The Android operating system is very focused on the concept of an activity. An activity is a task or unit of work that users can perform on their screen. For example, users would perform a phone activity for dialing a number and carry out a second activity for interacting with their address book to locate the number. Each Android application is a collection of one or more activities that users can launch and press the hardware's back key on their device to exit or cancel. The user's history is kept in the Android back stack, which you can manipulate from code in special cases. When a new activity starts, the previous one is paused and maintained in memory for later use, unless the operating system is running low on memory.
Activities are loosely coupled with each other; in some ways, you can think of them as having completely separate states from one another in memory. Static classes, properties, and fields will persist the life of the application, but the...