Once you provide a resource in your application (discussed in Providing Resources), you can apply it by referencing its resource ID. All resource IDs are defined in your project's {@code R} class, which the {@code aapt} tool automatically generates.
When your application is compiled, {@code aapt} generates the {@code R} class, which contains resource IDs for all the resources in your {@code res/} directory. For each type of resource, there is an {@code R} subclass (for example, {@code R.drawable} for all drawable resources), and for each resource of that type, there is a static integer (for example, {@code R.drawable.icon}). This integer is the resource ID that you can use to retrieve your resource.
Although the {@code R} class is where resource IDs are specified, you should never need to look there to discover a resource ID. A resource ID is always composed of:
There are two ways you can access a resource:
R.string.hello
{@code string} is the resource type and {@code hello} is the resource name. There are many Android APIs that can access your resources when you provide a resource ID in this format. See Accessing Resources in Code.
@string/hello
{@code string} is the resource type and {@code hello} is the resource name. You can use this syntax in an XML resource any place where a value is expected that you provide in a resource. See Accessing Resources from XML.
You can use a resource in code by passing the resource ID as a method parameter. For example, you can set an {@link android.widget.ImageView} to use the {@code res/drawable/myimage.png} resource using {@link android.widget.ImageView#setImageResource(int) setImageResource()}:
ImageView imageView = (ImageView) findViewById(R.id.myimageview); imageView.setImageResource(R.drawable.myimage);
You can also retrieve individual resources using methods in {@link android.content.res.Resources}, which you can get an instance of with {@link android.content.Context#getResources()}.
While uncommon, you might need access your original files and directories. If you do, then saving your files in {@code res/} won't work for you, because the only way to read a resource from {@code res/} is with the resource ID. Instead, you can save your resources in the {@code assets/} directory.
Files saved in the {@code assets/} directory are not given a resource ID, so you can't reference them through the {@code R} class or from XML resources. Instead, you can query files in the {@code assets/} directory like a normal file system and read raw data using {@link android.content.res.AssetManager}.
However, if all you require is the ability to read raw data (such as a video or audio file), then save the file in the {@code res/raw/} directory and read a stream of bytes using {@link android.content.res.Resources#openRawResource(int) openRawResource()}.
Here's the syntax to reference a resource in code:
[<package_name>.]R.<resource_type>.<resource_name>
See Resource Types for more information about each resource type and how to reference them.
There are many methods that accept a resource ID parameter and you can retrieve resources using methods in {@link android.content.res.Resources}. You can get an instance of {@link android.content.res.Resources} with {@link android.content.Context#getResources Context.getResources()}.
Here are some examples of accessing resources in code:
// Load a background for the current screen from a drawable resource
{@link android.app.Activity#getWindow()}.{@link
android.view.Window#setBackgroundDrawableResource(int)
setBackgroundDrawableResource}(R.drawable.my_background_image) ;
// Set the Activity title by getting a string from the Resources object, because
// this method requires a CharSequence rather than a resource ID
{@link android.app.Activity#getWindow()}.{@link android.view.Window#setTitle(CharSequence)
setTitle}(getResources().{@link android.content.res.Resources#getText(int)
getText}(R.string.main_title));
// Load a custom layout for the current screen
{@link android.app.Activity#setContentView(int)
setContentView}(R.layout.main_screen);
// Set a slide in animation by getting an Animation from the Resources object
mFlipper.{@link android.widget.ViewAnimator#setInAnimation(Animation)
setInAnimation}(AnimationUtils.loadAnimation(this,
R.anim.hyperspace_in));
// Set the text on a TextView object using a resource ID
TextView msgTextView = (TextView) findViewById(R.id.msg);
msgTextView.{@link android.widget.TextView#setText(int)
setText}(R.string.hello_message);
Caution: You should never modify the {@code R.java} file by hand—it is generated by the {@code aapt} tool when your project is compiled. Any changes are overridden next time you compile.
You can define values for some XML attributes and elements using a reference to an existing resource. You will often do this when creating layout files, to supply strings and images for your widgets.
For example, if you add a {@link android.widget.Button} to your layout, you should use a string resource for the button text:
<Button
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:text="@string/submit" />
Here is the syntax to reference a resource in an XML resource:
@[<package_name>:]<resource_type>/<resource_name>
See Resource Types for more information about each resource type and how to reference them.
In some cases you must use a resource for a value in XML (for example, to apply a drawable image to a widget), but you can also use a resource in XML any place that accepts a simple value. For example, if you have the following resource file that includes a color resource and a string resource:
<?xml version="1.0" encoding="utf-8"?> <resources> <color name="opaque_red">#f00</color> <string name="hello">Hello!</string> </resources>
You can use these resources in the following layout file to set the text color and text string:
<?xml version="1.0" encoding="utf-8"?>
<EditText xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:textColor="@color/opaque_red"
android:text="@string/hello" />
In this case you don't need to specify the package name in the resource reference because the resources are from your own package. To reference a system resource, you would need to include the package name. For example:
<?xml version="1.0" encoding="utf-8"?>
<EditText xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:textColor="@android:color/secondary_text_dark"
android:text="@string/hello" />
Note: You should use string resources at all times, so that your application can be localized for other languages. For information about creating alternative resources (such as localized strings), see Providing Alternative Resources. For a complete guide to localizing your application for other languages, see Localization.
You can even use resources in XML to create aliases. For example, you can create a drawable resource that is an alias for another drawable resource:
<?xml version="1.0" encoding="utf-8"?>
<bitmap xmlns:android="http://schemas.android.com/apk/res/android"
android:src="@drawable/other_drawable" />
This sounds redundant, but can be very useful when using alternative resource. Read more about Creating alias resources.
A style attribute resource allows you to reference the value of an attribute in the currently-applied theme. Referencing a style attribute allows you to customize the look of UI elements by styling them to match standard variations supplied by the current theme, instead of supplying a hard-coded value. Referencing a style attribute essentially says, "use the style that is defined by this attribute, in the current theme."
To reference a style attribute, the name syntax is almost identical to the normal resource format, but instead of the at-symbol ({@code @}), use a question-mark ({@code ?}), and the resource type portion is optional. For instance:
?[<package_name>:][<resource_type>/]<resource_name>
For example, here's how you can reference an attribute to set the text color to match the "primary" text color of the system theme:
<EditText id="text"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:textColor="?android:textColorSecondary"
android:text="@string/hello_world" />
Here, the {@code android:textColor} attribute specifies the name of a style attribute
in the current theme. Android now uses the value applied to the {@code android:textColorSecondary}
style attribute as the value for {@code android:textColor} in this widget. Because the system
resource tool knows that an attribute resource is expected in this context,
you do not need to explicitly state the type (which would be
?android:attr/textColorSecondary)—you can exclude the {@code attr} type.
Android contains a number of standard resources, such as styles, themes, and layouts. To
access these resource, qualify your resource reference with the
android package name. For example, Android provides a layout resource you can use for
list items in a {@link android.widget.ListAdapter}:
{@link android.app.ListActivity#setListAdapter(ListAdapter)
setListAdapter}(new {@link
android.widget.ArrayAdapter}<String>(this, android.R.layout.simple_list_item_1, myarray));
In this example, {@link android.R.layout#simple_list_item_1} is a layout resource defined by the platform for items in a {@link android.widget.ListView}. You can use this instead of creating your own layout for list items. For more information, see the List View developer guide.