Android &-8211; Useful Resources

The following resources contain additional information on Android. Please use them to get more in-depth knowledge on this topic.

Useful Links on Android

• Android Developers − Official Android website giving complete detail about latest Android releases, their features, APIs used by the Android Developers.

• Google Play &-8211; Developer Console − The biggest Android Marketplace to buy and sell Android Applications.

• Download latest Android SDK − The Android SDK provides you the API libraries and developer tools necessary to build, test, and debug apps for Android.

• Eclipse Download − One of the best IDE to develop Android Applications.

• Free Java Download − Download Java for your desktop computer now!

Useful Books on Android

To enlist your site on this page, please drop an email to [email protected]

Android Questions and Answers

Android Questions and Answers has been designed with a special intention of helping students and professionals preparing for various Certification Exams and Job Interviews. This section provides a useful collection of sample Interview Questions and Multiple Choice Questions (MCQs) and their answers with appropriate explanations.

Android &-8211; XML Parser

XML stands for Extensible Mark-up Language.XML is a very popular format and commonly used for sharing data on the internet. This chapter explains how to parse the XML file and extract necessary information from it.

Android provides three types of XML parsers which are DOM,SAX and XMLPullParser. Among all of them android recommend XMLPullParser because it is efficient and easy to use. So we are going to use XMLPullParser for parsing XML.

The first step is to identify the fields in the XML data in which you are interested in. For example. In the XML given below we interested in getting temperature only.

?xml version="1.0"?>
current>
   
   city id="2643743" name="London">
      coord lon="-0.12574" lat="51.50853"/>
      country>GB/country>
      sun rise="2013-10-08T06:13:56" set="2013-10-08T17:21:45"/>
   /city>
   
   temperature value="289.54" min="289.15" max="290.15" unit="kelvin"/>
   humidity value="77" unit="%"/>
   pressure value="1025" unit="hPa"/>
/current>

XML &-8211; Elements

An xml file consist of many components. Here is the table defining the components of an XML file and their description.

XML &-8211; Parsing

In the next step, we will create XMLPullParser object , but in order to create that we will first create XmlPullParserFactory object and then call its newPullParser() method to create XMLPullParser. Its syntax is given below −

private XmlPullParserFactory xmlFactoryObject = XmlPullParserFactory.newInstance();
private XmlPullParser myparser = xmlFactoryObject.newPullParser();

The next step involves specifying the file for XmlPullParser that contains XML. It could be a file or could be a Stream. In our case it is a stream.Its syntax is given below −

myparser.setInput(stream, null);

The last step is to parse the XML. An XML file consist of events, Name, Text, AttributesValue e.t.c. So XMLPullParser has a separate function for parsing each of the component of XML file. Its syntax is given below −

int event = myParser.getEventType();
while (event != XmlPullParser.END_DOCUMENT)  {
   String name=myParser.getName();
   switch (event){
      case XmlPullParser.START_TAG:
      break;
      
      case XmlPullParser.END_TAG:
      if(name.equals("temperature")){
         temperature = myParser.getAttributeValue(null,"value");
      }
      break;
   }		 
   event = myParser.next(); 					
}

The method getEventType returns the type of event that happens. e.g: Document start , tag start e.t.c. The method getName returns the name of the tag and since we are only interested in temperature , so we just check in conditional statement that if we got a temperature tag , we call the method getAttributeValue to return us the value of temperature tag.

Apart from the these methods, there are other methods provided by this class for better parsing XML files. These methods are listed below −

Example

Here is an example demonstrating the use of XML DOM Parser. It creates a basic application that allows you to parse XML.

To experiment with this example, you can run this on an actual device or in an emulator.

Following is the content of the modified main activity file MainActivity.java.

package com.example.sairamkrishna.myapplication;

import java.io.InputStream;
import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import org.w3c.dom.Document;
import org.w3c.dom.Element;
import org.w3c.dom.Node;
import org.w3c.dom.NodeList;
import android.app.Activity;
import android.os.Bundle;
import android.widget.TextView;

public class MainActivity extends Activity {
   TextView tv1;

   @Override
   public void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main);
      tv1=(TextView)findViewById(R.id.textView1);
		
      try {
         InputStream is = getAssets().open("file.xml");

         DocumentBuilderFactory dbFactory = DocumentBuilderFactory.newInstance();
         DocumentBuilder dBuilder = dbFactory.newDocumentBuilder();
         Document doc = dBuilder.parse(is);

         Element element=doc.getDocumentElement();
         element.normalize();

