Using Android Async Task to Perform Background Operations

Async Task is an android helper class that will allows you to run a background processes. If you are a Java programmer, may be you are familiar with thread. If you run a long task in the main thread, the current user interface will be blocked untill the task has finished. This is not a good practice. To avoid the user interface is being blocked, the long task must be perform in another thread.

Async Task Overview

In android, the only thread that can modify the user interface is called UI thread. This is means you can’t modify user interface from another thread. This is a problem, because in another side, we need to run a long task in separate thread and also we need to update the user interface to display the task progress. Actually, android provides a [cci]Handler[/cci] class that allows you to update the user interface from another thread. But we’ll discuss it in the next tutorial.

Rather than create new thread and handler, android has provides a helper class, [cci]AsyncTask[/cci], to do the same thing. The concept is same, [cci]AsyncTask[/cci] is a smart helper that encapsulates the creation of thread and handler.

[cci]AsyncTask[/cci] must be subclassed to be used. There are three methods that need to be overridden, [cci]doInbackground[/cci], [cci]onProgressUpdate[/cci] and [cci]onPostExecute[/cci]. The first method is mandatory to be overriden, the rest are optional.

[cci]doInbackground[/cci] is a method where the execution codes is placed. [cci]doProgressUpdate[/cci] and [cci]onPostExecute[/cci] run in UI thread, so user interface can be modified in both of methods. Put any codes for displaying the task progress on [cci]doProgressUpdate[/cci]. The [cci]doPostExecute[/cci] method will be called once the task has finished.

[cci]AsyncTask[/cci] is a generic class. [cci]AsyncTask[/cci] is subclassed in the form [cci]AsyncTask[/cci].

  1. [cci]Params[/cci], the type of the parameters sent to the task upon execution.
  2. [cci]Progress[/cci], the type of the progress units published during the background computation.
  3. [cci]Result[/cci], the type of the result of the background computation.

To give a better understanding, a sample application that use [cci]AsyncTask[/cci] will be given.

Creating Sample Application

We’ll create a sample application to demonstrate the use of [cci]AsyncTask[/cci] class to perform background processes. Our goal is to create a simple application which download a file from server and display the download progress in the progress bar.

Our application have very simple user interface that only contains a progress bar and a button.

AsyncTask Simple Application
AsyncTask Simple Application

If user click Start Button above, the application will start download a file from server and display the progress in progress bar. After the download process has completed, the file is saved into android root folder (/sdcard).

In this demonstration, I will use a file located on my server, http://semurjengkol.com/dl/files/CustomArrayAdapter.rar(445KB), this is an eclipse project from previous tutorial. You can also use this file or change the url as you want.

Now, open your Eclipse IDE and create new Android Application Project, then follow step by step below:

  1. Create new layout file [cci]res/layout/activity_main.xml[/cci]
    This file defines the application main layout which contain a progress bar and a button.

    <LinearLayout 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:orientation="vertical"
        android:paddingLeft="5dp"
        android:paddingRight="5dp" >
    
        <ProgressBar
            android:id="@+id/progress_bar"
            style="?android:attr/progressBarStyleHorizontal"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:indeterminate="false"
            android:max="100"
            android:progress="0" />
    
        <Button
            android:id="@+id/start_button"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="center_horizontal"
            android:text="@string/start_download" />
    
    </LinearLayout>
    

    The resource [cci]@string/start_download[/cci] we used above is defined in strings resources [cci]res/values/strings.xml[/cci]

    <?xml version="1.0" encoding="utf-8"?>
    <resources>
        <string name="app_name">AsyncTaskSample</string>
        <string name="action_settings">Settings</string>
        <string name="hello_world">Hello world!</string>
        <string name="start_download">Start Download</string>
    </resources>
    
  2. Create new java class [cci]src/DownloadListener.java[/cci]
    This is a listener that will be used by [cci]AsyncTask[/cci] to communicate with activity to change the user interface.

    package com.sj.asynctask;
    
    import java.io.File;
    
    public interface DownloadListener {
        public void onDownloadComplete(File filename);
        public void onProgressUpdate(int progress);
        public void onDownloadFailure(final String msg);
    }
    
  3. Create new java class [cci]src/DownloadTask.java[/cci]
    This file extends [cci]AsyncTask[/cci] and will do the download task and call the listener to display the progress result. The [cci]doInBackground[/cci] method retrieves two string parameters:

    1. URL of the file to be downloaded
    2. The destination storage location to save the file
    package com.sj.asynctask;
    
    import java.io.BufferedInputStream;
    import java.io.File;
    import java.io.FileOutputStream;
    import java.io.IOException;
    import java.io.InputStream;
    import java.io.OutputStream;
    import java.net.MalformedURLException;
    import java.net.URL;
    import java.net.URLConnection;
    
    import android.os.AsyncTask;
    import android.os.Environment;
    
    public class DownloadTask extends AsyncTask<String, Integer, Boolean> {
        /** This is our listener*/
        private final DownloadListener listener;
        /** Hold message if download failure*/
        private String msg;
        /** Save Destination */
        private File saveTo; 
        
        public DownloadTask(DownloadListener listener) {
            this.listener = listener;
        }
        
