The main activity for your app must be declared in the manifest with an <intent-filter> that includes theMAIN action and LAUNCHER category. For example:

<activity android:name=".MainActivity" android:label="@string/app_name">
    <intent-filter>
        <action android:name="android.intent.action.MAIN" />
        <category android:name="android.intent.category.LAUNCHER" />
    </intent-filter>
</activity>

Note: When you create a new Android project with the Android SDK tools, the default project files include anActivity class that's declared in the manifest with this filter.

If either the MAIN action or LAUNCHER category are not declared for one of your activities, then your app icon will not appear in the Home screen's list of apps.

Caution: Your activity will be destroyed and recreated each time the user rotates the screen. When the screen changes orientation, the system destroys and recreates the foreground activity because the screen configuration has changed and your activity might need to load alternative resources (such as the layout).

Once you’ve decided on the languages you will support, create the resource subdirectories and string resource files. For example:

MyProject/
    res/
       values/
           strings.xml
       values-es/
           strings.xml
       values-fr/
           strings.xml

For example, this project includes a default layout and an alternative layout for large screens:
MyProject/
    res/
        layout/              # default (portrait)
            main.xml
        layout-land/         # landscape
            main.xml
        layout-large/        # large (portrait)
            main.xml
        layout-large-land/   # large landscape
            main.xml


You should always provide bitmap resources that are properly scaled to each of the generalized density buckets: low, medium, high and extra-high density. This helps you achieve good graphical quality and performance on all screen densities.

To generate these images, you should start with your raw resource in vector format and generate the images for each density using the following size scale:

xhdpi: 2.0
hdpi: 1.5
mdpi: 1.0 (baseline)
ldpi: 0.75


This means that if you generate a 200x200 image for xhdpi devices, you should generate the same resource in 150x150 for hdpi, 100x100 for mdpi, and 75x75 for ldpi devices.

Then, place the files in the appropriate drawable resource directory:
MyProject/
    res/
        drawable-xhdpi/
            awesomeimage.png
        drawable-hdpi/
            awesomeimage.png
        drawable-mdpi/
            awesomeimage.png
        drawable-ldpi/
            awesomeimage.png


Any time you reference @drawable/awesomeimage, the system selects the appropriate bitmap based on the screen's density.

Note: Low-density (ldpi) resources aren’t always necessary. When you provide hdpi assets, the system scales them down by one half to properly fit ldpi screens.


Android provides a unique code for each platform version in the Build constants class. Use these codes within your app to build conditions that ensure the code that depends on higher API levels is executed only when those APIs are available on the system.
private void setUpActionBar() {
    // Make sure we're running on Honeycomb or higher to use ActionBar APIs
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {
        ActionBar actionBar = getActionBar();
        actionBar.setDisplayHomeAsUpEnabled(true);
    }
}