         NodeList nList = doc.getElementsByTagName("employee");
			
         for (int i=0; inList.getLength(); i++) {

            Node node = nList.item(i);
            if (node.getNodeType() == Node.ELEMENT_NODE) {
               Element element2 = (Element) node;
               tv1.setText(tv1.getText()+"nName : " + getValue("name", element2)+"n");
               tv1.setText(tv1.getText()+"Surname : " + getValue("surname", element2)+"n");
               tv1.setText(tv1.getText()+"-----------------------");
            }
         }
			
      } catch (Exception e) {e.printStackTrace();}

   }
	
   private static String getValue(String tag, Element element) {
      NodeList nodeList = element.getElementsByTagName(tag).item(0).getChildNodes();
      Node node = nodeList.item(0);
      return node.getNodeValue();
   }

}

Following is the content of Assets/file.xml.

?xml version="1.0"?>
records>
   employee>
      name>Sairamkrishna/name>
      surname>Mammahe/surname>
      salary>50000/salary>
   /employee>
	
   employee>
      name>Gopal /name>
      surname>Varma/surname>
      salary>60000/salary>
   /employee>
	
   employee>
      name>Raja/name>
      surname>Hr/surname>
      salary>70000/salary>
   /employee>
	
/records>

Following is the modified content of the xml res/layout/activity_main.xml.

?xml version="1.0" encoding="utf-8"?>
RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
   xmlns:tools="http://schemas.android.com/tools"
   android:layout_width="match_parent"
   android:layout_height="match_parent"
   android:paddingBottom="@dimen/activity_vertical_margin"
   android:paddingLeft="@dimen/activity_horizontal_margin"
   android:paddingRight="@dimen/activity_horizontal_margin"
   android:paddingTop="@dimen/activity_vertical_margin"
   tools:context=".MainActivity">

   TextView
      android:id="@+id/textView1"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content" />
/RelativeLayout>

Following is the content of AndroidManifest.xml file.

?xml version="1.0" encoding="utf-8"?>
manifest xmlns:android="http://schemas.android.com/apk/res/android"
   package="com.example.sairamkrishna.myapplication" >
   application
      android:allowBackup="true"
      android:icon="@mipmap/ic_launcher"
      android:label="@string/app_name"
      android:theme="@style/AppTheme" >
      
      activity
         android:name=".MainActivity"
         android:label="@string/app_name" >
         
         intent-filter>
            action android:name="android.intent.action.MAIN" />
            category android:name="android.intent.category.LAUNCHER" />
         /intent-filter>
      
      /activity>
   
   /application>
/manifest>

Let&-8217;s try to run our application we just modified. I assume you had created your AVD while doing environment setup. To run the app from Android studio, open one of your project&-8217;s activity files and click Run icon from the toolbar. Android studio installs the app on your AVD and starts it and if everything is fine with your setup and application, it will display following Emulator window −

Android &-8211; Widgets

A widget is a small gadget or control of your android application placed on the home screen. Widgets can be very handy as they allow you to put your favourite applications on your home screen in order to quickly access them. You have probably seen some common widgets, such as music widget, weather widget, clock widget e.t.c

Widgets could be of many types such as information widgets, collection widgets, control widgets and hybrid widgets. Android provides us a complete framework to develop our own widgets.

Widget &-8211; XML file

In order to create an application widget, first thing you need is AppWidgetProviderInfo object, which you will define in a separate widget XML file. In order to do that, right click on your project and create a new folder called xml. Now right click on the newly created folder and create a new XML file. The resource type of the XML file should be set to AppWidgetProvider. In the xml file, define some properties which are as follows −

appwidget-provider 
   xmlns:android="http://schemas.android.com/apk/res/android" 
   android:minWidth="146dp" 
   android:updatePeriodMillis="0" 
   android:minHeight="146dp" 
   android:initialLayout="@layout/activity_main">
/appwidget-provider>

Widget &-8211; Layout file

Now you have to define the layout of your widget in your default XML file. You can drag components to generate auto xml.

Widget &-8211; Java file

After defining layout, now create a new JAVA file or use existing one, and extend it with AppWidgetProvider class and override its update method as follows.

In the update method, you have to define the object of two classes which are PendingIntent and RemoteViews. Its syntax is −

PendingIntent pending = PendingIntent.getActivity(context, 0, intent, 0);
RemoteViews views = new RemoteViews(context.getPackageName(), R.layout.activity_main);

In the end you have to call an update method updateAppWidget() of the AppWidgetManager class. Its syntax is −

appWidgetManager.updateAppWidget(currentWidgetId,views);		

A part from the updateAppWidget method, there are other methods defined in this class to manipulate widgets. They are as follows −

Widget &-8211; Manifest file

You also have to declare the AppWidgetProvider class in your manifest file as follows:

receiver android:name="ExampleAppWidgetProvider" >
   
   intent-filter>
      action android:name="android.appwidget.action.APPWIDGET_UPDATE" />
   /intent-filter>
   
   meta-data android:name="android.appwidget.provider"
      android:resource="@xml/example_appwidget_info" />
/receiver>

Example

Here is an example demonstrating the use of application Widget. It creates a basic widget applications that will open this current website in the browser.