        @Override
        protected Boolean doInBackground(String... params) {
            /** Params should be array of string with length 2. 
             * 1. url
             * 2. filename destination*/
            if(params == null || params.length < 2) {
                msg = "Incomplete parameters";
                return false;
            }
            
            String sUrl = params[0];
            String filename = params[1]; 
    
            /** get root directory: /sdcard */
            File rootDir = Environment.getExternalStorageDirectory(); 
            /** create destination file*/
            saveTo = new File(rootDir, filename);
        
            try {
                /** define url*/
                URL url = new URL(sUrl);
                /** open the connection*/
                URLConnection conn = url.openConnection();
                conn.connect();
                
                /** get the size of file that will be downloaded. It will be used to measure the progress*/
                int fileLength = conn.getContentLength(); 
                
                /** create input and outpout stream*/
                InputStream is = new BufferedInputStream(url.openStream());
                OutputStream os = new FileOutputStream(saveTo);
                
                /** create buffer*/
                byte buffer[] = new byte[512];
                /** hold total of downloaded bytes*/
                long totalDownloaded = 0;
                int count;
                
                while ((count = is.read(buffer)) != -1) {
                    totalDownloaded += count;
                    
                    /**cause call to onProgressUpdate, which is run on UI thread*/
                    publishProgress((int) (totalDownloaded * 100 / fileLength));                 
                    os.write(buffer, 0, count);
                }
                
                /** closing stream*/
                os.flush();
                os.close();
                is.close();
                return true;
            } 
            catch (MalformedURLException e) {
                msg = "Invalid URL";
            }
            catch (IOException e) {
                msg = "No internet connection";
            }
            
            return false;
        }
        
        @Override
        protected void onProgressUpdate(Integer... values) {
            if(listener != null) listener.onProgressUpdate(values[0]);
        }
        
        @Override
        protected void onPostExecute(Boolean result) {
            if(!result) {
                if(listener != null) listener.onDownloadFailure(msg);
                return;
            } 
            
            if(listener != null) listener.onDownloadComplete(saveTo);
        }
    }
    

  4. Create main activity [cci]src/MainActivity.java[/cci]
    This is our main activity that will inflate the [cci]res/layout/activity_main.xml[/cci] to build our main user interface. The activity will create and execute [cci]DownloadTask[/cci] object when Start Button is clicked. Our activity also implements the [cci]DownloadListener[/cci] and set itself as a listener to [cci]DownloadTask[/cci] object. The [cci]DownloadTask[/cci] object will call the listener method implementation in the activity when the progress updated, download failure and when download completed.

    package com.sj.asynctask;
    
    import java.io.File;
    
    import android.app.Activity;
    import android.os.Bundle;
    import android.view.View;
    import android.view.View.OnClickListener;
    import android.widget.Button;
    import android.widget.ProgressBar;
    import android.widget.Toast;
    
    public class MainActivity extends Activity implements OnClickListener, DownloadListener{
        private ProgressBar progressBar;
        private Button startButton;
        
        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);
            initView();
        }
        
        private void initView() {
            progressBar = (ProgressBar) findViewById(R.id.progress_bar);
            startButton = (Button)findViewById(R.id.start_button);
            
            if(startButton != null) startButton.setOnClickListener(this);
        }
    
        @Override
        public void onClick(View v) {
            switch (v.getId()) {
            case R.id.start_button:
                /** file url to be downloaded*/
                String url = "http://semurjengkol.com/dl/files/CustomArrayAdapter.rar";
                /** destination filename*/
                String filename = "CustomArrayAdapter.rar";
                
                if(startButton != null) startButton.setEnabled(false);
                
                DownloadTask task = new DownloadTask(this);
                task.execute(url, filename);
                break;
            default:
                break;
            }        
        }
    
        @Override
        public void onDownloadComplete(File filename) {
            if(filename != null) {
                Toast.makeText(this, "Download complete. File is saved to " + filename.getPath(), 
                        Toast.LENGTH_LONG).show();
            }
            
            if(startButton != null) startButton.setEnabled(true);
        }
    
        @Override
        public void onProgressUpdate(int progress) {
            if(progressBar != null) progressBar.setProgress(progress);
        }
    
        @Override
        public void onDownloadFailure(String msg) {
            Toast.makeText(this, msg, Toast.LENGTH_LONG).show();
            if(progressBar != null) progressBar.setProgress(0);
            if(startButton != null) startButton.setEnabled(true);
        }
    }
    
  5. Adding permission to [cci]AndroidManifest.xml[/cci]
    This simple application has two main features:

    • Connecting to internet to download a file
    • Save the file into sdcard storage

    In order to our application runs well, we must add [cci]uses-permission[/cci] in our [cci]AndroidManifest.xml[/cci] related to above features. Add the following lines into [cci]AndroidManifest.xml[/cci]:

    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
    <uses-permission android:name="android.permission.INTERNET" />    
    

    Below is our final [cci]AndroidManifest.xml[/cci]:

    <?xml version="1.0" encoding="utf-8"?>
    <manifest xmlns:android="http://schemas.android.com/apk/res/android"
        package="com.sj.asynctask"
        android:versionCode="1"
        android:versionName="1.0" >
    
        <uses-sdk
            android:minSdkVersion="8"
            android:targetSdkVersion="17" />
    
        <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
        <uses-permission android:name="android.permission.INTERNET" />
        
        <application
            android:allowBackup="true"
            android:icon="@drawable/ic_launcher"
            android:label="@string/app_name"
            android:theme="@style/AppTheme" >
            <activity
                android:name="com.sj.asynctask.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>
    

Now run the project and click the start button, it will download the file and display the progress on the progress bar

Async Task Download Progress
Async Task Download Progress

Using Custom Array Adapter on Android List View

List View is an element that displays a collection of items in single column direction. It has an internal vertical scroll bar that enable user to scroll if it’s height bigger than display height. The type of single item in the list is any of java object.

