Android Project : -Folder, File & Description

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S.N. Folder, File & Description
1 srcThis contains the .java source files for your project. By default, it includes an source file having an activity class that runs when your app is launched using the app icon.
2 genThis contains the .R file, a compiler-generated file that references all the resources found in your project. You should not modify this file.
3 binThis folder contains the Android package files .apk built by the ADT during the build process and everything else needed to run an Android application.
4 res/drawable-hdpiThis is a directory for drawable objects that are designed for high-density screens.
5 res/layoutThis is a directory for files that define your app’s user interface.
6 res/valuesThis is a directory for other various XML files that contain a collection of resources, such as strings and colours definitions.
7 AndroidManifest.xmlThis is the manifest file which describes the fundamental characteristics of the app and defines each of its components.

Following section will give a brief overview few of the important application files.

The Main Activity File

The main activity code is a Java file This is the actual application file which ultimately gets converted to a Dalvik executable and runs your application. Following is the default code generated by the application wizard for Hello World! application −

package com.example.helloworld;

import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;

public class MainActivity extends Activity {

   public void onCreate(Bundle savedInstanceState) {
   public boolean onCreateOptionsMenu(Menu menu) {
      getMenuInflater().inflate(, menu);
      return true;

Here, R.layout.activity_main refers to the activity_main.xml file located in theres/layout folder. The onCreate() method is one of many methods that are figured when an activity is loaded.

The Manifest File

Whatever component you develop as a part of your application, you must declare all its components in a manifest.xml which resides at the root of the application project directory. This file works as an interface between Android OS and your application, so if you do not declare your component in this file, then it will not be considered by the OS. For example, a default manifest file will look like as following file −

<manifest xmlns:android=""
   android:versionName="1.0" >
      android:targetSdkVersion="22" />
       android:theme="@style/AppTheme" >
          android:label="@string/title_activity_main" >
             <action android:name="android.intent.action.MAIN" />
             <category android:name="android.intent.category.LAUNCHER"/>

Here <application>…</application> tags enclosed the components related to the application. Attribute android:icon will point to the application icon available under res/drawable-hdpi. The application uses the image named ic_launcher.png located in the drawable folders

The <activity> tag is used to specify an activity and android:name attribute specifies the fully qualified class name of the Activity subclass and theandroid:label attributes specifies a string to use as the label for the activity. You can specify multiple activities using <activity> tags.

The action for the intent filter is named android.intent.action.MAIN to indicate that this activity serves as the entry point for the application. The category for the intent-filter is named android.intent.category.LAUNCHER to indicate that the application can be launched from the device’s launcher icon.

The @string refers to the strings.xml file explained below. Hence,@string/app_name refers to the app_name string defined in the strings.xml file, which is “HelloWorld”. Similar way, other strings get populated in the application.

Following is the list of tags which you will use in your manifest file to specify different Android application components:

  • <activity>elements for activities
  • <service> elements for services
  • <receiver> elements for broadcast receivers
  • <provider> elements for content providers

The Strings File

The strings.xml file is located in the res/values folder and it contains all the text that your application uses. For example, the names of buttons, labels, default text, and similar types of strings go into this file. This file is responsible for their textual content. For example, a default strings file will look like as following file −

   <string name="app_name">HelloWorld</string>
   <string name="hello_world">Hello world!</string>
   <string name="menu_settings">Settings</string>
   <string name="title_activity_main">MainActivity</string>

The R File

The gen/com.example.helloworld/ file is the glue between the activity Java files like and the resources like strings.xml. It is an automatically generated file and you should not modify the content of the file. Following is a sample of file −

 * This class was automatically generated by the
 * aapt tool from the resource data it found.  It
 * should not be modified by hand.

package com.example.helloworld;

public final class R {
   public static final class attr {
   public static final class dimen {
      public static final int padding_large=0x7f040002;
      public static final int padding_medium=0x7f040001;
      public static final int padding_small=0x7f040000;
   public static final class drawable {
      public static final int ic_action_search=0x7f020000;
      public static final int ic_launcher=0x7f020001;
   public static final class id {
      public static final int menu_settings=0x7f080000;
   public static final class layout {
      public static final int activity_main=0x7f030000;
   public static final class menu {
      public static final int activity_main=0x7f070000;
   public static final class string {
      public static final int app_name=0x7f050000;
      public static final int hello_world=0x7f050001;
      public static final int menu_settings=0x7f050002;
      public static final int title_activity_main=0x7f050003;
   public static final class style {
      public static final int AppTheme=0x7f060000;

The Layout File

The activity_main.xml is a layout file available in res/layout directory, that is referenced by your application when building its interface. You will modify this file very frequently to change the layout of your application. For your “Hello World!” application, this file will have following content related to default layout −

<RelativeLayout xmlns:android=""
   android:layout_height="match_parent" >
      tools:context=".MainActivity" />

This is an example of simple RelativeLayout which we will study in a separate chapter. The TextView is an Android control used to build the GUI and it have various attributes like android:layout_width, android:layout_height etc which are being used to set its width and height etc.. The @string refers to the strings.xml file located in the res/values folder. Hence, @string/hello_world refers to the hello string defined in the strings.xml file, which is “Hello World!”.

Running the Application

Let’s try to run our Hello World! application we just created. I assume you had created your AVD while doing environment set-up. To run the app from Eclipse, open one of your project’s activity files and click Run Eclipse Run Icon icon from the tool bar. Eclipse installs the app on your AVD and starts it and if everything is fine with your set-up and application, it will display following Emulator window −

Android Hello World

Congratulations!!! you have developed your first Android Application and now just keep following rest of the tutorial step by step to become a great Android Developer. All the very best.

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