To experiment with this example, you need to run this on an actual device on which internet is running.

Following is the content of the modified MainActivity.java.

package com.example.sairamkrishna.myapplication;

import android.app.PendingIntent;
import android.appwidget.AppWidgetManager;
import android.appwidget.AppWidgetProvider;
import android.content.Context;
import android.content.Intent;
import android.net.Uri;
import android.widget.RemoteViews;
import android.widget.Toast;

public class MainActivity extends AppWidgetProvider{
   public void onUpdate(Context context, AppWidgetManager appWidgetManager,int[] appWidgetIds) {
      for(int i=0; iappWidgetIds.length; i++){
         int currentWidgetId = appWidgetIds[i];
         String url = "http://webforums.club";
         
         Intent intent = new Intent(Intent.ACTION_VIEW);
         intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
         intent.setData(Uri.parse(url));
         
         PendingIntent pending = PendingIntent.getActivity(context, 0,intent, 0);
         RemoteViews views = new RemoteViews(context.getPackageName(),R.layout.activity_main);
         
         views.setOnClickPendingIntent(R.id.button, pending);
         appWidgetManager.updateAppWidget(currentWidgetId,views);
         Toast.makeText(context, "widget added", Toast.LENGTH_SHORT).show();
      }
   }
}

Following is the modified content of the xml res/layout/activity_main.xml.

?xml version="1.0" encoding="utf-8"?>
RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
   xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
   android:layout_height="match_parent" android:paddingLeft="@dimen/activity_horizontal_margin"
   android:paddingRight="@dimen/activity_horizontal_margin"
   android:paddingTop="@dimen/activity_vertical_margin"
   android:paddingBottom="@dimen/activity_vertical_margin"
   tools:context=".MainActivity"
   android:transitionGroup="true">
   
   TextView
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:text="Tutorials point"
      android:id="@+id/textView"
      android:layout_centerHorizontal="true"
      android:textColor="-ff3412ff"
      android:textSize="35dp" />
      
   Button
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:text="Widget"
      android:id="@+id/button"
      android:layout_centerHorizontal="true"
      android:layout_marginTop="61dp"
      android:layout_below="@+id/textView" />

/RelativeLayout>

Following is the content of the res/xml/mywidget.xml.

?xml version="1.0" encoding="utf-8"?>
appwidget-provider 
   xmlns:android="http://schemas.android.com/apk/res/android" 
   android:minWidth="146dp" 
   android:updatePeriodMillis="0" 
   android:minHeight="146dp" 
   android:initialLayout="@layout/activity_main">
/appwidget-provider>

Following is the content of the res/values/string.xml.

resources>
   string name="app_name">My Application/string>
/resources>

Following is the content of AndroidManifest.xml file.

?xml version="1.0" encoding="utf-8"?>
manifest xmlns:android="http://schemas.android.com/apk/res/android"
   package="com.example.sairamkrishna.myapplication" >
   
   application
      android:allowBackup="true"
      android:icon="@mipmap/ic_launcher"
      android:label="@string/app_name"
      android:theme="@style/AppTheme" >
      receiver android:name=".MainActivity">
      
      intent-filter>
         action android:name="android.appwidget.action.APPWIDGET_UPDATE">/action>
      /intent-filter>
      
      meta-data android:name="android.appwidget.provider"
         android:resource="@xml/mywidget">/meta-data>
      
      /receiver>
   
   /application>
/manifest>

Let&-8217;s try to run your application. I assume you have connected your actual Android Mobile device with your computer. To run the app from Android studio, open one of your project&-8217;s activity files and click Run icon from the tool bar. Before starting your application, Android studio will display following window to select an option where you want to run your Android application.

Select your mobile device as an option and then check your mobile device which will display your default screen −

Go to your widget section and add your created widget to the desktop or home screen. It would look something like this −

Now just tap on the widget button that appears, to launch the browser. But before that please make sure that you are connected to the internet. After pressing the button , the following screen would appear −

Note. By just changing the url in the java file, your widget will open your desired website in the browser.

Android &-8211; Wi-Fi

Android allows applications to access to view the access the state of the wireless connections at very low level. Application can access almost all the information of a wifi connection.

