How to Develop Android Apps

8/3/2019 How to Develop Android Apps

1/67

1. Android Development

1.1. Android Operation System

Android is an operating system based onLinuxwith aJavaprogramming interface. It

provides tools, e.g. a compiler, debugger and a device emulator as well as its own Java

Virtual machine (Dalvik Virtual Machine - DVM).Androidis created by the Open

Handset Alliance which is lead by Google.

Android uses a special virtual machine, e.g. the Dalvik Virtual Machine. Dalvik uses

special bytecode. Therefore you cannot run standard Java bytecode on Android.

Android provides a tool "dx" which allows to convert Java Class files into "dex" (Dalvik

Executable) files. Android applications are packed into an .apk (Android Package) file bythe program "aapt" (Android Asset Packaging Tool) To simplify development Google

provides the Android Development Tools (ADT) forEclipse. The ADT performs

automatically the conversion from class to dex files and creates the apk during

deployment.

Android supports 2-D and 3-D graphics using the OpenGL libraries and supports data

storage in aSQLitedatabase.

EveryAndroidapplications runs in its own process and under its own userid which isgenerated automatically by the Android system during deployment. Therefore the

application is isolated from other running applications and a misbehaving application

cannot easily harm other Android applications.

1.2. Important Android components

An Android application consists out of the following parts:

Activity - Represents the presentation layer of an Android application, e.g. ascreen which the user sees. An Android application can have several activities and

it can be switched between them during runtime of the application.

Views - The User interface of an Activities is build with widgets classes which

inherent from "android.view.View". The layout of the views is managed by

"android.view.ViewGroups".
http://www.vogella.de/articles/Ubuntu/article.htmlhttp://www.vogella.de/articles/Ubuntu/article.htmlhttp://www.vogella.de/articles/Ubuntu/article.htmlhttp://www.vogella.de/articles/JavaIntroduction/article.htmlhttp://www.vogella.de/articles/JavaIntroduction/article.htmlhttp://www.vogella.de/articles/JavaIntroduction/article.htmlhttp://www.vogella.de/articles/Android/article.htmlhttp://www.vogella.de/articles/Android/article.htmlhttp://www.vogella.de/articles/Android/article.htmlhttp://www.vogella.de/articles/Eclipse/article.htmlhttp://www.vogella.de/articles/Eclipse/article.htmlhttp://www.vogella.de/articles/Eclipse/article.htmlhttp://www.vogella.de/articles/AndroidSQLite/article.htmlhttp://www.vogella.de/articles/AndroidSQLite/article.htmlhttp://www.vogella.de/articles/Android/article.htmlhttp://www.vogella.de/articles/Android/article.htmlhttp://www.vogella.de/articles/Android/article.htmlhttp://www.vogella.de/articles/Android/article.htmlhttp://www.vogella.de/articles/AndroidSQLite/article.htmlhttp://www.vogella.de/articles/Eclipse/article.htmlhttp://www.vogella.de/articles/Android/article.htmlhttp://www.vogella.de/articles/JavaIntroduction/article.htmlhttp://www.vogella.de/articles/Ubuntu/article.html


2/67

Services- perform background tasks without providing an UI. They can notify

the user via the notification framework in Android.

Content Provider- provides data to applications, via a content provider your

application can share data with other applications. Android contains a SQLite DB

which can serve as data provider

Intentsare asynchronous messages which allow the application to request

functionality from other services or activities. An application can call directly a

service or activity (explicit intent) or ask the Android system for registered

services and applications for an intent (implicit intents). For example the

application could ask via an intent for a contact application. Application register

themself to an intent via an IntentFilter. Intents are a powerful concept as they

allow to create loosely coupled applications.

Broadcast Receiver - receives system messages and implicit intents, can be usedto react to changed conditions in the system. An application can register as a

broadcast receiver for certain events and can be started if such an event occurs.

Other Android parts areAndroid WidgetsorLive FoldersandLive Wallpapers. Live

Folders display any source of data on the homescreen without launching the

corresponding application.

1.3. Security and permissions

Android defines certain permissions for certain tasks. For example if the application

want to access the Internet it must define in its configuration file that it would like to

use the related permission. During the installation of an Android application the user

get a screen in which he needs to confirm the required permissions of the application.