[cci]ListView[/cci] needs an adapter to works. This adapter behaves as a data resources for the list. The adapter also defines how each items in the list is displayed. The adapter is attached to the list via [cci]setAdapter[/cci] method on the [cci]ListView[/cci] object. There are two commons used adapters:

  • Array Adapter: An adapter that is designed to work with arrays data resources.
  • Cursor Adapter: An adapter that is designed to handle database related data.

This tutorial will focuses on array adapter.

Simple Array Adapter

A simple array adapter is an array adapter that is built using array of string as items. The array of string is passed to the constructor of [cci]ArrayAdapter[/cci] object.

List<String> apps = new ArrayList<String>();
apps.add("Twitter");
apps.add("Whatsapp");
apps.add("Facebook");
ArrayAdapter<String> adapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, apps);

The constructor of array adapter have 3 arguments:

  1. context: this is the application context
  2. layout resource id: the layout resource id for item view
  3. array items: this is array object for items

We will attach the created adapter into the list by using [cci]setAdapter[/cci] method on [cci]ListView[/cci] object.

List Activity

[cci]ListActivity[/cci] is a subclass of [cci]Activity[/cci] that will make our work with [cci]ListView[/cci] easier. [cci]ListActivity[/cci] has internal implementation method that is related to [cci]ListView[/cci]. You must have list view object with id is [cci]android:id/list[/cci] in the layout file to use List Activity.

<ListView
   android:id="@android:id/list"
   android:layout_width="match_parent"
   android:layout_height="wrap_content" >
</ListView>

Now we will create a sample list view application using simple array adapter. Open your Eclipse IDE and create new android project by selecting menu: File -> New -> Android Application Project. Fill out required fields.

  1. Create new xml file: [cci]activity_main.xml[/cci]
    <LinearLayout 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" >
    
        <ListView
            android:id="@android:id/list"
            android:layout_width="match_parent"
            android:layout_height="wrap_content">
        </ListView>
    
    </LinearLayout>
    
  2. Create new activity class: [cci]MainActivity.java[/cci]
    package com.sj.customlistview;
    
    import java.util.ArrayList;
    import java.util.List;
    
    import android.app.ListActivity;
    import android.os.Bundle;
    import android.widget.ArrayAdapter;
    
    public class MainActivity extends ListActivity{    
    
        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);        
            setContentView(R.layout.activity_main);        
            initView();   
        }
    
        private void initView() {
            List<String> apps = new ArrayList<String>();
            apps.add("Twitter");
            apps.add("Whatsapp");
            apps.add("Facebook");
            ArrayAdapter<String> adapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, apps);
            setListAdapter(adapter);
        }
    }
    
  3. Put your activity in [cci]AndroidManifest.xml[/cci] file
  4. <?xml version="1.0" encoding="utf-8"?>
    <manifest xmlns:android="http://schemas.android.com/apk/res/android"
        package="com.sj.customlistview"
        android:versionCode="1"
        android:versionName="1.0" >
    
        <uses-sdk
            android:minSdkVersion="8"
            android:targetSdkVersion="17" />
    
        <application
            android:allowBackup="true"
            android:icon="@drawable/ic_launcher"
            android:label="@string/app_name"
            android:theme="@style/AppTheme" >
            <activity
                android:name="com.sj.customlistview.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>
    

Run your project, your list view will be displayed like image below:

Simple Android List View
Simple Android List View


Customize Array Adapter

By default, array adapter displays a [cci]TextView[/cci] for each items and set the text of it by calling [cci]toString[/cci] method of item object. We can customize the default display of array adapter by overriding [cci]getView[/cci] method.
For a demo, we will modify previous project and use custom array adapter:

  1. Create new java class: [cci]Application.java[/cci].
    This class represents a single object of list item.

    package com.sj.customlistview;
    
    public class Application {
        private String title;
        private long totalDl;
        private int rating;
        private String icon;
        
        public String getTitle() {
            return title;
        }
        public void setTitle(String title) {
            this.title = title;
        }
        public long getTotalDl() {
            return totalDl;
        }
        public void setTotalDl(long totalDl) {
            this.totalDl = totalDl;
        }
        public int getRating() {
            return rating;
        }
        public void setRating(int rating) {
            this.rating = rating;
        }
        public String getIcon() {
            return icon;
        }
        public void setIcon(String icon) {
            this.icon = icon;
        }
    }
    
  2. Create new xml file: [cci]app_custom_list.xml[/cci]
    This file defines a view for list item

    <?xml version="1.0" encoding="utf-8"?>
    <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical" >
    
