Common Tasks and How to Do Them in Android

• Creating an Android Application using the Eclipse plugin
• Creating an Android Application without the Eclipse plugin
• Adding an External Library (.jar) using Eclipse
• Implementing Activity callbacks (Android calls your activity at various key moments in its life cycle. You must know how to handle each of these to draw your screen, initialize class members, and acquire data.)
• Opening a new screen
• Listening for button clicks
• Configuring general window properties
• Referring to localhost from the emulated environment
• Storing and retrieving state
• Storing and retrieving preferences
• Storing and retrieving larger or more complex persistent data (files and data)
• Playing audio, video, still, or other media files
• Listening for and broadcasting global messages and setting alarms
• Displaying alerts
• Displaying a progress bar
• Adding items to the screen menu
• Display a web page
• Binding to data
• Getting a Handle to a Screen Element
• Capture images from the phone camera
• Handling expensive operations in the UI thread
• Selecting, highlighting, or styling portions of text
• Utilizing attributes in a Map query
• List of files for an Android application
• Print messages to a log file

The ApiDemos sample application includes many, many examples of common tasks and UI features. See the code inside <sdk>samples/ApiDemos and the other sample applications under the samples/ folder in the SDK.

Creating an Android Application using the Eclipse Plugin

Using the Android Eclipse plugin is the fastest and easiest way to start creating a new Android application. The plugin automatically generates the correct project structure for your application, and keeps the resources compiled for you automatically.

It is still a good idea to know what is going on though. Take a look at Application Fundamentals to understand the basics of how an Android application works.

You should also take a look at the ApiDemos application and the other sample applications included in the SDK, in the <sdk>/samples/ folder in the SDK.

Finally, a great way to started with Android development in Eclipse is to follow both the Hello, World and Notepad code tutorials. In particular, the start of the Hello Android tutorial is an excellent introduction to creating a new Android application in Eclipse.

Creating an Android Application without the Eclipse Plugin

This topic describes the manual steps in creating an Android application. Before reading this, you should read Application Fundamentals to understand the basics of how an Android application works. You might also want to look at the sample code included with the Android SDK, in the <sdk>/samples/ directory.

Here is a list of the basic steps in building an application.

1. Create your required resource files   This includes the AndroidManifest.xml global description file, string files that your application needs, and layout files describing your user interface. A full list of optional and required files and syntax details for each is given in File List for an Android Application.
2. Design your user interface   See User Interface for details on elements of the Android screen.
3. Implement your Activity (this page)   You will create one class/file for each screen in your application. Screens will inherit from an android.app class, typically android.app.Activity for basic screens, android.app.ListActivity for list screens, or android.app.Dialog for dialog boxes. You will implement the required callbacks that let you draw your screen, query data, and commit changes, and also perform any required tasks such as opening additional screens or reading data from the device. Common tasks, such as opening a new screen or reading data from the device, are described below. The list of files you'll need for your application are described in List of Files for an Android Application.
4. Build and install your package. The Android SDK has some nice tools for generating projects and debugging code.

Adding an External Library (.jar) using Eclipse

You can use a third party JAR in your application by adding it to your Eclipse project as follows:

1. In the Package Explorer panel, right-click on your project and select Properties.
2. Select Java Build Path, then the tab Libraries.
3. Press the Add External JARs... button and select the JAR file.

Alternatively, if you want to include third party JARs with your package, create a new directory for them within your project and select Add Library... instead.

It is not necessary to put external JARs in the assets folder.

Implementing Activity Callbacks

Android calls a number of callbacks to let you draw your screen, store data before pausing, and refresh data after closing. You must implement at least some of these methods. Read the Activities document to learn when and in what order these methods are called. Here are some of the standard types of screen classes that Android provides:

• android.app.Activity - This is a standard screen, with no specialization.
• android.app.ListActivity - This is a screen that is used to display a list of something. It hosts a ListView object, and exposes methods to let you identify the selected item, receive callbacks when the selected item changes, and perform other list-related actions.
• android.app.Dialog - This is a small, popup dialog-style window that isn't intended to remain in the history stack. (It is not resizeable or moveable by the user.)

