This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
Note that an Activity is created, started, resumed,paused, stopped or destroyed automatically by the system depending on events generated by user. There are callbacks with similar names that help you setup your activity and do what you wish to do across activity life cycle.
What you are referring to is refreshing your data set/data store. There is a subtle difference between the two.
You can refresh your data set any time you wish(by calling appropriate method on your data store instance). And if you do it inside correct Activity life cycle method, your users will see updated data every time.
For e.g.: onResume() is called whenever an Activity is brought to the front. So, if you refresh your data set in onResume() the user will see updated view every time.