1.4. AndroidManifest.xml

An Android application is described the file "AndroidManifest.xml". This file must

declare all activities, services, broadcast receivers and content provider of the

application. It must also contain 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.
http://www.vogella.de/articles/AndroidServices/article.htmlhttp://www.vogella.de/articles/AndroidServices/article.htmlhttp://www.vogella.de/articles/Android/article.html#contentproviderhttp://www.vogella.de/articles/Android/article.html#contentproviderhttp://www.vogella.de/articles/AndroidIntent/article.htmlhttp://www.vogella.de/articles/AndroidIntent/article.htmlhttp://www.vogella.de/articles/AndroidWidgets/article.htmlhttp://www.vogella.de/articles/AndroidWidgets/article.htmlhttp://www.vogella.de/articles/AndroidWidgets/article.htmlhttp://android-developers.blogspot.com/2009/04/live-folders.htmlhttp://android-developers.blogspot.com/2009/04/live-folders.htmlhttp://android-developers.blogspot.com/2009/04/live-folders.htmlhttp://developer.android.com/resources/articles/live-wallpapers.htmlhttp://developer.android.com/resources/articles/live-wallpapers.htmlhttp://developer.android.com/resources/articles/live-wallpapers.htmlhttp://developer.android.com/resources/articles/live-wallpapers.htmlhttp://android-developers.blogspot.com/2009/04/live-folders.htmlhttp://www.vogella.de/articles/AndroidWidgets/article.htmlhttp://www.vogella.de/articles/AndroidIntent/article.htmlhttp://www.vogella.de/articles/Android/article.html#contentproviderhttp://www.vogella.de/articles/AndroidServices/article.html


3/67

The "package" attribute defines the base package for the following Java elements. It also

must be unique as the Android Marketplace only allows application for a specfic package

once. Therefore a good habit is to use your reverse domain name as a package to avoid

collisions with other developers.

"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 and the Android Market uses this to determine if you provided a

newer version to trigger the update on devices which have your application installed.


4/67

You typically start with "1" and increase this value by one if you roll-out a new version of

your application.

"activity" defines an activity in this example pointing to the class

"de.vogella.android.temperature.Convert". For this class an intent filter is registeredwhich 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 @ values refer to resource

files which contain the actual values. This 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" defines the minimal SDK version

your application is valid for. This will prevent your application being installed on devices

with older SDK versions.

1.5. R.java, Resources and Assets

The directory "gen" in an Android project contains generated values. "R.java" is a

generated class which contains references to resources of the "res" folder in the project.

These resources are defined in the "res" directory and can be values, menus, layouts,

icons or pictures or animations. For example a resource can be an image or an XML files

which defines strings.

If you create a new resources, the corresponding reference is automatically created in

"R.java". The references are static int values, the Android system provides methods to

access the corresponding resource. For example to access a String with the reference id

"R.string.yourString" use the method getString(R.string.yourString)); Please do not try

to modify "R.java" manually.

While the directory"res" contains structured values which are known to the Android

platform the directory "assets" can be used to store any kind of data. In Java you can

access this data via the AssetsManager and the method getAssets().

1.6. Activities and Layouts


5/67

The user interface for Activities is defined via layouts. Layouts are at runtime instances

of "android.view.ViewGroups". The layout defines the UI elements, their properties and

their arragement. UI elements are based on the class "android.view.View". ViewGroup is

a subclass of View A and a layout can contain UI components (Views) or other layouts

(ViewGroups). You should not nestle ViewGroups to deeply as this has a negativ impact

on performance.

A layout can be defined via Java code or via XML. You typically uses Java code to

generate the layout if you don't know the content until runtime; for example if your

layout depends on content which you read from the internet.

XML based layouts are defined via a resource file in the folder "/res/layout". This file

specifies the view groups, views, their relationship and their attributes for a specific

layout. If a UI element needs to be accessed via Java code you have to give the UI

element an unique id via the "android:id" attribute. To assign a new id to an UI element

use "@+id/yourvalue". By conversion this will create and assign a new id "yourvalue" to

the corresponding UI element. In your Java code you can later access these UI elements

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 to define different layouts

for different devices. You can also mix both approaches.

1.7. Activities and Lifecyle

The operating system controls the life cycle 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 life cycle for an activities via pre-defined methods. The most

important methods are:

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


6/67

The activity will also be restarted if a so called "configuration change" happens. A

configuration change for examples happens if the user changes the orientation of the

device (vertical or horizontal). The activity is in this case restarted to enable the Android

platform to load different resources for these configuration, e.g. layouts for vertical or

horizontal mode. In the emulator you can simulate the change of the orientation via

CNTR+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).

1.8. Context

The class android.content.Context provides the connections to the Android system. It is

the interface to global information about the application environment. Context also

provides the method which allows to receive Android services, e.g. theLocation Service.

As Activities and Services extend the class "Context" you can directly access the context

via "this".

2. Installation

The following assume that you have alreadyEclipse installed. For details please

seeEclipse Tutorial .