        <ImageView
            android:id="@+id/appIcon"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignParentLeft="true"
            android:layout_alignParentTop="true"        
            android:layout_marginLeft="3dp"
            android:layout_marginTop="3dp"
            android:src="@drawable/facebook" />
        <TextView
            android:id="@+id/titleTxt"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignTop="@id/appIcon"
            android:layout_toRightOf="@id/appIcon"
            android:layout_marginLeft="3dp"
            android:text="Application Title"
            android:textSize="22dp"/>
    
        <LinearLayout
            android:id="@+id/ratingCntr"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_toRightOf="@id/appIcon"
            android:layout_below="@id/titleTxt"
            android:layout_marginLeft="5dp" >
        </LinearLayout>
    
        <TextView
            android:id="@+id/dlTxt"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_below="@id/titleTxt"
            android:layout_alignParentRight="true"
            android:layout_marginRight="3dp"
            android:text="0 dl" />    
    </RelativeLayout>
    
  3. Create new java class: [cci]AppAdapter.java[/cci]
    This is our adapter that will inflate [cci]app_custom_list.xml[/cci] and set the data into it.

    package com.sj.customlistview;
    
    import java.text.NumberFormat;
    import java.util.List;
    
    import android.content.Context;
    import android.content.res.Resources;
    import android.view.LayoutInflater;
    import android.view.View;
    import android.view.ViewGroup;
    import android.widget.ArrayAdapter;
    import android.widget.ImageView;
    import android.widget.LinearLayout;
    import android.widget.TextView;
    
    public class AppAdapter extends ArrayAdapter<Application>{
        private List<Application> items;
        
        public AppAdapter(Context context, List<Application> items) {
            super(context, R.layout.app_custom_list, items);
            this.items = items;
        }
        
        @Override
        public int getCount() {
            return items.size();
        }
        
        @Override
        public View getView(int position, View convertView, ViewGroup parent) {
            View v = convertView;
            
            if(v == null) {
                LayoutInflater li = LayoutInflater.from(getContext());
                v = li.inflate(R.layout.app_custom_list, null);            
            }
            
            Application app = items.get(position);
            
            if(app != null) {
                ImageView icon = (ImageView)v.findViewById(R.id.appIcon);
                TextView titleText = (TextView)v.findViewById(R.id.titleTxt);
                LinearLayout ratingCntr = (LinearLayout)v.findViewById(R.id.ratingCntr);
                TextView dlText = (TextView)v.findViewById(R.id.dlTxt);
                
                if(icon != null) {
                    Resources res = getContext().getResources();
                    String sIcon = "com.sj.customlistview:drawable/" + app.getIcon();
                    icon.setImageDrawable(res.getDrawable(res.getIdentifier(sIcon, null, null)));
                }
                
                if(titleText != null) titleText.setText(app.getTitle());
                
                if(dlText != null) {
                    NumberFormat nf = NumberFormat.getNumberInstance();
                    dlText.setText(nf.format(app.getTotalDl())+" dl");            
                }
                
                if(ratingCntr != null && ratingCntr.getChildCount() == 0) {        
                    /*
                     * max rating: 5
                     */
                    for(int i=1; i<=5; i++) {
                        ImageView iv = new ImageView(getContext());
                        
                        if(i <= app.getRating()) {
                            iv.setImageDrawable(getContext().getResources().getDrawable(R.drawable.start_checked));
                        }
                        else {                
                            iv.setImageDrawable(getContext().getResources().getDrawable(R.drawable.start_unchecked));
                        }
                        
                        ratingCntr.addView(iv);
                    }
                }
            }
            
            return v;
        }
    }
    
  4. Open your [cci]MainActivity.java[/cci] and modify several lines.
    package com.sj.customlistview;
    
    import java.util.ArrayList;
    import java.util.List;
    
    import android.app.ListActivity;
    import android.os.Bundle;
    
    public class MainActivity extends ListActivity{    
    
        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);        
            setContentView(R.layout.activity_main);        
            initView();   
        }
    
        private void initView() {
            List<Application> apps = populateSampleApplication();
            AppAdapter adapter = new AppAdapter(this, apps);
            setListAdapter(adapter);        
        }
        
        private List<Application> populateSampleApplication(){        
            String[] apps = new String[] {                
                "Twitter,450000,5,twitter",
                "Skype,300002,2,skype",
                "Facebook,500560,4,facebook"
            };
            
            List<Application> list = new ArrayList<Application>();
            
            for(String app:apps) {
                String[] rApp = app.split(","); 
                Application ap = new Application();
                ap.setTitle(rApp[0]);
                ap.setTotalDl(Integer.parseInt(rApp[1]));
                ap.setRating(Integer.parseInt(rApp[2]));
                ap.setIcon(rApp[3]);
                list.add(ap);
            }
            
            return list;
        }
    }
    

I’ve used some social icons in this project. You can download the icons here: social icons
Now run your project, it will display list view below:

Custom Array Adapter
Custom Array Adapter

Android Gravity and Layout Gravity

Gravity is an android method for aligning view in a layout. There are two kinds of gravity, gravity and layout gravity. Basically a gravity is set in xml layout files, but you can also set a gravity of any view in java source code. To set gravity in xml use [cci]android:layout_gravity[/cci] and [cci]android:gravity[/cci] attributes. The values of both attributes is combination of the following constants:

top : Push object to the top of its container, not changing its size.
bottom : Push object to the bottom of its container, not changing its size.
left : Push object to the left of its container, not changing its size.
right : Push object to the right of its container, not changing its size.
center_vertical : Place object in the vertical center of its container, not changing its size.
fill_vertical : Grow the vertical size of the object if needed so it completely fills its container.
center_horizontal : Place object in the horizontal center of its container, not changing its size.
fill_horizontal : Grow the horizontal size of the object if needed so it completely fills its container.
center : Place the object in the center of its container in both the vertical and horizontal axis, not changing its size.
fill : Grow the horizontal and vertical size of the object if needed so it completely fills its container.
clip_vertical : Additional option that can be set to have the top and/or bottom edges of the child clipped to its container’s bounds. The clip will be based on the vertical gravity: a top gravity will clip the bottom edge, a bottom gravity will clip the top edge, and neither will clip both edges.
clip_horizontal : Additional option that can be set to have the left and/or right edges of the child clipped to its container’s bounds. The clip will be based on the horizontal gravity: a left gravity will clip the right edge, a right gravity will clip the left edge, and neither will clip both edges.
start : Push object to the beginning of its container, not changing its size.
end : Push object to the end of its container, not changing its size.

android:gravity attribute

[cci]android:gravity[/cci] will set alignment to the content of a view. This aligment will only take effect if size of view is bigger than view’s content. Maybe you got confuse, what is the different between view and view content. Consider a button view below.

Button View
Button View

As you can see from image above, the button view is the rectangle boundary of a button, while the content of the button view is the text “button” it self.

Now, we will align the content of the button to bottom right of the button. To align content to bottom right we add [cci]android:gravity=”bottom|right”[/cci] attribute.

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >

    <Button
        android:id="@+id/button1"
        android:layout_width="match_parent"
        android:layout_height="70dp"
        android:text="Button"
        android:gravity="bottom|right" />

</LinearLayout>

Will result as follow:

Content of button view aligned to bottom right
Content of button view aligned to bottom right

android:layout_gravity attribute

[cci]android:layout_gravity[/cci] is used to align view child to its parent. Note that this attribute will only take effect if the parent is [cci]LinearLayout[/cci] and the size of child smaller than parent size.


Now we will align a button to the right of parent. To do this, we must add [cci]android:layout_gravity=”right”[/cci] attribute.

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Button"
        android:layout_gravity="right" />

</LinearLayout>

The result is as follow:

Button view is aligned to right
Button view is aligned to right

Advance Android Relative Layout Example

Relative Layout is an android view group to align all childrens relative to other view or its parent. Relative layout give you flexibiliy to place childs wherever you want. In the previous example, I have give some basic example of Relative Layout. Now, I will give you how to create a more complex relative layout example.

Our goal is to create layout like this:

Advance relative layout example
Advance relative layout example

See the following step by step to create above layout:

1. Aligning child to top left of parent

Actually, by default every childs is aligned to top left of parent in relative layout. So we needn’t add any special attribute to align chilc in top left of parent. But you can align child to top left of parent by adding [cci]android:layout_alignParentLeft=”true”[/cci] and [cci]android:layout_alignParentTop=”true”[/cci] attributes.

<Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/button1"
        android:layout_alignParentLeft="true"
        android:layout_alignParentTop="true" />

2. Aligning child in top right of parent

To align child in top right of parent, add the following attributes [cci]android:layout_alignParentRight=”true”[/cci] and [cci]android:layout_alignParentTop=”true”[/cci].

<Button
        android:id="@+id/button2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/button2"
        android:layout_alignParentRight="true"
        android:layout_alignParentTop="true" />

3. Aligning child in bottom left of parent

To align child in bottom left of parent, add the following attributes [cci]android:layout_alignParentBottom=”true”[/cci] and [cci]android:layout_alignParentLeft=”true”[/cci].

<Button
        android:id="@+id/button2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/button2"
        android:layout_alignParentBottom="true"
        android:layout_alignParentLeft="true" />

4. Aligning child in bottom right of parent

To align child in bottom right of parent, add the following attributes [cci]android:layout_alignParentBottom=”true”[/cci] and [cci]android:layout_alignParentRight=”true”[/cci].

<Button
        android:id="@+id/button2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/button2"
        android:layout_alignParentBottom="true"
        android:layout_alignParentRight="true" />

5. Align child in the center of parent

This is our center child and you must give it an [cci]id[/cci] as our other 4 childs aligned relative this child. To align a child center of the parent, add the following attribute [cci]android:layout_centerInParent=”true”[/cci]. This attribute will align our child center to the parent, vertically and horizontally.

<Button
        android:id="@+id/button5"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/button5"
        android:layout_centerInParent="true"/>

6. Align child in top left of the center child

To align child in top left of the center child, add the following attributes [cci]android:layout_toLeftOf=”@id/button5″[/cci] and [cci]android:layout_above=”@id/button5″[/cci].

<Button
        android:id="@+id/button6"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/button6"
        android:layout_toLeftOf="@id/button5"
        android:layout_above="@id/button5"/>


7. Align child in top right of the center child

To align child in top right of the center child, add the following attributes [cci]android:layout_toRightOf=”@id/button5″[/cci] and [cci]android:layout_above=”@id/button5″[/cci].