Opening a New Screen

Your Activity will often need to open another Activity screen as it progresses. This new screen can be part of the same application or part of another application, the new screen can be floating or full screen, it can return a result, and you can decide whether to close this screen and remove it from the history stack when you are done with it, or to keep the screen open in history. These next sections describe all these options.

Floating or full?

When you open a new screen you can decide whether to make it transparent or floating, or full-screen. The choice of new screen affects the event sequence of events in the old screen (if the new screen obscures the old screen, a different series of events is called in the old screen). See the Activities document for details.

Transparent or floating windows are implemented in three standard ways:

• Create an app.Dialog class
• Create an app.AlertDialog class
• Set the Theme_Dialog theme attribute to @android:style/Theme.Dialog in your AndroidManifest.xml file. For example:

  <activity class="AddRssItem" android:label="Add an item" android:theme="@android:style/Theme.Dialog"/>

Calling startActivity() or startActivityForResult() will open a new screen in whatever way it defines itself (if it uses a floating theme it will be floating, otherwise it will be full screen).

Opening a Screen

When you want to open a new screen, you can either explicitly specify the activity class to open, or you can let the operating system decide which screen to open, based upon the data and various parameters you pass in. A screen is opened by calling startActivity and passing in an Intent object, which specifies the criteria for the handling screen. To specify a specific screen, call Intent.setClass or setClassName with the exact activity class to open. Otherwise, set a variety of values and data, and let Android decide which screen is appropriate to open. Android will find one or zero Activities that match the specified requirements; it will never open multiple activities for a single request. More information on Intents and how Android resolves them to a specific class is given in the Intent topic.

Some Intent examples

The following snippet loads the com.android.samples.Animation1 class, and passes it some arbitrary data.:

Intent myIntent = new Intent();
myIntent.setClassName("com.android.samples", "com.android.samples.Animation1");
myIntent.putExtra("com.android.samples.SpecialValue", "Hello, Joe!"); // key/value pair, where key needs current package prefix.
startActivity(myIntent);    

The next snippet requests that a Web page be opened by specifying the VIEW action, and a URI data string starting with "http://" schema:

Intent myIntent = new Intent(Intent.VIEW_ACTION, Uri.parse("http://www.google.com"));

Here is the intent filter from the AndroidManifest.xml file for com.android.browser:

<intent-filter>
    <action android:name="android.intent.action.VIEW" />
    <category android:name="android.intent.category.DEFAULT" />
    <scheme android:name="http" />
    <scheme android:name="https" />
    <scheme android:name="file" />
</intent-filter> 

Android defines a number of standard values, for instance the action constants defined by Intent. You can define custom values, but both the caller and handler must use them. See the <intent-filter> tag description in The AndroidManifest.xml File for more information on the manifest syntax for the handling application.

Returning a Result from a Screen

A window can return a result after it closes. This result will be passed back into the calling Activity's onActivityResult() method, which can supply an Intent containing arbitrary data, along with the request code passed to startActivityForResult(). Note that you must call the startActivityForResult() method that accepts a request code parameter to get this callback. The following code demonstrates opening a new screen and retrieving a result.