2.1. Android SDK
http://www.vogella.de/articles/AndroidLocationAPI/article.htmlhttp://www.vogella.de/articles/AndroidLocationAPI/article.htmlhttp://www.vogella.de/articles/AndroidLocationAPI/article.htmlhttp://www.vogella.de/articles/Eclipse/article.htmlhttp://www.vogella.de/articles/Eclipse/article.htmlhttp://www.vogella.de/articles/Eclipse/article.htmlhttp://www.vogella.de/articles/Eclipse/article.htmlhttp://www.vogella.de/articles/Eclipse/article.htmlhttp://www.vogella.de/articles/Eclipse/article.htmlhttp://www.vogella.de/articles/Eclipse/article.htmlhttp://www.vogella.de/articles/Eclipse/article.htmlhttp://www.vogella.de/articles/AndroidLocationAPI/article.html


7/67

Download the Android SDK from the Android homepage underAndroid 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". Avoid using spaces in the path name

otherwise you may experience problems later.

2.2. Eclipse

Use theEclipse update managerto install all available plugins for the Android

Development Tools (ADT) from the URLhttps://dl-ssl.google.com/android/eclipse/.

2.3. Configuration

In Eclipse open the Preferences dialog via Windows -> Preferences. Select Android and

enter the installation path of the Android SDK.

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

Select "Available packages" and open the "Third Party Add-ons". Select the Google API

10 (Android 2.3.4) version of the SDK. We are using the Google version as this includes

also the Google Maps which we might want to use later at a certain point.
http://developer.android.com/sdk/index.htmlhttp://developer.android.com/sdk/index.htmlhttp://developer.android.com/sdk/index.htmlhttp://www.vogella.de/articles/Eclipse/article.html#updatemanagerhttp://www.vogella.de/articles/Eclipse/article.html#updatemanagerhttp://www.vogella.de/articles/Eclipse/article.html#updatemanagerhttps://dl-ssl.google.com/android/eclipse/https://dl-ssl.google.com/android/eclipse/https://dl-ssl.google.com/android/eclipse/https://dl-ssl.google.com/android/eclipse/http://www.vogella.de/articles/Eclipse/article.html#updatemanagerhttp://developer.android.com/sdk/index.html


8/67

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

restart Eclipse.

2.4. Android Source Code

The following step is optional.

During Android development it is very useful to have the Android source code available

as Android uses a lot of defaults.

Haris Pecomaintains plugins which provides access to the Android Source code code.

Use the Eclipse update manager to install the Android Source plugin from the following

update site: "http://adt-

addons.googlecode.com/svn/trunk/source/com.android.ide.eclipse.source.update".

More details can be found on theproject website.

3. Create an Android Emulator Device
http://twitter.com/snpe60http://twitter.com/snpe60http://code.google.com/p/adt-addons/http://code.google.com/p/adt-addons/http://code.google.com/p/adt-addons/http://code.google.com/p/adt-addons/http://twitter.com/snpe60


9/67

The Android tools include an emulator. This emulator behaves like a real Android device

in most cases and allow you to test your application without having a real device. You

can emulate one or several devices with different configurations. Each configuration is

defined via an "Android Virtual Device" (AVD).

To define an AVD press the device manager button, press "New" and enter the following.


10/67

We will select the box "Enabled" for Snapshots. This will make the second start of the

virtual device much faster.

At the end press the button "Create AVD".This will create the device and display it under

the "Virtual devices". To test if your setup is correct, select your device and press "Start".

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


11/67

4. Error handling

Things are not always working as they should be. Several users report that get the

following errors:

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.

To solve this error select from the menu Project -> Clean.


12/67

If you having problems with your own code you can use the LogCat viewer as described

inLogCat Viewer .

5. Your first Android project

5.1. Create Project

This app is also available on the Android Marketplace. Search for "vogella" for find this

example.

Select File -> New -> Other -> Android -> Android Project and create the Android

project "de.vogella.android.temperature". Enter the following.
http://www.vogella.de/articles/Android/article.html#viewshttp://www.vogella.de/articles/Android/article.html#viewshttp://www.vogella.de/articles/Android/article.html#viewshttp://www.vogella.de/articles/Android/article.html#views


13/67


14/67

Press "Finish". This should create the following directory structure.

While "res" contains structured values which are known to the Android platform the

directory "assets" can be used to store any kind of data. In Java you can access this data

via the AssetsManager and the method getAssets().

5.2. Two faces of things

The Android SDK allows to define 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 both things by clicking on the


15/67

tab on the lower part of the screen. For example in the Package Explorer select

"res/layout/main.xml".

5.3. Create attributes

Android allows you 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 enter

"myColor" as the name and "#3399CC" as the value.


16/67

Add also the following "String" attributes. String attributes allow to translate the

application at a later point.

Table 1. String Attributes

Name ValuemyClickHandler myClickHandler

celsius to Celsius

fahrenheit to Fahrenheit

calc Calculate

Switch to the XML representation and validate the values.