<Button
        android:id="@+id/button6"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/button6"
        android:layout_toRightOf="@id/button5"
        android:layout_above="@id/button5"/>

8. Align child in bottom right of the center child

To align child in bottom right of the center child, add the following attributes [cci]android:layout_toRightOf=”@id/button5″[/cci] and [cci]android:layout_below=”@id/button5″[/cci].

<Button
        android:id="@+id/button6"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/button6"
        android:layout_toRightOf="@id/button5"
        android:layout_below="@id/button5"/>

9. Align child in bottom left of the center child

To align child in bottom left of the center child, add the following attributes [cci]android:layout_toLeftOf=”@id/button5″[/cci] and [cci]android:layout_below=”@id/button5″[/cci].

<Button
        android:id="@+id/button6"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/button6"
        android:layout_toLeftOf="@id/button5"
        android:layout_below="@id/button5"/>

Final Source Codes

Below is the final source codes:

<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" >

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/button1"
        android:layout_alignParentLeft="true"
        android:layout_alignParentTop="true" />

    <Button
        android:id="@+id/button2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/button2"
        android:layout_alignParentRight="true"
        android:layout_alignParentTop="true" />

    <Button
        android:id="@+id/button3"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/button3"
        android:layout_alignParentBottom="true"
        android:layout_alignParentLeft="true" />
    
    <Button
        android:id="@+id/button4"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/button4"
        android:layout_alignParentBottom="true"
        android:layout_alignParentRight="true" />
    
    <Button
        android:id="@+id/button5"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/button5"
        android:layout_centerInParent="true"/>
    
    <Button
        android:id="@+id/button6"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/button6"
        android:layout_toLeftOf="@id/button5"
        android:layout_above="@id/button5"/>
    <Button
        android:id="@+id/button7"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/button7"
        android:layout_toRightOf="@id/button5"
        android:layout_above="@id/button5"/>
    <Button
        android:id="@+id/button8"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/button8"
        android:layout_toRightOf="@id/button5"
        android:layout_below="@id/button5"/>
    <Button
        android:id="@+id/button9"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/button9"
        android:layout_toLeftOf="@id/button5"
        android:layout_below="@id/button5"/>
</RelativeLayout>

Android Linear Layout Example

Linear Layout is an android View Group that aligns all children in single oriented manner, vertically or horizontally. A Linear layout can have only one orientation, only vertical or only horizontal. The orientation can be specified by using attribute [cci]android:orientation[/cci].

Horizontal Orientation

By default, if you don’t specify the attribute [cci]android:orientation[/cci], the orientation of linear layout will be horizontal. But you can put [cci]android:orientation=”horizontal”[/cci] to set horizontal orientation. For example:

<LinearLayout 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:orientation="horizontal" >

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/hello_world"
         />
    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/hello_world1"
         />
    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/hello_world2"
         />
</LinearLayout>

The result of above layout is:

Linear layout example
Linear layout example

Vertical Orientation

To set the orieantation of linear layout to vertical, add [cci]android:orientation=”vertical”[/cci] to the View Group. For example:

<LinearLayout 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:orientation="vertical" >

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/hello_world"
         />
    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/hello_world1"
         />
    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/hello_world2"
         />
</LinearLayout>

The result of above layout is:

Linear layout example
Linear layout example


Flexible Width or Hight of Children

Linear Layout support flexibility of width or height for its child to consume remaining space in the parent. Flexible width only supported in horizontal orientation and flexible height only supported in vertical orietation. Use [cci]android:layout_weight[/cci] to make a child flexible. The value of the attribute is an integer value. The larger number of [cci]android:layout_weight[/cci] of a child, it consumes more extra space in the parent. Default value of the attributes in 0.

Flexible Width in Horizontal Orientation

To make width of a child flexible, you must set it width to [cci]0dp[/cci] and layout_weight greater than 0.

<LinearLayout 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:orientation="horizontal" >

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/button1" />

    <Button
        android:id="@+id/button2"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_weight="1"
        android:text="@string/button2" />

    <Button
        android:id="@+id/button3"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/button3" />

         />
</LinearLayout>

Here is the result:

Layout weight example
Layout weight example

Flexible Height in Vertical Orientation

To make height of a child flexible, you must set it height to [cci]0dp[/cci] and layout_weight greater than 0.

<LinearLayout 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:orientation="vertical" >

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/button1" />

    <Button
        android:id="@+id/button2"
        android:layout_width="wrap_content"
        android:layout_height="0dp"
        android:layout_weight="1"
        android:text="@string/button2" />

    <Button
        android:id="@+id/button3"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/button3" />

         />
</LinearLayout>

Here is the result:

Layout weight example on vertical orientation
Layout weight example on vertical orientation

Setting margin to child

Linear Layout respects to margin attribute on child. To set margin, using [cci]android:layout_margin[/cci]. This attribute will apply to all 4 sides, top, right,bottom and left. If you need just apply margin with specified side, you can use [cci]android:layout_marginLeft[/cci],[cci]android:layout_marginTop[/cci],[cci]android:layout_marginRight[/cci],[cci]android:layout_marginBottom[/cci]. Note, you must put margin value in [cci]dp[/cci] unit.
Here is an example:

<LinearLayout 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:orientation="vertical" >

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/button1" />

    <Button
        android:id="@+id/button2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_margin="10dp"
        android:text="@string/button2" />

    <Button
        android:id="@+id/button3"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/button3" />

         />
</LinearLayout>

Here is the result:

Layout margin example
Layout margin example

Setting gravity to child

Another feature of Linear Layout is gravity of a child. Gravity is an aligment of child with its parent. I will give tutorial about gravity in the next tutorial.