// Open the new screen.
public void onClick(View v){
    // Start the activity whose result we want to retrieve.  The
    // result will come back with request code GET_CODE.
    Intent intent = new Intent(this, com.example.app.ChooseYourBoxer.class);
    startActivityForResult(intent, CHOOSE_FIGHTER);
}
// Listen for results.
protected void onActivityResult(int requestCode, int resultCode, Intent data){
    // See which child activity is calling us back.
    switch (requestCode) {
        case CHOOSE_FIGHTER:
            // This is the standard resultCode that is sent back if the
            // activity crashed or didn't doesn't supply an explicit result.
            if (resultCode == RESULT_CANCELED){
                myMessageboxFunction("Fight cancelled");
            } 
            else {
                myFightFunction(data);
            }
        default:
            break;
    }
}
// Class SentResult
// Temporary screen to let the user choose something.
    private OnClickListener mLincolnListener = new OnClickListener(){
        public void onClick(View v) {
            Bundle stats = new Bundle();
            stats.putString("height","6\'4\""); 
            stats.putString("weight", "190 lbs");
            stats.putString("reach", "74\"");
            setResult(RESULT_OK, "Lincoln", stats);
            finish();
        }
    };
    private OnClickListener mWashingtonListener = new OnClickListener() {
        public void onClick(View v){
            Bundle stats = new Bundle();
            stats.putString("height","6\'2\""); 
            stats.putString("weight", "190 lbs");
            stats.putString("reach", "73\"");
            setResult(RESULT_OK, "Washington", stats);
            finish();
        }
    };
	

Lifetime of the new screen

An activity can remove itself from the history stack by calling Activity.finish() on itself, or the activity that opened the screen can call Activity.finishActivity() on any screens that it opens to close them.

Listening for Button Clicks

Button click and other UI event capturing are covered in Input Events.

Configuring General Window Properties

You can set a number of general window properties, such as whether to display a title, whether the window is floating, and whether it displays an icon, by calling methods on the Window member of the underlying View object for the window. Examples include calling getWindow().requestFeature() (or the convenience method requestWindowFeature(some_feature)) to hide the title. Here is an example of hiding the title bar:

//Hide the title bar
requestWindowFeature(Window.FEATURE_NO_TITLE);

A better way to achieve the same end is to specify a theme in your Android Manifest file:

<application android:icon="@drawable/icon" android:theme="@android:style/Theme.NoTitleBar">

This is preferable because it tells the system not to show a title bar while your application is starting up. With the explicit method call, your application will have a title bar visible to the user until onCreate runs.

(Note that this can be applied to either the <application> tag or to individual <activity> tags.)

Caution: This theme will also hide the Action Bar on Android 3.0 and higher. If you want to keep the Action Bar, but hide the title bar, see how you can select a theme based on platform version.

Referring to localhost from the emulated environment

If you need to refer to your host computer's localhost, such as when you want the emulator client to contact a server running on the same host, use the alias 10.0.2.2 to refer to the host computer's loopback interface. From the emulator's perspective, localhost (127.0.0.1) refers to its own loopback interface.

Storing and Retrieving State

If your application is dumped from memory because of space concerns, it will lose all user interface state information such as checkbox state and text box values as well as class member values. Android calls Activity.onSaveInstanceState before it pauses the application. This method hands in a Bundle that can be used to store name/value pairs that will persist and be handed back to the application even if it is dropped from memory. Android will pass this Bundle back to you when it calls onCreate(). This Bundle only exists while the application is still in the history stack (whether or not it has been removed from memory) and will be lost when the application is finalized. See the topics for onSaveInstanceState(Bundle) and onCreate(Bundle) for examples of storing and retrieving state.

Read more about the lifecycle of an activity in Activities document.

Storing and Retrieving Larger or More Complex Persistent Data

Your application can store files or complex collection objects, and reserve them for private use by itself or other activities in the application, or it can expose its data to all other applications on the device. See Storing, Retrieving, and Exposing Data to learn how to store and retrieve private data, how to store and retrieve common data from the device, and how to expose your private data to other applications.

Playing Media Files

Please see the document Audio and Video for more details.

Listening For and Broadcasting Global Messages, and Setting Alarms

You can create a listening class that can be notified or even instantiated whenever a specific type of system message is sent.

The listening classes, called broadcast receivers, extend BroadcastReceiver. If you want Android to instantiate the object whenever an appropriate intent notification is sent, define the receiver with a <receiver> element in the AndroidManifext.xml file. If the caller is expected to instantiate the object in preparation to receive a message, this is not required. The receiver will get a call to their BroadcastReceiver.onReceive() method. A receiver can define an <intent-filter> tag that describes the types of messages it will receive. Just as Android's IntentResolver will look for appropriate Activity matches for a startActivity() call, it will look for any matching Receivers (but it will send the message to all matching receivers, not to the "best" match).