Hello World, Convert!

Temperature Converter


17/67

#3399CC

myClickHandler

to Celsius

to Fahrenheit

Calculate

5.4. Add UI Elements

Select "res/layout/main.xml" and open the Android editor via a double-click. This editor

allows you to create the UI via drag and drop or via the XML source code. You canswitch 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.

The following shows a screenshot of the Palette view from which you can drag and drop

new UI elements into your layout. Please note that the "Palette" view changes frequently

so your view might be a bit different.


18/67

Right-click on the text object Hello World, Hello! in the layout. Select Delete on the

popup menu to remove the text object. Then, from the Palette view, select Text Fields

and locate Plain Text. Drag this onto the layout to create a text input field. All object

types in the section "Text Fields derive from the class "EditText", they just specify via

an additional attribute which text type can be used.

Now select the Palette section Form Widgets and drag a RadioGroup object onto 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.

From the Palette section Form Widgets, drag a Button object onto the layout.

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.


19/67

Switch to "main.xml" and verify that your XML looks like the following.


20/67

android:layout_width="wrap_content"

android:id="@+id/radio1"

android:layout_height="wrap_content">

5.5. Edit UI properties

If you select a UI element you can change its properties via the properties view. Most of

the properties can also be changed via the right mouse menu. Select the EditText field,

right mouse click on it, select Properties-> Text and delete the content. This means no

text will be initially shown in the text field.


21/67

Assign the "celsius" string attribute to your "text" property of the first radio button and

"fahrenheit" to the second.


22/67


23/67

From now on I assume you are able to use the properties menu on the UI elements. Set

the property "Checked" to true for the first RadioButton. Assign "calc" to the text

property of your button and assign "myClickHandler" to the "onClick" property. Set the

"Input type" property to "numberSigned" and "numberDecimal" on your EditText.

Right-click on the view in Graphical Layout mode, then select

Properties/Background... from the popup menu. Select Color and then myColor

in the list.


24/67

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


25/67

android:id="@+id/radio1"

android:layout_height="wrap_content"

android:text="@string/fahrenheit">

5.6. Code your application

Change your code in "Convert.java" to the following. Note that the "myClickHandler"

will be called based on 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;

publicclass Convert extends Activity {

private EditText text;


26/67

@Override

publicvoid onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(R.layout.main);

text = (EditText) findViewById(R.id.editText1);

}

// This method is called at button click because we assigned the name

to the

// "On Click property" of the button

publicvoid myClickHandler(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


27/67

.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

privatefloat convertFahrenheitToCelsius(float fahrenheit) {

return ((fahrenheit - 32) * 5 / 9);

}

// Converts to fahrenheit

privatefloat convertCelsiusToFahrenheit(float celsius) {

return ((celsius * 9) / 5) + 32;

}

}

5.7. Start Project


28/67

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.

5.8. Using the home menu

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


29/67


30/67

6. Menus

6.1. Menus

To use menusAndroidprovides two ways. First is the option menu which can be opened

via the menu button. The option menu of your activity is filled in the method

onCreateOptionsMenu() of your activity. You can register here a menu via your code or

use a XML menu resources which you inflate via a "MenuInflator". You get a

MenuInflator via your activity with the method getMenuInflator().

onCreateContextMenu() is only called once. If you want to influence the menu later you

have to use the method onPrepareOptionsMenu().
http://www.vogella.de/articles/Android/article.htmlhttp://www.vogella.de/articles/Android/article.htmlhttp://www.vogella.de/articles/Android/article.htmlhttp://www.vogella.de/articles/Android/article.html


31/67

The second option to display a menu is to use the context menu for a UI widget (view). A

context menu is activated if the user "long presses" the view.

A context menu for a view is registered via the method registerForContextMenu(view).

The method onCreateContextMenu() is called every time a context menu is activated asthe context menu is discarded after its usage. The Android platform may also add

options to your view, e.g. "EditText" provides context options to select text, etc.

6.2. Project

This chapter will demonstrate how to create and evaluate a option menu, how to define

preferences and how to navigate between activities via anintent . Create a project

"de.vogella.android.preferences" with the activity "HelloPreferences". Change the UI in

the file "/res/layout/main.xml" to the following:

6.3. Add a menu XML resource
http://www.vogella.de/articles/AndroidIntent/article.htmlhttp://www.vogella.de/articles/AndroidIntent/article.htmlhttp://www.vogella.de/articles/AndroidIntent/article.htmlhttp://www.vogella.de/articles/AndroidIntent/article.html


32/67

Menus can be defined via XML files. Select your project, right click on it and select New

-> Other -> Android -> "Android XML File". Select the option "Menu", enter as File

"menu.xml" and press the button "Finish".

