• Activity - A screen in the Android application

• Services - Background activities without UI

• Content Provider - provides data to applications, Android contains a SQLLite DB which can serve as data provider

• Broadcast Receiver - receives system messages, can be used to react to changed conditions in the system

• onSaveInstanceState() - called if the activity is stopped. Used to save data so that the activity can restore its states if re-started

• onPause() - always called if the Activity ends, can be used to release ressource or save data

• onResume() - called if the Activity is re-started, can be used to initiaze fields

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" .

Use the Eclipse update manager to install all available plugins for the Android Development Tools (ADT) from the URL https://dl-ssl.google.com/android/eclipse/ .

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

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

1. Project ... is missing required source folder: 'gen'

2. The project could not be built until build path errors are resolved.

3. Unable to open class file R.java.

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

The Android SDK allows to maintain certain artifacts, e.g. strings and UI's, in two ways, via a rich editor and directly via XML. The following description tries to use the rich UI but for validation lists also the XML. You can switch between the two things the the tab on the lower part of the screen. For example:

Modifying "string.xml" gives currently a NPE, for details see the Bug report . As a workaround you can select Window -> Preferences -> XML -> XML Files -> Editor. Uncheck "Use inferred grammar in absence of DTD/Schema".

Android allows to create attributes for resources, e.g. for strings and / or colors. These attributes can be used in your UI definition via XML or in your Java source code.

Select the file "res/values/string.xml" and press "Add". Select "Color" and maintain "myColor" as the name and "#3399CC" as the value.

Add also the following "String" attributes. String attributes allow to translate the application at a later point.

Switch to the XML representation and validate that you maintained the values correctly.

				<?xml version="1.0" encoding="utf-8"?>
<resources>
	<string name="hello">Hello World, Convert!</string>
	<string name="app_name">Temperature Converter</string>
	<color name="myColor">#3399CC</color>
	<string name="buttonHandler">myClickHandler</string>
	<string name="celsius">to Celsius</string>
	<string name="fahrenheit">to Fahrenheit</string>
	<string name="calc">Calculate</string>
</resources>
			

Select "res/layout/main.xml" and open the Android editor via double-click. This editor allows to maintain the UI via drag and drop or directly via the XML source code. You can switch between both representations via the tabs at the bottom of the editor. For changing the postion and grouping elements you can use the outline view.

Delete the "Hello World, Hello!" via a right mouse click. From the "Views" bar, drag in an "EditText". Add from the layout a "RadioGroup" and then two RadioButtons, add one "Button". The result should look like the following and the corresponding XML is listed below. Make sure that your code is the same as listed below.

Switch to "main.xml" and verify that your XML looks 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">

<EditText android:text="@+id/EditText01" android:id="@+id/EditText01" android:layout_width="wrap_content" android:layout_height="wrap_content"></EditText>
	<RadioGroup android:id="@+id/RadioGroup01"
		android:layout_width="wrap_content" android:layout_height="wrap_content">
		<RadioButton android:text="@+id/RadioButton01" android:id="@+id/RadioButton01"
			android:layout_width="wrap_content" android:layout_height="wrap_content"></RadioButton>
		<RadioButton android:text="@+id/RadioButton02" android:id="@+id/RadioButton02"
			android:layout_width="wrap_content" android:layout_height="wrap_content"></RadioButton>
	</RadioGroup>
	<Button android:text="@+id/Button01" android:id="@+id/Button01"
		android:layout_width="wrap_content" android:layout_height="wrap_content"></Button>
</LinearLayout>
			

If you select a UI element you can change its properties via the properties view. Select EditText and change the property "Layout Width" to "fill_parent".

Assign the "celsius" string attribute to your "text" property of the first radio button and "fahrenheit" to the second. Set the property "Checked" to true for the first RadioButton. Assign "calc" to the text property of your button and assign "buttonHandler" to the "onClick" property. Delete the text property in the EditText (this means no text will be initially shown) and set the "Input type" property to "numberSigned" and "number decimal".

Select the complete widget and use the Properties view to set the property "background" to the color attribute "@color/myColor".

Switch to the "main.xml" tab and verify that the XML is correctly maintained.

				<?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/myColor">

<EditText android:id="@+id/EditText01" android:layout_height="wrap_content" android:layout_width="fill_parent" android:inputType="numberSigned|numberDecimal"></EditText>
	<RadioGroup android:id="@+id/RadioGroup01"
		android:layout_width="wrap_content" android:layout_height="wrap_content">
		<RadioButton android:id="@+id/RadioButton01"
			android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="@string/celsius" android:checked="true"></RadioButton>
		<RadioButton android:id="@+id/RadioButton02"
			android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="@string/fahrenheit"></RadioButton>
	</RadioGroup>
	<Button android:id="@+id/Button01"
		android:layout_height="wrap_content" android:onClick="@string/buttonHandler" android:layout_width="wrap_content" android:text="@string/calc"></Button>
</LinearLayout>
			

Change your code in "Convert.java" to the following. Note that the "myClickHandler" will be called basedon the "On Click" property of your button.