To send a notification, the caller creates an Intent object and calls Context.sendBroadcast() with that Intent. Multiple recipients can receive the same message. You can broadcast an Intent message to an intent receiver in any application, not only your own. If the receiving class is not registered using <receiver> in its manifest, you can dynamically instantiate and register a receiver by calling Context.registerReceiver().

Receivers can include intent filters to specify what kinds of intents they are listening for. Alternatively, if you expect a single known caller to contact a single known receiver, the receiver does not specify an intent filter, and the caller specifies the receiver's class name in the Intent by calling Intent.setClassName() with the recipient's class name. The recipient receives a Context object that refers to its own package, not to the package of the sender.

Note:   If a receiver or broadcaster enforces permissions, your application might need to request permission to send or receive messages from that object. You can request permission by using the <uses-permission> tag in the manifest.

Here is a code snippet of a sender and receiver. This example does not demonstrate registering receivers dynamically. For a full code example, see the AlarmService class in the ApiDemos project.

Sending the message

// We are sending this to a specific recipient, so we will
// only specify the recipient class name. 
Intent intent = new Intent(this, AlarmReceiver.class);
intent.putExtra("message","Wake up.");
sendBroadcast(intent);

Receiving the message

Receiver AndroidManifest.xml (because there is no intent filter child, this class will only receive a broadcast when the receiver class is specified by name, as is done in this example):

<receiver class=".AlarmReceiver" />

Receiver Java code:

public class AlarmReceiver extends BroadcastReceiver{
    // Display an alert that we've received a message.    
    @Override 
    public void onReceive(Context context, Intent intent){
	    // Send a text notification to the screen.
        NotificationManager nm = (NotificationManager)
        context.getSystemService(Context.NOTIFICATION_SERVICE);
        nm.notifyWithText(R.id.alarm,
                          "Alarm!!!",
                          NotificationManager.LENGTH_SHORT,
                          null);
   }
}   

Other system messages

You can listen for other system messages sent by Android as well, such as USB connection/removal messages, SMS arrival messages, and timezone changes. See Intent for a list of broadcast messages to listen for. Messages are marked "Broadcast Action" in the documentation.

Listening for phone events

The android.telephony package overview page describes how to register to listen for phone events.

Setting Alarms

Android provides an AlarmManager service that will let you specify an Intent to send at a designated time. This intent is typically used to start an application at a preset time. (Note: If you want to send a notification to a sleeping or running application, use Handler instead.)

Displaying Alerts

There are two major kinds of alerts that you may display to the user: (1) Normal alerts are displayed in response to a user action, such as trying to perform an action that is not allowed. (2) Out-of-band alerts, called notifications, are displayed as a result of something happening in the background, such as the user receiving new e-mail.

Normal Alerts

Android provides a number of ways for you to show popup notifications to your user as they interact with your application.

Class	Description
app.Dialog	A generic floating dialog box with a layout that you design.
app.AlertDialog	A popup alert dialog with two buttons (typically OK and Cancel) that take callback handlers. See the section after this table for more details.
ProgressDialog	A dialog box used to indicate progress of an operation with a known progress value or an indeterminate length (setProgress(bool)). See Views > Progress Bar in ApiDemos for examples.
Activity	By setting the theme of an activity to android:theme="@android:style/Theme.Dialog", your activity will take on the appearance of a normal dialog, floating on top of whatever was underneath it. You usually set the theme through the android:theme attribute in your AndroidManifest.xml. The advantage of this over Dialog and AlertDialog is that Application has a much better managed life cycle than dialogs: if a dialog goes to the background and is killed, you cannot recapture state, whereas Application exposes a Bundle of saved values in onCreate() to help you maintain state.