Press Add and select "Item". Maintain the following value. This defines the entries in

your menu. We will have only one entry.


33/67

Change your class "HelloPreferences" to the following. The OnCreateOptionsMenu

method is used to create the menu. The behavior in "onOptionsItemSelected" is

currently hard-coded to show a Toast and will soon call the preference settings. In case

you want to disable or hide menu items you can use the method

"onPrepareOptionsMenu" which is called every time the menu is called.

package de.vogella.android.preferences;



import android.view.Menu;

import android.view.MenuInflater;

import android.view.MenuItem;



34/67

publicclass HelloPreferences extends Activity {

/** Called when the activity is first created. */

@Override




}

@Override

publicboolean onCreateOptionsMenu(Menu menu) {

MenuInflater inflater = getMenuInflater();

inflater.inflate(R.menu.menu, menu);

return true;

}

@Override

publicboolean onOptionsItemSelected(MenuItem item) {

Toast.makeText(this, "Just a test",

Toast.LENGTH_SHORT).show();

return true;

}

}


35/67

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

displayed. If you select the menu entry you should see a small info message.

The two "Preference" buttons are not yet active. We will use them in the next chapter.

7. Preferences and Intents

7.1. Overview

Preferences allow you to save data for your application. Preferences are stored as key

values.Intentsallow you to start Activities from other Activities.

7.2. Using preferences
http://www.vogella.de/articles/AndroidIntent/article.htmlhttp://www.vogella.de/articles/AndroidIntent/article.htmlhttp://www.vogella.de/articles/AndroidIntent/article.htmlhttp://www.vogella.de/articles/AndroidIntent/article.html


36/67

We will continue using the example project "de.vogella.android.preferences" from the

last chapter.

Preference values can also be stored as a XML resource. Create another Android XML

File "preferences.xml" this time of type "Preference".


37/67

Press Add, add a category and add two preferences "EditTextPreferences" to this

category : "User" and "Password".

To allow the user to enter the preference value you can define a Activity with extends

PreferenceActivity. This activity can load a preference definition resources via the


38/67

method addPreferencesFromResource(). Create the class "Preferences" which will load

the "preference.xml".



import android.preference.PreferenceActivity;

publicclass Preferences extends PreferenceActivity {


@Override



addPreferencesFromResource(R.xml.preferences);

}

}

To make this class available as an activity for Android you need to register it in your

"AndroidManifest.xml" file. Select "AndroidManifest.xml" and the tab "Application".

Add the activity "Preferences".


39/67

The first button will show the current values of the preferences via a Toast and the

second button will revert the maintained user name to demonstrate how you couldchange the preferences via code.



40/67


import android.content.Intent;

import android.content.SharedPreferences;

import android.content.SharedPreferences.Editor;


import android.preference.PreferenceManager;

import android.view.Menu;

import android.view.MenuInflater;

import android.view.MenuItem;


import android.view.View.OnClickListener;

import android.widget.Button;


publicclass HelloPreferences extends Activity {

SharedPreferences preferences;


@Override




Button button = (Button) findViewById(R.id.Button01);

// Initialize preferences


41/67

preferences =PreferenceManager.getDefaultSharedPreferences(this);

button.setOnClickListener(new OnClickListener() {

publicvoid onClick(View v) {

String username =

preferences.getString("username", "n/a");

String password =

preferences.getString("password", "n/a");

Toast.makeText(

HelloPreferences.this,

"You entered user: " + username+ " and password: "

+ password,


}

});

Button buttonChangePreferences = (Button)

findViewById(R.id.Button02);

buttonChangePreferences.setOnClickListener(new

OnClickListener() {

publicvoid onClick(View v) {

Editor edit = preferences.edit();

String username =

preferences.getString("username", "n/a");

// We will just revert the current user name

and save again

StringBuffer buffer = new StringBuffer();

for (int i = username.length() - 1; i >= 0; i--

) {


42/67

buffer.append(username.charAt(i));

}

edit.putString("username", buffer.toString());

edit.commit();

// A toast is a view containing a quick little

message for the

// user. We give a little feedback

Toast.makeText(HelloPreferences.this,

"Reverted string sequence of

user name.",


}

});

}

}

We will update the method onOptionsItemSelected() to open the activity "Preferences"

once you select the option menu. Even though we currently have only one option in our

menu we use a switch to be ready for several new menu entries. To see the current

values of the preferences we define a button and use the class "PreferenceManager" to

get the sharedPreferences.