The information that an application can access includes connected network&-8217;s link speed,IP address, negotiation state, other networks information. Applications can also scan, add, save, terminate and initiate Wi-Fi connections.

Android provides WifiManager API to manage all aspects of WIFI connectivity. We can instantiate this class by calling getSystemService method. Its syntax is given below −

WifiManager mainWifiObj;
mainWifiObj = (WifiManager) getSystemService(Context.WIFI_SERVICE); 

In order to scan a list of wireless networks, you also need to register your BroadcastReceiver. It can be registered using registerReceiver method with argument of your receiver class object. Its syntax is given below −

class WifiScanReceiver extends BroadcastReceiver {
   public void onReceive(Context c, Intent intent) {
   }
}

WifiScanReceiver wifiReciever = new WifiScanReceiver();
registerReceiver(wifiReciever, new IntentFilter(WifiManager.SCAN_RESULTS_AVAILABLE_ACTION));  

The wifi scan can be start by calling the startScan method of the WifiManager class. This method returns a list of ScanResult objects. You can access any object by calling the get method of list. Its syntax is given below −

ListScanResult> wifiScanList = mainWifiObj.getScanResults();
String data = wifiScanList.get(0).toString();

Apart from just Scanning, you can have more control over your WIFI by using the methods defined in WifiManager class. They are listed as follows −

Example

Here is an example demonstrating the use of WIFI. It creates a basic application that open your wifi and close your wifi

To experiment with this example, you need to run this on an actual device on which wifi is turned on.

Following is the content of the modified main activity file src/MainActivity.java.

package com.example.sairamkrishna.myapplication;

import android.net.wifi.WifiManager;
import android.os.Bundle;
import android.app.Activity;
import android.content.Context;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;

public class MainActivity extends Activity {
   Button enableButton,disableButton;
   @Override
   protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main);

      enableButton=(Button)findViewById(R.id.button1);
      disableButton=(Button)findViewById(R.id.button2);

      enableButton.setOnClickListener(new OnClickListener(){
         public void onClick(View v){
            WifiManager wifi = (WifiManager) getSystemService(Context.WIFI_SERVICE);
            wifi.setWifiEnabled(true);
         }
      });
		
      disableButton.setOnClickListener(new OnClickListener(){
         public void onClick(View v){
            WifiManager wifi = (WifiManager) getSystemService(Context.WIFI_SERVICE);
            wifi.setWifiEnabled(false);
         }
      });
   }
}

Following is the modified content of the xml res/layout/activity_main.xml.

?xml version="1.0" encoding="utf-8"?>
RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
   xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
   android:layout_height="match_parent"
   android:paddingLeft="@dimen/activity_horizontal_margin"
   android:paddingRight="@dimen/activity_horizontal_margin"
   android:paddingTop="@dimen/activity_vertical_margin"
   android:paddingBottom="@dimen/activity_vertical_margin" tools:context=".MainActivity">
	
   ImageView
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:id="@+id/imageView"
      android:src="@drawable/abc"
      android:layout_alignParentTop="true"
      android:layout_centerHorizontal="true" />
		
   Button
      android:id="@+id/button1"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:layout_marginLeft="76dp"
      android:text="Enable Wifi"
      android:layout_centerVertical="true"
      android:layout_alignEnd="@+id/imageView" />

   Button
      android:id="@+id/button2"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:text="Disable Wifi"
      android:layout_marginBottom="93dp"
      android:layout_alignParentBottom="true"
      android:layout_alignStart="@+id/imageView" />

/RelativeLayout>

Following is the content of AndroidManifest.xml file.

?xml version="1.0" encoding="utf-8"?>
manifest xmlns:android="http://schemas.android.com/apk/res/android"
   package="com.example.sairamkrishna.myapplication" >
   uses-permission android:name="android.permission.ACCESS_WIFI_STATE" />
   uses-permission android:name="android.permission.CHANGE_WIFI_STATE" />
   
   application
      android:allowBackup="true"
      android:icon="@mipmap/ic_launcher"
      android:label="@string/app_name"
      android:theme="@style/AppTheme" >
      
      activity
         android:name=".MainActivity"
         android:label="@string/app_name" >
         
         intent-filter>
            action android:name="android.intent.action.MAIN" />
            category android:name="android.intent.category.LAUNCHER" />
         /intent-filter>
         
      /activity>
      
   /application>
/manifest>

Let&-8217;s try to run your application. I assume you have connected your actual Android Mobile device with your computer. To run the app from Android studio, open one of your project&-8217;s activity files and click Run icon from the toolbar. Before starting your application, Android studio will display following window to select an option where you want to run your Android application.

Select your mobile device as an option and It will shows the following image−

Now click on disable wifi button.then the sample output should be like this −