Android Relative Layout example

[cci]RelativeLayout[/cci] is one of android container or [cci]ViewGroup[/cci]. Like it’s name, the main feature of [cci]Relativelayout[/cci] is to place one [cci]View[/cci] relative to other [cci]View[/cci] or with it’s parent it’s self.

Place a view relative to other view

In order to make a relative position of one view to another view, an [cci]id[/cci] is must be defined in referred view. Says you have two [cci]TextView[/cci] in layout, you want second [cci]TextView[/cci] placed to the right of first [cci]TextView[/cci] but the top of second [cci]TextView[/cci] is place to bottom of first [cci]TextView[/cci]. To do this, you must define an [cci]id[/cci] to the first [cci]TextView[/cci].

<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">
 
    <TextView
        android:id="@+id/textView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/hello_world"
         />
 
    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_toRightOf="@id/textView1"
        android:layout_below="@id/textView1"        
        android:text="@string/hay"
         />
</RelativeLayout>

In above source code, we have put 2 [cci]TextView[/cci]. The frist [cci]TextView[/cci] have an id [cci]textView1[/cci]. Now, take a look at hightlighted line of the above source code. The line [cci]android:layout_toRightOf=”@id/textView1″[/cci] means set the position of [cci]TextView[/cci] to the right of first [cci]TextView[/cci]. While [cci]android:layout_below=”@id/textView1″[/cci] means place second [cci]TextView[/cci] to the bottom of first [cci]TextView[/cci].
Here the result of above layout:

Relative layuot exampl
Relative layuot exampl

There are more attributes that can be used to make relative position between views. Here are the complete list:

  • [cci]android:layout_toLeftOf[/cci]
    Positions the right edge of this view to the left of the given anchor view ID.
  • [cci]android:layout_toRightOf[/cci]
    Positions the left edge of this view to the right of the given anchor view ID.
  • [cci]android:layout_above[/cci]
    Positions the bottom edge of this view above the given anchor view ID.
  • [cci]android:layout_below[/cci]
    Positions the bottom edge of this view below the given anchor view ID.
  • [cci]android:layout_alignBaseline[/cci]
    Positions the baseline of this view on the baseline of the given anchor view ID.
  • [cci]android:layout_alignLeft[/cci]
    Makes the left edge of this view match the left edge of the given anchor view ID.
  • [cci]android:layout_alignTop[/cci]
    Makes the top edge of this view match the top edge of the given anchor view ID.
  • [cci]android:layout_alignRight[/cci]
    Makes the right edge of this view match the right edge of the given anchor view ID.
  • [cci]android:layout_alignBottom[/cci]
    Makes the bottom edge of this view match the bottom edge of the given anchor view ID.
  • [cci]android:layout_toStartOf[/cci]
    Positions the end edge of this view to the start of the given anchor view ID.
  • [cci]android:layout_toEndOf[/cci]
    Positions the start edge of this view to the end of the given anchor view ID.
  • [cci]android:layout_alignStart[/cci]
    Makes the start edge of this view match the start edge of the given anchor view ID.
  • [cci]android:layout_alignEnd[/cci]
    Makes the end edge of this view match the end edge of the given anchor view ID.

Place a view relative to it’s parent

Another relative layout capability is place a view relative to it’s parent. This means you can make bottom edge of a view match bottom edge of its parent. Just add simple attribute [cci]android:layout_alignParentBottom=”true”[/cci] to your view then your view automatically aligned to its parent. I will give an example how to do this in code.

<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">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/hello_world"
        android:layout_alignParentBottom="true"/>

</RelativeLayout>

Here the result of above layout:

Relative layout example
Relative layout example

Except [cci]android:layout_alignParentBottom=”true”[/cci], there are another attributes for aligning to parent. Here are complete list

  • [cci]android:layout_alignParentTop=”true”[/cci]
    If true, makes the top edge of this view match the top edge of the parent.
  • [cci]android:layout_alignParentLeft=”true”[/cci]
    If true, makes the left edge of this view match the left edge of the parent.
  • [cci]android:layout_alignParentRight=”true”[/cci]
    If true, makes the right edge of this view match the right edge of the parent.
  • [cci]android:layout_alignParentBottom=”true”[/cci]
    If true, makes the bottom edge of this view match the bottom edge of the parent.
  • [cci]android:layout_alignParentTop=”true”[/cci]
    If true, makes the top edge of this view match the top edge of the parent.
  • [cci]android:layout_alignParentStart=”true”[/cci]
    If true, makes the start edge of this view match the start edge of the parent.
  • [cci]android:layout_alignParentEnd=”true”[/cci]
    If true, makes the end edge of this view match the end edge of the parent.

Give an ID to your view in android xml file

Give an ID to your view object is very useful if you want to refer to that view later in source code. For example you want to set text of a TextView in source code. To do that, put [cci]android:id[/cci] attibute to your view.

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

In above code, you can see I have put an [cci]android:id=”@+id/textView1″[/cci]. This line defines an [cci]id[/cci] of our [cci]TextView[/cci] to [cci]textView1[/cci].