@Override

publicboolean onCreateOptionsMenu(Menu menu) {

MenuInflater inflater = getMenuInflater();

inflater.inflate(R.menu.menu, menu);

return true;


43/67

}

// This method is called once the menu is selected

@Override

publicboolean 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);

// Some feedback to the user

Toast.makeText(HelloPreferences.this,

"Here you can enter your user

credentials.",


break;

}

return true;

}

7.3. Run

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. The saved values should be displayed in


44/67

a small message windows (Toast) if you press your first button. If you press the second

button the username should be reversed.

8. Dialogs via the AlertDialog

We have already used a "Toast" which is a small message window which does not take

the focus. In this chapter we will use the class "AlertDialog". AlertDialog is used to open

a dialog from our activity. This modal dialog gets the focus until the user closes it.

An instance of this class can be created by the builder pattern, e.g. you can chain your

method calls.


45/67

You should always open a dialog from the class onCreateDialog(int) as the Android

system manages the dialog in this case for you. This method is automatically called by

Android if you call showDialog(int).

Create a new Android project "de.vogella.android.alertdialog" with the activity"ShowMyDialog". Maintain the following layout for "main.xml".

Change the code of your activity to the following.

package de.vogella.android.alertdialog;


import android.app.AlertDialog;

import android.app.AlertDialog.Builder;

import android.app.Dialog;

import android.content.DialogInterface;


46/67




publicclass ShowMyDialog extends Activity {


@Override




}

publicvoid openMyDialog(View view) {

showDialog(10);

}

@Override

protected Dialog onCreateDialog(int id) {

switch (id) {

case 10:

// Create out AlterDialog

Builder builder = new AlertDialog.Builder(this);

builder.setMessage("This will end the activity");

builder.setCancelable(true);


47/67

builder.setPositiveButton("I agree", new

DialogInterface.OnClickListener() {

publicvoid onClick(DialogInterface dialog, int which) {

ShowMyDialog.this.finish();

}

});

builder.setNegativeButton("No, no", new

DialogInterface.OnClickListener() {

publicvoid onClick(DialogInterface dialog, int which) {

Toast.makeText(getApplicationContext(),"Activity will

continue",Toast.LENGTH_LONG).show();

}

});

AlertDialog dialog = builder.create();

dialog.show();

}

returnsuper.onCreateDialog(id);

}

}

If you run your application and click your button you should see your dialog.


48/67

More on dialogs can be found onAndroid Dialogs standard documentation.

9. TableLayout

9.1. Overview

In earlier chapter we have used the LinearLayout which allows you to stack widgets

vertical or horizontal. LinearLayout can be nested to achieve nice effects. This chapter

will demonstrate the usage of "TableLayout".

This layout allows you to organize a view into a table format. You specify via the view

group "TableRow" rows for your table. Afterwards you put widgets into the individual

rows.
http://developer.android.com/guide/topics/ui/dialogs.htmlhttp://developer.android.com/guide/topics/ui/dialogs.htmlhttp://developer.android.com/guide/topics/ui/dialogs.htmlhttp://developer.android.com/guide/topics/ui/dialogs.htmlhttp://developer.android.com/guide/topics/ui/dialogs.html


49/67

On the "TableLayout" you can define which column should take additional space via the

"android:stretchColumns" attribute. If several columns should take the available space

you can specify them as a comma-separated list. Similar you can use the attribute

"android:shrinkColumn", which will try to word-wrap the content of the specified

widgets and the attribute "android:collapseColums" to define initially hidden columns.

Via Java you can display / hide these columns via the method setColumnVisible().

Columns will be automatically created based on the maximum number of widgets in one

row. Per default each widgets creates a new column in the row. You can specific via

"android:layout_column" the column a widget should go and via "android:layout_span"

how many columns a widget should take.

You can also put non TableRows in a table. This way you can for example add dividers

between your columns.

9.2. Example

Create the project "de.vogella.android.layout.table" with the activity

"DemoTableLayout". Change "main.xml" to the following.


50/67

android:layout_width="wrap_content"

android:layout_height="wrap_content">


51/67

Change the activity "DemoTableLayout" to the following to use the button to hide the

second column in the table.

package de.vogella.android.layout.table;




importandroid.widget.Button;

import android.widget.TableLayout;

publicclass DemoTableLayout extends Activity {

private TableLayout layout;

private Button button;


@Override




layout = (TableLayout) findViewById(R.id.tableLayout1);

button = (Button) findViewById(R.id.collapse);

}


52/67

publicvoid toogleHiddenRows(View view) {

// Second row has index 1

layout.setColumnCollapsed(1, !layout.isColumnCollapsed(1));

if (layout.isColumnCollapsed(1)) {

button.setText("Show second column");

} else {

button.setText("Hide second column");

}

}

}

10. ContentProvider

10.1. Overview

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".

10.2. Create contacts on your emulator

For this example we need a few maintained contacts. Select the home menu and then

the menu entry "Contacts" to create contacts.