AlertDialog

This is a basic warning dialog box that lets you configure a message, button text, and callback. You can create one by calling using the AlertDialog.Builder class, as shown here.

private Handler mHandler = new Handler() {
    public void handleMessage(Message msg) {
        switch (msg.what) {
            case ACCEPT_CALL:
            answer(msg.obj);
            break;
            case BOUNCE_TO_VOICEMAIL:
            voicemail(msg.obj);
            break;
        }
    }
};
private void IncomingMotherInlawCall(Connection c) {
    String Text;
    // "Answer" callback.
    Message acceptMsg = Message.obtain();
    acceptMsg.target = mHandler;
    acceptMsg.what = ACCEPT_CALL;
    acceptMsg.obj = c.getCall();
    // "Cancel" callback.
    final Message rejectMsg = Message.obtain();
    rejectMsg.target = mHandler;
    rejectMsg.what = BOUNCE_TO_VOICEMAIL;
    rejectMsg.obj = c.getCall();
    new AlertDialog.Builder(this)
      .setMessage("Phyllis is calling")
      .setPositiveButton("Answer", acceptMsg)
      .setOnCanceListener(new OnCancelListener() {
        public void onCancel(DialogInterface dialog) {
          rejectMsg.sendToTarget();
        }});
      .show();
}    

Notifications

Out-of-band alerts should always be displayed using the NotificationManager, which allows you to tell the user about something they may be interested in without disrupting what they are currently doing. A notification can be anything from a brief pop-up box informing the user of the new information, through displaying a persistent icon in the status bar, to vibrating, playing sounds, or flashing lights to get the user's attention. In all cases, the user must explicitly shift their focus to the notification before they can interact with it.

The following code demonstrates using NotificationManager to display a basic text popup when a new SMS message arrives in a listening service, and provides the current message count. You can see several more examples in the ApiDemos application, under app/ (named notification*.java).

static void setNewMessageIndicator(Context context, int messageCount){
   // Get the static global NotificationManager object.
   NotificationManager nm = NotificationManager.getDefault();

   // If we're being called because a new message has been received, 
   // then display an icon and a count. Otherwise, delete the persistent
   // message.
   if (messageCount > 0) {
      nm.notifyWithText(myApp.NOTIFICATION_GUID,      // ID for this notification.
                messageCount + " new message" + messageCount > 1 ? "s":"", // Text to display.
                NotificationManager.LENGTH_SHORT); // Show it for a short time only.
   }
}

To display a notification in the status bar and have it launch an intent when the user selects it (such as the new text message notification does), call NotificationManager.notify(), and pass in vibration patterns, status bar icons, or Intents to associate with the notification.

Displaying a Progress Bar

An activity can display a progress bar to notify the user that something is happening. To display a progress bar in a screen, call Activity.requestWindowFeature(Window.FEATURE_PROGRESS). To set the value of the progress bar, call Activity.getWindow().setFeatureInt(Window.FEATURE_PROGRESS, level). Progress bar values are from 0 to 9,999, or set the value to 10,000 to make the progress bar invisible.

You can also use the ProgressDialog class, which enables a dialog box with an embedded progress bar to send a "I'm working on it" notification to the user.

Adding Items to the Screen Menu

See Menus.

Display a Web Page

Use the webkit.WebView object.

Binding to Data

You can bind a ListView to a set of underlying data by using a shim class called ListAdapter (or a subclass). ListAdapter subclasses bind to a variety of data sources, and expose a common set of methods such as getItem() and getView(), and uses them to pick View items to display in its list. You can extend ListAdapter and override getView() to create your own custom list items. There are essentially only two steps you need to perform to bind to data:

1. Create a ListAdapter object and specify its data source
2. Give the ListAdapter to your ListView object.

That's it!

Here's an example of binding a ListActivity screen to the results from a cursor query. (Note that the setListAdapter() method shown is a convenience method that gets the page's ListView object and calls setAdapter() on it.)

