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What?s the difference between file, class and activity in android?

File – It is a block of arbitrary information, or resource for storing information. It can be of any type.
  Class – Its a compiled form of .Java file . Android finally used this .class files to produce an executable apk
  Activity – An activity is the equivalent of a Frame/Window in GUI toolkits. It is not a file or a file type it is just a class that can be extended in Android for loading UI elements on view.