53/67

Press Menu and select "New Contact".


54/67

As a result you should have a few new contacts.


55/67

10.3. Using the Contact Content Provider

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

"ContactsView".

Rename the id of 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.


56/67

android:layout_height="fill_parent">

Access to the contact content provider require a certain permission as not all

applications should have access to the contact information. Open the

AndroidManifest.xml, and select the Permissions tab. On that tab click the "Add" button,

and select "Uses Permission". From the drop-down list select the entry

"android.permission.READ_CONTACTS".

Change the coding of the activity.

package de.vogella.android.contentprovider;


import android.database.Cursor;

import android.net.Uri;


import android.provider.ContactsContract;

import android.widget.TextView;

publicclass ContactsView extends Activity {


@Override


57/67




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;


58/67

String sortOrder = ContactsContract.Contacts.DISPLAY_NAME

+ " COLLATE LOCALIZED ASC";

return managedQuery(uri, projection, selection, selectionArgs,

sortOrder);

}

}

Typically you would display such data in aListView. Please see theListView Tutorialfordetails.

11. ScrollView

ScrollViews can be used to contain one view that might be to big to fit on one screen. If

the view is to big the ScrollView will display a scroll bar to scroll the context. Of course

this view can be a layout which can then contain other elements.

Create an android project "de.vogella.android.scrollview" with the activity "ScrollView".

Create the following layout and class.


59/67

android:text="This is a header"

android:textAppearance="?android:attr/textAppearanceLarge"

android:paddingLeft="8dip" android:paddingRight="8dip"

android:paddingTop="8dip">

package de.vogella.android.scrollview;




importandroid.widget.TextView;

publicclass ScrollView extends Activity {



60/67

@Override




TextView view = (TextView) findViewById(R.id.TextView02);

String s="";

for (int i=0; i < 100; i++) {

s += "vogella.de ";

}

view.setText(s);

}

}

The attribute "android:fillViewport="true"" ensures that the the scrollview is set to the

full screen even if the elements are smaller then one screen and the "layout_weight" tell

the android system that these elements should be extended.


61/67

12. DDMS perspective and important views

12.1. DDMS - Dalvik Debug Monitor Server

Eclipse provides aperspectivefor interacting with your device and program.Open it to

see the possible options. This perspective includes the following views which can also be

used independently and allows to place calls and send SMS to the device. It also allow to

set the current geo position and toperform a performance traceof your application.

12.2. LogCat View

You can see the log (including System.out.print() statements) via the LogCat view.
http://www.vogella.de/articles/Eclipse/article.html#perspectivehttp://www.vogella.de/articles/Eclipse/article.html#perspectivehttp://www.vogella.de/articles/Eclipse/article.html#perspectivehttp://www.vogella.de/articles/Eclipse/article.html#perspectivehttp://www.vogella.de/articles/Eclipse/article.html#perspectivehttp://www.vogella.de/articles/Eclipse/article.html#perspectivehttp://www.vogella.de/articles/AndroidPerformance/article.html#traceviewhttp://www.vogella.de/articles/AndroidPerformance/article.html#traceviewhttp://www.vogella.de/articles/AndroidPerformance/article.html#traceviewhttp://www.vogella.de/articles/AndroidPerformance/article.html#traceviewhttp://www.vogella.de/articles/Eclipse/article.html#perspectivehttp://www.vogella.de/articles/Eclipse/article.html#perspective


62/67

12.3. File explorer

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

13. Shell


63/67

13.1. Android Debugging Bridge - Shell

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

You can also copy file from and to your device via the following commands.

// Assume the gesture file exists on your Android device

adb pull /sdcard/gestures ~/test

// Now copy it back

adb push ~/test/gesture /sdcard/gestures2

This will connect you to your device and give youLinux 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
http://www.vogella.de/articles/Ubuntu/article.htmlhttp://www.vogella.de/articles/Ubuntu/article.htmlhttp://www.vogella.de/articles/Ubuntu/article.html


64/67

# Issue a command to a specific device

adb -s emulator-5554 shell

13.2. Uninstall an application via adb

You can uninstall an android application via the shell. Switch the the data/app directory

(cd /data/app) and simply delete your android application.

13.3. Emulator Console via telnet

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

geo fix 48 51


65/67

For more information on the emulator console please seeEmulator Console manual

14. Deploy your application on a real device

Turn on "USB Debugging" on your device in the settings. Select in the settings

Applications > Development, then enable USB debugging. You also need to install the

driver for your mobile phone. For details please seeDeveloping on a Device . Please note

that the Android version you are developing for must be the installed version on your

phone.

To select your phone, select the "Run Configurations", select "Manual" selection and

select your device.