// Run a query and get a Cursor pointing to the results.
Cursor c = People.query(this.getContentResolver(), null);
startManagingCursor(c);
// Create the ListAdapter. A SimpleCursorAdapter lets you specify two interesting things:
// an XML template for your list item, and
// The column to map to a specific item, by ID, in your template.
ListAdapter adapter = new SimpleCursorAdapter(this,  
                android.R.layout.simple_list_item_1,  // Use a template that displays a text view
                c,                                    // Give the cursor to the list adapter
                new String[] {People.NAME} ,          // Map the NAME column in the people database to...
                new String[] {"text1"});              // The "text1" view defined in the XML template
setListAdapter(adapter);

See view/List4 in the ApiDemos project for an example of extending ListAdapter for a new data type.

Getting a Handle to a Screen Element

You can get a handle to a screen element by calling Activity.findViewById. You can then use the handle to set or retrieve any values exposed by the object.

Capture Images from the Phone Camera

You can hook into the device's camera onto your own Canvas object by using the Camera class. See that class's documentation, and the ApiDemos project's Camera Preview application (Graphics/Camera Preview) for example code.

Handling Expensive Operations in the UI Thread

Avoid performing long-running operations (such as network I/O) directly in the UI thread — the main thread of an application where the UI is run — or your application may be blocked and become unresponsive. Here is a brief summary of the recommended approach for handling expensive operations:

1. Create a Handler object in your UI thread
2. Spawn off worker threads to perform any required expensive operations
3. Post results from a worker thread back to the UI thread's handler either through a Runnable or a Message
4. Update the views on the UI thread as needed

The following outline illustrates a typical implementation:

public class MyActivity extends Activity {
    [ . . . ]
    // Need handler for callbacks to the UI thread
    final Handler mHandler = new Handler();
    // Create runnable for posting
    final Runnable mUpdateResults = new Runnable() {
        public void run() {
            updateResultsInUi();
        }
    };
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        [ . . . ]
    }
    protected void startLongRunningOperation() {
        // Fire off a thread to do some work that we shouldn't do directly in the UI thread
        Thread t = new Thread() {
            public void run() {
                mResults = doSomethingExpensive();
                mHandler.post(mUpdateResults);
            }
        };
        t.start();
    }
    private void updateResultsInUi() {
        // Back in the UI thread -- update our UI elements based on the data in mResults
        [ . . . ]
    }
}

For further discussions on this topic, see Designing for Responsiveness and the Handler documentation.

Selecting, Highlighting, or Styling Portions of Text

You can highlight or style the formatting of strings or substrings of text in a TextView object. There are two ways to do this:

• If you use a string resource, you can add some simple styling, such as bold or italic using HTML notation. The currently supported tags are: B (bold), I (italic), U (underline), TT (monospace), BIG, SMALL, SUP (superscript), SUB (subscript), and STRIKE (strikethrough). So, for example, in res/values/strings.xml you could declare this:
  <resource>
    <string id="@+id/styled_welcome_message">We are <b><i>so</i></b> glad to see you.</string>
</resources>
• To style text on the fly, or to add highlighting or more complex styling, you must use the Spannable object as described next.

To style text on the fly, you must make sure the TextView is using Spannable storage for the text (this will always be true if the TextView is an EditText), retrieve its text with getText(), and call setSpan(Object, int, int, int), passing in a new style class from the android.text.style package and the selection range.

The following code snippet demonstrates creating a string with a highlighted section, italic section, and bold section, and adding it to an EditText object.

