Android is an OS specifically designed for resource constrained environment .

Android is an OS specifically designed for resource constrained environment. Android manages resources intelligently, and will kill applications when other applications need to use memory, and resurrect them later to restore service. Unlike some other phones, which sacrifices basic smartphone functionality, like multitasking, by only allowing one running application at a time). As for how Android determine how to resurrect apps, it's mainly determined by Intents. Applications can register to be notified by the OS when certain events occur, e.g. the phone starting up, you're entering a particular location, the wifi gets turned on, etc. When handling notifications (Intents), the application (or parts of it) may get resurrected. In a sense, conceptually in Android, all installed applications are always running all the time and you have conceptually infinite memory. The OS will kill and resurrect applications as needed to maintain this illusion.