Designing the User Interface(Chapter 3 of Hello, Android)

　　A layout is a container for one or more child objects and a behavior to position them on the screen within the rectangle of the parent object.
　　Here is a list of the most common layouts provided by Android:
　　FrameLayout: classArranges its children so they all start at the top left of the screen. This is used for tabbed views and image switchers.
　　LinearLayout: Arranges its children in a single column or row. This is the most common layout you will use.
　　RelativeLayout: Arranges its children in relation to each other or to the parent. This is often used in forms.
　　TableLayout: Arranges its children in rows and columns, similar to an HTML table.

　　The Log class provides several static methods to print messages of various severity levels to the Android system log:
　　• Log.e( ): Errors
　　• Log.w( ): Warnings
　　• Log.i( ): Information
　　• Log.d( ): Debugging
　　• Log.v( ): Verbose
　　Users will never see this log, but as a developer you can view it in a couple ways. In Eclipse, open the LogCat view by selecting Window > Show View > Other... > Android > LogCat.