Each application has 2 bars at the top: status bar and title bar. Sometimes it's useful to remove/hide them (especially in gaming apps).
Sometimes we need to have a static top and bottom with the rest space taken by the main content.
ListView can be used for viewing only with no click. We can disable the orange (default) color on the item when clicked.
Sometimes we need to save some data when the activity is re-created.
Sometimes we need no support the rotation of the screen. It must be either portrait or landscape. There is a solution for each activity
Explaining how to access any resource by variable, not directly. Example with "drawables"
Sometimes we need to restart the Activity. Here is the solution.