Android OS (operating system) is an operating system for mobile devices, smartphones and tablets. It is developed by Google's mobile phone alliance, which consists of an operating system, middleware, user interface and application software, and is claimed to be the first truly open and complete mobile software for mobile devices.
The original meaning of the word Android refers to "robot", which is an operating system based on the open kernel of Linux. It is a mobile operating system announced by Google on November 5, 2007. Android was originally developed by a company originally called "Android". After Google acquired "Android.Inc" in 2005, it continued to develop and operate the Android system. It adopted a software stack (aka software stack) architecture. Mainly divided into three parts. The underlying Linux kernel only provides basic functionality, and other applications are developed by companies themselves, some of which are written in Java.
Android uses Java as the programming language, from interface to function, there are endless changes, Activity is equivalent to J2ME MIDlet, an Activity class (class) is responsible for creating windows (window), an activity in the activity is in the foreground (foreground) The mode, the background running program is called Service. The two are connected by ServiceConnection and AIDL to achieve the effect of running multiple programs at the same time. If the entire screen of the running Activity is replaced by another Activity, the Activity is stopped (stopped) and even killed by the system.