Refers view from activity

After you have defined an id, now you can refer to your [cci]TextView[/cci] in an activity like this

TextView textView = (TextView) findViewById(R.id.textView1);

Now, with your [cci]TextView[/cci] in hand, you can set text to it as you want like this

if(textView != null) textView.setText("new text");

Refers view in xml file

Sometimes we need to refers to our view in the xml layout file. For example you want to align top another [cci]TextView[/cci] to other. To achieve this, you can refer to our last [cci]TextView[/cci] like this [cci]@id/textView1[/cci]. Note you must replace ‘+’ sign when referring to already defined [cci]id[/cci]. Use ‘+’ sign only in first time when you define an [cci]id[/cci].

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

    <TextView
        android:id="@+id/textView2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignTop="@id/textView1"/>

Android hello word application

This tutorial will guide you to create a simple hello word application in android. Through this tutorial you will learn about the basic understanding of android activity and layout. Most common tool to create android application is Eclipse ADT plugin. You can follow step by step how to install eclipse ADT Plugin in the official developer site here.

After your eclipse ADT plugin was installed, now you’re ready to create android Hello Word project. In eclipse, select menu

File->New->Other

This will open a wizard.

android project wizard
android project wizard

On the wizard, open  Android sub tree and choose Android Applicaton Project then click Next.

android new project
android new project

The wizard will prompt you to fill below informations:

  1. Application Name: this name will shown in the google play store
  2. Project Name: name used by eclipse, must be unique in a workspace
  3. Package Name: this is your application identifier, must be unique for each application.

The next screen will allow you to put icon for your application. Choose your application icon or just click next until you see this window:

create android activity
create android activity

Activity in android just like a screen that is visible to the user. For now, just choose Blank Activity. 

create android activity
create android activity

Now you must set Activity Name and Layout Name. Activity name is a class name for your activity and layout name is your layout file name. Click finish to close the wizard. Now, you will see a HelloWord project on eclipse project explorer. Here is the folder structure of HelloWord project.

android folder structure
android folder structure

Here is basic description about  project sub folders:

  1. src: contains your java source files
  2. bin: contains your compiled java class
  3. gen: contains android generated files
  4. libs: here you can place third party library (jar file)
  5. res: contains your projects’ resources, like images, icons, layouts, strings, etc

Now open [cci]res/layout/activity_main.xml[/cci] file, it will looks like this:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity" >

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/hello_world"
         />

</RelativeLayout>

This file defines your layout structure. You can place your view object here. There are two categories of view in android, [cci]View[/cci] and [cci]ViewGroup[/cci] or container. [cci]View[/cci] object can be [cci]TextView, EditText, Spinner[/cci], etc while [cci]ViewGroup[/cci] acts as the [cci]View[/cci] container.

As you can see from above source code, it uses [cci]RelativeLayout[/cci] as main container. This layout places it’s childs relative to other childs. With this layout you can place any child overlapping to others.  Another most common layout is [cci]LinearLayout.[/cci] The main function of [cci]LinearLayout[/cci] is to  place their child in orientation manner, vertical or horizontal.

Either [cci]View[/cci] and [cci]ViewGroup[/cci] must have [cci]android:layout_width[/cci] and [cci]android:layout_height[/cci], these attributes respectively defines [cci]View[/cci] width and height. You can put value for both attributes using integer value in [cci]dp[/cci] such as [cci]0dp, 30dp, 100dp[/cci], etc. [cci]dp[/cci] or [cci]dip[/cci] is android unit which means [cci]density independent pixels[/cci]. Also you can put following constant value :

  1. [cci]MATCH_PARENT: [/cci]make the size of [cci]View[/cci] as big as it’s parent (container) minus padding.
  2. [cci]WRAP_CONTENT: [/cci]make the size of[cci] View [/cci]to match content size.

Now take a look inside [cci]RelativeLayout[/cci] at above code. There is one TextView inside with an attribute [cci]android:text:”@string/hello_word”[/cci], this attribute defines the text for [cci]TextView[/cci], but as you can see, the attribute value is not direct string but refers to string resources. You can find the string resource in folder [cci]res/values/string.xml[/cci].

<?xml version="1.0" encoding="utf-8"?>
<resources>

    <string name="app_name">HelloWord</string>
    <string name="action_settings">Settings</string>
    <string name="hello_world">Hello world!</string>
</resources>

Android will search in the string resources for string that has name [cci]hello_word[/cci] and then use the value. Actually you can put direct string like this [cci]android:text:”Hello world!”[/cci], but the recommended way is using string resource to put your string.

Now how to tell android to use this layout? Goto [cci]src[/cci] folder, and open [cci]MainActivity.java[/cci],

package com.semurjengkol.helloword;

import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;

public class MainActivity extends Activity {

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
	}

	@Override
	public boolean onCreateOptionsMenu(Menu menu) {
		// Inflate the menu; this adds items to the action bar if it is present.
		getMenuInflater().inflate(R.menu.main, menu);
		return true;
	}

}

To use you layout in activity, you must override [cci]onCreate[/cci] method of the [cci]Activity[/cci] class and then call [cci]setContentView(R.layout.activity_main);[/cci] now you can run your [cci]HelloWord[/cci] application, it’s will looks like this.

Android hello word app
Android hello word app

Download source code HelloWord