W2A.java file starts with a package statement that names the package (
ca.javajeff.w2a) that stores the
W2A class. This is followed by a series of import statements that import various Android API types. Next, the code describes the
W2A class, which extends
W2A first declares an
androidAnimation instance field of type
android.graphics.drawable.AnimationDrawable. Objects of type
AnimationDrawable describe frame-by-frame animations, in which the current drawable is replaced with the next drawable in the animation sequence.
A drawable is something that can be drawn, such as an image.
AnimationDrawable indirectly extends the abstract
android.graphics.drawable.Drawable class, which is a general abstraction for a drawable.
The onCreate() method
All of the app's work takes place in
onCreate(Bundle) method: no other methods are required, which helps to keep this app simple.
onCreate(Bundle) first invokes its same-named superclass method, a rule that must be followed by all overriding activity methods.
This method then executes
setContentView(R.layout.main) to establish the app's user interface.
R.layout.main is an identifier (ID) for an application resource, which resides in a separate file. You interpret this ID as follows:
- R is the name of a class that's generated when the app is being built. This class is named
Rbecause its content identifies various kinds of application resources, including layouts, images, strings, and colors.
- layout is the name of a class that's nested within
R. An application resource whose ID is stored in this class describes a specific layout resource. Each kind of application resource is associated with a nested class that's named in a similar fashion. For example,
stringidentifies string resources.
- main is the name of an
int-based constant declared within
layout. This resource ID identifies the main layout resource. Specifically,
mainrefers to a
main.xmlfile that stores the main activity's layout information.
W2A's only layout resource.
void setContentView(int layoutResID) method instructs Android to create a user interface screen using the layout information stored in
main.xml. Behind the scenes, Android creates the user interface components described in
main.xml and positions them on the device screen as specified by
main.xml's layout data.
The screen is based on views (abstractions of user interface components) and view groups (views that group related user interface components). Views are instances of classes that subclass the
android.view.View class and are analogous to AWT/Swing components. View groups are instances of classes that subclass the abstract
android.view.ViewGroup class and are analogous to AWT/Swing containers. Android refers to specific views (such as buttons or spinners) as widgets.
ImageView androidImage = (ImageView) findViewById(R.id.android);. This statement first calls
View findViewById(int id) method to find the
android.widget.ImageView element declared in
main.xml and identified as
android. It instantiates
ImageView and initializes it to the values declared in the
main.xml file. The statement then saves this object's reference in local variable
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