15. Thank you

Please help me to support this article:
http://developer.android.com/guide/developing/tools/emulator.html#consolehttp://developer.android.com/guide/developing/tools/emulator.html#consolehttp://developer.android.com/guide/developing/tools/emulator.html#consolehttp://developer.android.com/guide/developing/device.htmlhttp://developer.android.com/guide/developing/device.htmlhttp://developer.android.com/guide/developing/device.htmlhttp://developer.android.com/guide/developing/device.htmlhttp://developer.android.com/guide/developing/tools/emulator.html#console


66/67

16. Questions and Discussion

Before posting questions, please see thevogella FAQ. If you have questions or find an

error in this article please use thewww.vogella.de Google Group. I have created a short

listhow to create good questionswhich might also help you.

17. Links and Literature

17.1. Source Code

Source Code of Examples

17.2. Android Resources

Android 2 (German Book)

Android ListView and ListActivity

Android Location API and Google Maps

Android Intents

Android and Networking

Android Homepage

Android Developer Homepage

Android Issues / Bugs

Android Google Groups
http://www.vogella.de/faq.htmlhttp://www.vogella.de/faq.htmlhttp://www.vogella.de/faq.htmlhttp://groups.google.com/group/vogellahttp://groups.google.com/group/vogellahttp://groups.google.com/group/vogellahttp://www.vogella.de/blog/2010/03/09/asking-community-questions/http://www.vogella.de/blog/2010/03/09/asking-community-questions/http://www.vogella.de/code.htmlhttp://www.vogella.de/code.htmlhttp://dpunkt.de/s/androidhttp://dpunkt.de/s/androidhttp://www.vogella.de/articles/AndroidListView/article.htmlhttp://www.vogella.de/articles/AndroidListView/article.htmlhttp://www.vogella.de/articles/AndroidLocationAPI/article.htmlhttp://www.vogella.de/articles/AndroidLocationAPI/article.htmlhttp://www.vogella.de/articles/AndroidIntent/article.htmlhttp://www.vogella.de/articles/AndroidIntent/article.htmlhttp://www.vogella.de/articles/AndroidNetworking/article.htmlhttp://www.vogella.de/articles/AndroidNetworking/article.htmlhttp://code.google.com/intl/de-DE/android/http://code.google.com/intl/de-DE/android/http://developer.android.com/http://developer.android.com/http://code.google.com/p/android/issues/listhttp://code.google.com/p/android/issues/listhttp://groups.google.com/group/android-developershttp://groups.google.com/group/android-developershttp://flattr.com/thing/49772/Tutorials-for-Eclipse-Java-Android-and-Webprogramminghttp://flattr.com/thing/49772/Tutorials-for-Eclipse-Java-Android-and-Webprogramminghttp://groups.google.com/group/android-developershttp://code.google.com/p/android/issues/listhttp://developer.android.com/http://code.google.com/intl/de-DE/android/http://www.vogella.de/articles/AndroidNetworking/article.htmlhttp://www.vogella.de/articles/AndroidIntent/article.htmlhttp://www.vogella.de/articles/AndroidLocationAPI/article.htmlhttp://www.vogella.de/articles/AndroidListView/article.htmlhttp://dpunkt.de/s/androidhttp://www.vogella.de/code.htmlhttp://www.vogella.de/blog/2010/03/09/asking-community-questions/http://groups.google.com/group/vogellahttp://www.vogella.de/faq.html


67/67

Android Assets Studio

17.3. vogella Resources

Eclipse RCP Training (German) Eclipse RCP Training with Lars Vogel

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
http://android-ui-utils.googlecode.com/hg/asset-studio/dist/index.htmlhttp://android-ui-utils.googlecode.com/hg/asset-studio/dist/index.htmlhttp://www.vogella.de/training/eclipsercp.htmlhttp://www.vogella.de/training/eclipsercp.htmlhttp://www.vogella.de/articles/Android/article.htmlhttp://www.vogella.de/articles/Android/article.htmlhttp://www.vogella.de/articles/GWT/article.htmlhttp://www.vogella.de/articles/EclipseRCP/article.htmlhttp://www.vogella.de/articles/JUnit/article.htmlhttp://www.vogella.de/articles/Git/article.htmlhttp://www.vogella.de/articles/Git/article.htmlhttp://www.vogella.de/articles/JUnit/article.htmlhttp://www.vogella.de/articles/EclipseRCP/article.htmlhttp://www.vogella.de/articles/GWT/article.htmlhttp://www.vogella.de/articles/Android/article.htmlhttp://www.vogella.de/training/eclipsercp.htmlhttp://android-ui-utils.googlecode.com/hg/asset-studio/dist/index.html

Documents

How to Develop Android Apps