// Get our EditText object.
EditText vw = (EditText)findViewById(R.id.text);
// Set the EditText's text.
vw.setText("Italic, highlighted, bold.");
// If this were just a TextView, we could do:
// vw.setText("Italic, highlighted, bold.", TextView.BufferType.SPANNABLE);
// to force it to use Spannable storage so styles can be attached.
// Or we could specify that in the XML.
// Get the EditText's internal text storage
Spannable str = vw.getText();
// Create our span sections, and assign a format to each.
str.setSpan(new StyleSpan(android.graphics.Typeface.ITALIC), 0, 7, Spannable.SPAN_EXCLUSIVE_EXCLUSIVE);
str.setSpan(new BackgroundColorSpan(0xFFFFFF00), 8, 19, Spannable.SPAN_EXCLUSIVE_EXCLUSIVE);
str.setSpan(new StyleSpan(android.graphics.Typeface.BOLD), 21, str.length() - 1, Spannable.SPAN_EXCLUSIVE_EXCLUSIVE);

Utilizing attributes in a Map query

When using a search intent to ask the Maps activity to search for something, the Maps activity responds to the following attributes in the optional context bundle:

               float "centerLatitude" default 0.0f
               float "centerLongitude" default 0.0f
               float "latitudeSpan" default 0.0f
               float "longitudeSpan" default 0.0f
               int "zoomLevel" default 10

This context information is used to center the search result in a particular area, and is equivalent to adjusting the Map activity to the described location and zoom level before issuing the query.

If the latitudeSpan, longitudeSpan, and zoomLevel attributes are not consistent, then it is undefined which one takes precedence.

List of Files for an Android Application

The following list describes the structure and files of an Android application. Many of these files can be built for you (or stubbed out) by the android tool shipped in the tools/ menu of the SDK.

MyApp/
AndroidManifest.xml	(required) Advertises the screens that this application provides, where they can be launched (from the main program menu or elsewhere), any content providers it implements and what kind of data they handle, where the implementation classes are, and other application-wide information. Syntax details for this file are described in The AndroidManifest.xml File.
src/         /myPackagePath/.../MyClass.java	(required) This folder holds all the source code files for your application, inside the appropriate package subfolders.
res/	(required) This folder holds all the resources for your application. Resources are external data files or description files that are compiled into your code at build time. Files in different folders are compiled differently, so you must put the proper resource into the proper folder. (See Resources for details.)
anim/                animation1.xml                         ...	(optional) Holds any animation XML description files that the application uses. The format of these files is described in Resources.
drawable/                       some_picture.png                       some_stretchable.9.png                       some_background.xml                                     ...	(optional) Zero or more files that will be compiled to android.graphics.drawable resources. Files can be image files (png, gif, or other) or XML files describing other graphics such as bitmaps, stretchable bitmaps, or gradients. Supported bitmap file formats are PNG (preferred), JPG, and GIF (discouraged), as well as the custom 9-patch stretchable bitmap format. These formats are described in Resources.
layout/                       screen_1_layout.xml                                  ...	(optional) Holds all the XML files describing screens or parts of screens. Although you could create a screen in Java, defining them in XML files is typically easier. A layout file is similar in concept to an HTML file that describes the screen layout and components. See User Interface for more information about designing screens, and Available Resource Types for the syntax of these files.
values/                       arrays                       classes.xml                       colors.xml                       dimens.xml                       strings.xml                       styles.xml                       values.xml	(optional) XML files describing additional resources such as strings, colors, and styles. The naming, quantity, and number of these files are not enforced--any XML file is compiled, but these are the standard names given to these files. However, the syntax of these files is prescribed by Android, and described in Resources.
xml/	(optional) XML files that can be read at run time on the device.
raw/	(optional) Any files to be copied directly to the device.

Print Messages to a Log File

To write log messages from your application:

1. Import android.util.Log.
2. Use Log.v(), Log.d(), Log.i(), Log.w(), or Log.e() to log messages. (See the Log class.)
   E.g., Log.e(this.toString(), "error: " + err.toString())
3. Launch DDMS from a terminal by executing ddms in your Android SDK /tools path.
4. Run your application in the Android emulator.
5. From the DDMS application, select the emulator (e.g., "emulator-5554") and click Device > Run logcat... to view all the log data.

Note: If you are running Eclipse and encounter a warning about the VM debug port when opening DDMS, you can ignore it if you're only interested in logs. However, if you want to further inspect and control your processes from DDMS, then you should close Eclipse before launching DDMS so that it may use the VM debugging port.