				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 Convert extends Activity {
	private EditText text;

	@Override
	public void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.main);
		text = (EditText) findViewById(R.id.EditText01);

	}

	// This method is called at button click because we assigned the name to the
	// "On Click property" of the button
	public void myClickHandler(View view) {
		switch (view.getId()) {
		case R.id.Button01:
			RadioButton celsiusButton = (RadioButton) findViewById(R.id.RadioButton01);
			RadioButton fahrenheitButton = (RadioButton) findViewById(R.id.RadioButton02);
			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(convertFahrenheitToCelcius(inputValue)));
			} else {
				text.setText(String
						.valueOf(convertCelciusToFahrenheit(inputValue)));
			}
			// Switch to the other button
			if (fahrenheitButton.isChecked()) {
				fahrenheitButton.setChecked(false);
				celsiusButton.setChecked(true);
			} else {
				fahrenheitButton.setChecked(true);
				celsiusButton.setChecked(false);
			}
			break;
		}
	}

	// Converts to celcius
	private float convertFahrenheitToCelcius(float fahrenheit) {
		return ((fahrenheit - 32) * 5 / 9);
	}

	// Converts to fahrenheit
	private float convertCelciusToFahrenheit(float celsius) {
		return ((celsius * 9) / 5) + 32;
	}
}
			

To start the Android Application, select your project, right click on it, Run-As-> Android Application Be patient, the emulator starts up very slow. You should get the following result.

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

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

List can be used to display a scrollable list of items. You can either use lists in your layout or if the purpose of your Activity is to show primary a list you can extend ListActivities which provides nice hocks for typical actions for lists.

A ListActivity extends Activity and simplifies the approach to show several objects in a list. It extends the standard Activity with a standard ListView Elements, callbacks for list events, e..g for selecting a list element and helper methods to access the current list position and the selected element(s).

To test this create a new Android project "de.vogella.android.listactivity" with the activity "MyList". You do not need to change the default layout "main.xml". Create the following activity.

				package de.vogella.android.listactivity;

import android.app.ListActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.Toast;

public class MyList extends ListActivity {

	/** Called when the activity is first created. */
	public void onCreate(Bundle icicle) {
		super.onCreate(icicle);
		// Create an array of Strings, that will be put to our ListActivity
		String[] names = new String[] { "Linux", "Windows7", "Eclipse", "Suse", "Ubuntu", "Solaris", "Android", "iPhone"};
		// Create an ArrayAdapter, that will actually make the Strings above
		// appear in the ListView
		this.setListAdapter(new ArrayAdapter<String>(this,
				android.R.layout.simple_list_item_checked, names));
	}

	@Override
	protected void onListItemClick(ListView l, View v, int position, long id) {
		super.onListItemClick(l, v, position, id);
		// Get the item that was clicked
		Object o = this.getListAdapter().getItem(position);
		String keyword = o.toString();
		Toast.makeText(this, "You selected: " + keyword, Toast.LENGTH_LONG)
				.show();
	}
}
			

The example above is boring, as only text is shown. You can also define your own layout for the rows and assign this layout to your row adapter. We will add a graphic to each list entry.

Create the following layout file "rowlayout.xml" in the res/layout folder of your project "de.vogella.android.listactivity".

				
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
	android:layout_width="wrap_content" android:layout_height="wrap_content">
	<ImageView android:id="@+id/icon" android:layout_height="wrap_content" android:src="@drawable/icon" android:layout_width="22px" android:layout_marginTop="4px" android:layout_marginRight="4px" android:layout_marginLeft="4px">
		</ImageView>
	<TextView android:text="@+id/TextView01" android:layout_width="wrap_content" android:layout_height="wrap_content" android:id="@+id/label" android:textSize="30px"></TextView>
</LinearLayout>

			

Change your activity "MyList" to the following. This is almost the same coding as in the previous example, the only difference is that we are using our own layout in the ArrayAdapter and telling the adapter which UI element should contains the text.

				
package de.vogella.android.listactivity2;

import android.app.Activity;
import android.app.ListActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.Toast;

public class MyLayoutList extends ListActivity {

	/** Called when the activity is first created. */
	public void onCreate(Bundle icicle) {
		super.onCreate(icicle);
		// Create an array of Strings, that will be put to our ListActivity
		String[] names = new String[] { "Linux", "Windows7", "Eclipse", "Suse",
				"Ubuntu", "Solaris", "Android", "iPhone" };
		// Use your own layout and point the adapter to the UI elements which contains the label
		this.setListAdapter(new ArrayAdapter<String>(this, R.layout.rowlayout,
				R.id.label, names));
	}

	@Override
	protected void onListItemClick(ListView l, View v, int position, long id) {
		super.onListItemClick(l, v, position, id);
		// Get the item that was clicked
		Object o = this.getListAdapter().getItem(position);
		String keyword = o.toString();
		Toast.makeText(this, "You selected: " + keyword, Toast.LENGTH_LONG)
				.show();

	}
}
			

The above example uses one layout for all rows. If you want to influence the display of the different rows you can also define your own adapter and implement your own getView() method. This method is responsible for creating the listview. In this method we will read the pre-defined layout via LayoutInflator and return one individual view per row.

