROUP + " = '"				+ ("1") + "'";		String[] selectionArgs = null;		String sortOrder = ContactsContract.Contacts.DISPLAY_NAME				+ " COLLATE LOCALIZED ASC";		return managedQuery(uri, projection, selection, selectionArgs,				sortOrder);	}}			

Android allows to access the network via the the java.net.URL class.

To set the proxy you can use the class Settings. For example you could add the following line to your onCreate method in your activity.

				Settings.System.putString(getContentResolver(), Settings.System.HTTP_PROXY, "myproxy:8080");			

You also have to give your application the right to change the settings "android.permission.WRITE_SETTINGS" in "AndroidManifest.xml".

Create the project "de.vogella.android.network.html". Add the following elements to your activity:

Create the following code to read a webpage and show the HTML code in the TextView.

This example also demonstrate the usage of Android preferences to store user data. The URL which the user has typed is stored in the preferences in the method onPause(). This method is called whenever the Activity is send into the background.

				package de.vogella.android.network.html;import java.io.BufferedReader;import java.io.InputStreamReader;import java.net.URL;import android.app.Activity;import android.content.SharedPreferences;import android.content.SharedPreferences.Editor;import android.os.Bundle;import android.view.View;import android.widget.EditText;import android.widget.TextView;public class ReadWebpage extends Activity {	private static final String PREFERENCES = "PREFERENCES";	private static final String URL = "url";	private String lastUrl;	private EditText urlText;	private TextView textView;	/** Called when the activity is first created. */	@Override	public void onCreate(Bundle savedInstanceState) {		super.onCreate(savedInstanceState);		setContentView(R.layout.main);		urlText = (EditText) findViewById(R.id.address);		textView = (TextView) findViewById(R.id.pagetext);		loadPreferences();		urlText.setText(lastUrl);	}		/**	 * Demonstrates loading of preferences The last value in the URL string will	 * be loaded	 */	private void loadPreferences() {		SharedPreferences preferences = getSharedPreferences(PREFERENCES,				Activity.MODE_PRIVATE);		// Set this to the Google Homepage		lastUrl = preferences.getString(URL, "http://209.85.229.147");	}	@Override	protected void onPause() {		super.onPause();		SharedPreferences preferences = getSharedPreferences(PREFERENCES,				Activity.MODE_PRIVATE);		Editor preferenceEditor = preferences.edit();		preferenceEditor.putString(URL, urlText.getText().toString());		// You have to commit otherwise the changes will not be remembered		preferenceEditor.commit();	}		// Will be connected with the buttons via XML	public void myClickHandler(View view) {		switch (view.getId()) {		case R.id.ReadWebPage:			try {				textView.setText("");				// Perform action on click				URL url = new URL(urlText.getText().toString());				// Get the response				BufferedReader rd = new BufferedReader(						new InputStreamReader(url.openStream()));				String line = "";				while ((line = rd.readLine()) != null) {					textView.append(line);				}			}			catch (Exception e) {				System.out.println("Nay, did not work");				textView.setText(e.getMessage());			}			break;		}	}}			

Assign the handler "buttonHandler" to the button in the property "on Click". via your XML.

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

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

You can access your Android emulator also via the console. Open a shell, switch to your "android-sdk" installation directory into the folder "tools".

Start the shell via the command "adb shell".

The emulator console lets you dynamically access your simulated device. Use "telnet localhost 5554" to conntect to your simulated device. To exit the console session, use the command "quit" or "exit".

For example you can set your geolocation in the emulator via "geo fix -121.45356 46.51119 4392"

For more information on the emulator console please see Emulator Console manual

You can uninstall an android application via the shell. Switch the the data/app directory (cd /data/app) and simply delete your android application.

Create a new device which supports the Google API's. During device creation select the target Google API's in the version of your SDK.

Create a new project "de.vogella.android.locationapi". Make sure to select the Google API

Add the following permissions to your application in Android.xml.

You need to add the Google maps library to your application. Open Android.xml, tab Application and add a "Uses library".

Define your view layout like the following.

				<?xml version="1.0" encoding="utf-8"?><RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"    android:id="@+id/mainlayout"    android:orientation="vertical"    android:layout_width="fill_parent"    android:layout_height="fill_parent" >    <com.google.android.maps.MapView        android:id="@+id/mapview"        android:layout_width="fill_parent"        android:layout_height="fill_parent"        android:clickable="true"        android:apiKey="Your Maps API Key"    /></RelativeLayout>			

Create the following activity. This activity use an LocationListner to update the map with the current location.

				package de.vogella.android.locationapi;import android.content.Context;import android.location.Location;import android.location.LocationListener;import android.location.LocationManager;import android.os.Bundle;import android.widget.RelativeLayout;import android.widget.ZoomControls;import com.google.android.maps.GeoPoint;import com.google.android.maps.MapActivity;import com.google.android.maps.MapController;import com.google.android.maps.MapView;public class CurrentLocation extends MapActivity {	private MapController mapController;	private MapView mapView;	private LocationManager locationManager;	public void onCreate(Bundle bundle) {		super.onCreate(bundle);		setContentView(R.layout.main); // bind the layout to the activity		// create a map view		RelativeLayout linearLayout = (RelativeLayout) findViewById(R.id.mainlayout);		mapView = (MapView) findViewById(R.id.mapview);		ZoomControls mZoom = (ZoomControls) mapView.getZoomControls();		linearLayout.addView(mZoom);		mapController = mapView.getController();		// Zoon 1 is world view				mapController.setZoom(14);		locationManager = (LocationManager) getSystemService(Context.LOCATION_SERVICE);		locationManager.requestLocationUpdates(LocationManager.GPS_PROVIDER, 0,				0, new GeoUpdateHandler());	}	@Override	protected boolean isRouteDisplayed() {		return false;	}	public class GeoUpdateHandler implements LocationListener {		@Override		public void onLocationChanged(Location location) {			int lat = (int) (location.getLatitude() * 1E6);			int lng = (int) (location.getLongitude() * 1E6);			GeoPoint point = new GeoPoint(lat, lng);			mapController.setCenter(point);//			setContentView(mapView);		}		@Override		public void onProviderDisabled(String provider) {		}		@Override		public void onProviderEnabled(String provider) {		}		@Override		public void onStatusChanged(String provider, int status, Bundle extras) {		}	}}			

Run and test your application. You should be able to zoon in and out. Use Emulator console to send geo-coordinates to your device for example

				geo fix 13.24 52.31			

Source Code of Examples

Android Homepage

Android Developer Homepage

Android Issues / Bugs

Android Market

SQL light database homepage

Content Provider Tutorial by Google

Notepad Tutorial from Google

Android Core Snippets / Tutorial page

Marakana Android Tutorials

http://www.ibm.com/developerworks/opensource/library/os-android-networking/index.html Networking with Android from Frank Ableson

http://www.ibm.com/developerworks/opensource/library/os-android-sensor/index.html Androids Sensors from Frank Ableson

Eclipse Tutorials

Web development Tutorials

Android Development Tutorial

GWT Tutorial

Eclipse RCP Tutorial