Create the following class "MyArrayAdapter.java".

				
package de.vogella.android.listactivity;

import android.app.Activity;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.ImageView;
import android.widget.TextView;

public class MyArrayAdapter extends ArrayAdapter<String> {
	private final Activity context;
	private final String[] names;

	public MyArrayAdapter(Activity context, String[] names) {
		super(context, R.layout.rowlayout, names);
		this.context = context;
		this.names = names;
	}

	@Override
	public View getView(int position, View convertView, ViewGroup parent) {
		LayoutInflater inflater = context.getLayoutInflater();
		View rowView = inflater.inflate(R.layout.rowlayout, null, true);
		
		TextView label = (TextView) rowView.findViewById(R.id.label);
		label.setText(names[position]);
		System.out.println(names[position]);
		// Change the icon for Windows and iPhone
		if (names[position].startsWith("Windows7") ||  names[position].startsWith("iPhone") ) {
			ImageView imageView = (ImageView) rowView.findViewById(R.id.icon);
			imageView.setImageResource(R.drawable.alt_window_16);
		} 
		
		return rowView;
	}

}

			

				
package de.vogella.android.listactivity;

import android.app.ListActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.ListView;
import android.widget.Toast;

public class MyList extends ListActivity {

	/** Called when the activity is first created. */
	public void onCreate(Bundle icicle) {
		super.onCreate(icicle);
		// Create an array of Strings, that will be put to our ListActivity
		String[] names = new String[] { "Linux", "Windows7", "Eclipse", "Suse", "Ubuntu", "Solaris", "Android", "iPhone"};
		this.setListAdapter(new MyArrayAdapter(this, names));
	}

	@Override
	protected void onListItemClick(ListView l, View v, int position, long id) {
		super.onListItemClick(l, v, position, id);
		// Get the item that was clicked
		Object o = this.getListAdapter().getItem(position);
		String keyword = o.toString();
		Toast.makeText(this, "You selected: " + keyword, Toast.LENGTH_LONG)
				.show();
	}
}
			

This chapter will demonstrate how to create and evaluate a menu, how to define preferences and how to navigate between activities via an intent . Create a project "de.vogella.android.preferences" with the activity "HelloPreferences". Change the UI in the file "/res/layout/main.xml" to 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">
	<Button android:id="@+id/Button01" android:layout_width="wrap_content"
		android:layout_height="wrap_content" android:text="Show Preferences"></Button>
</LinearLayout>

			

Menus can be defined via XML files. Select your project, right click on it and select New -> Other -> Android -> "Android XML File".

Press Add and select "Item". Maintain the following value. This defines the entries in your menu. We will have only one entry.

Change your class "HelloPreferences" to the following. The OnCreateOptionsMenu method is used to create the menu. Please note that at the moment nothing happens if you select this menu. The behavior will be later implemented in the method "onOptionsItemSelected".

				
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;
    }
}
			

Run your application and press "Menu" on the emulator. Your menu should be displayed.

Preference values can also be stored as a XML resource. Create another Android XML File "preferences.xml" this time of type preferences.

Press Add, add a category and add two preferences "EditTextPreferences" to this category : "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. Press the "menu" hardware button and then select your menu item "Preferences". You should be able to enter your user settings then press the back hardware button to return to your main activity and press the button. The saved values 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".

Select the home menu and then the menu entry "Contacts" to create 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);
	}

}
			

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 following command "adb shell".

				adb shell

			

This will connect you to your device and give you Linux command line access to the underlying file system, e.g. ls, rm, mkdir, etc. The application data is stored in the directory "/data/data/package_of_your_app".

If you have several devices running you can issue commands to one individuel device.

				
# Lists all devices
adb devices
#Result
List of devices attached
emulator-5554 attached
emulator-5555 attached
# Issue a command to a specific device
adb -s emulator-5554 shell

			

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

Alternatively to adb you can also use telnet to connect to the device. This allows you to simulate certain things, e.g. incoming call, change the network "stability", set your current geocodes, etc. Use "telnet localhost 5554" to conntect to your simulated device. To exit the console session, use the command "quit" or "exit".

For example to change the power settings of your phone, to receive an sms and to get an incoming call make the following.

				
# connects to device
telnet localhost 5554
# set the power level
power status full
power status charging
# make a call to the device
gsm call 012041293123
# send a sms to the device
sms send 12345 Will be home soon
# set the geo location

			

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